Free NPK Fertilizer Calculator - Elemental N, P, K Breakdown
Break down any NPK fertilizer into elemental nitrogen, phosphorus, and potassium by weight, and see what target N rate your plants need.
Enter your fertilizer bag size and N-P-K numbers to see the actual elemental N, P, K weight and a recommended liquid dose.
The NPK Fertilizer Calculator helps you turn a fertilizer label into numbers you can actually use. A bag that says 20-20-20 looks simple, but those three numbers are not a dose, a schedule, or a promise that every plant needs the same strength. They are percentages by weight, and the phosphorus and potassium numbers are usually label-standard oxide equivalents rather than pure elemental P and K. The difference matters when you are comparing products, mixing liquid feed, or trying to avoid pushing a stressed plant too hard.
Use this calculator when you have a fertilizer product in hand and want to know what its analysis means in practical terms: how much nitrogen is in the bag, how much phosphate and potash the label guarantees, how the oxide numbers convert to elemental phosphorus and potassium, and what a target nitrogen concentration means in grams per litre. It is especially useful for houseplants because small pots leave little room for error. A mistake that barely matters in a garden bed can turn into root burn, salt buildup, or weak growth in a 6-inch pot.
The three numbers on a fertilizer label are the guaranteed analysis. In North American fertilizer labeling, they represent the percentage by weight of nitrogen, available phosphate as P2O5, and soluble potash as K2O; the University of Maryland Extension gives the same definition and a 10-5-5 example by bag weight in its garden fertilizer basics. A 1 kg bag of 20-20-20 therefore contains 200 g of total nitrogen, 200 g of available phosphate equivalent, and 200 g of soluble potash equivalent by label convention.
That does not mean the bag is 60 percent plant food and 40 percent useless filler. The remaining weight may include carrier material, conditioners that reduce caking or dust, water, coating material, or other ingredients listed elsewhere on the label. It also does not mean the fertilizer is automatically right for every plant. The number describes concentration, not need.
The calculator starts with the same label logic: bag weight multiplied by the percentage on the label. If your bag is 2 kg and the label reads 10-5-8, the label-standard nutrient weights are 0.20 kg nitrogen, 0.10 kg phosphate equivalent, and 0.16 kg potash equivalent. That makes product comparison much clearer than comparing ratios alone.
Nitrogen is normally stated as elemental N, but phosphorus and potassium are traditionally stated as oxide equivalents. The Association of American Plant Food Control Officials label guide says primary nutrient guarantees are reported as Total Nitrogen (N), Available Phosphate (P2O5), and Soluble Potash (K2O), and that most other guaranteed nutrients are stated on an elemental basis in the AAPFCO product label guide. This is why a label can look like it is giving elemental N-P-K while actually mixing one elemental number with two oxide-equivalent numbers.
For everyday feeding, you can usually work from the label numbers because product directions, soil-test recommendations, and garden advice often use the same convention. For nutrient accounting, hydroponics, research notes, or direct elemental comparison, convert the oxide forms. The common conversions are:
Those conversion factors are also shown by extension guidance; Maryland Extension lists P2O5 x 0.44 = P and K2O x 0.83 = K in the same fertilizer basics resource. So a 20-20-20 fertilizer is 20 percent N, about 8.7 percent elemental P, and about 16.6 percent elemental K. It is still sold as 20-20-20 because fertilizer labels use the conventional oxide basis.
The calculator does three jobs. First, it multiplies the total package weight by each label percentage so you can see the nutrient guarantee in the same weight unit as the bag. Second, it helps you interpret the phosphorus and potassium values by reminding you when the label is probably using P2O5 and K2O rather than pure P and K. Third, it converts a target nitrogen concentration for liquid feeding into grams of fertilizer per litre of water.
That third job is the most practical for indoor growers. Many fertilizer mistakes happen because the label scoop is designed for broad use, while a houseplant owner is feeding a small root system in a closed container. A target based on nitrogen gives you a cleaner way to compare a 20 percent soluble powder with a 5 percent liquid concentrate. The math asks one direct question: how many grams of product are needed to deliver the target nitrogen concentration?
This tool does not replace the product label, a soil test, or crop-specific guidance. It will not tell you whether a nutrient deficiency is actually present. It will not correct for hard water, potting mix chemistry, slow-release coating behavior, fertilizer age, or the way organic products release nutrients over time. It gives you a transparent starting point so your next step is based on arithmetic rather than guesswork.
For bag weight, the formula is simple:
Nutrient weight = bag weight x label percentage / 100.
If a 500 g pouch is labeled 20-20-20, the label-standard calculation is:
If you want elemental phosphorus and potassium from that same product, apply the conversion factors:
For liquid feed, the calculator uses nitrogen as the anchor. Virginia Cooperative Extension notes that fertilizer recommendations are generally given in ppm nitrogen in the final solution and gives a grams-per-litre formula based on desired ppm and fertilizer nutrient percentage in its fertilizer calculations guide. One ppm is one milligram per litre for water-based mixing at ordinary horticultural precision. The working formula is:
Grams fertilizer per litre = target ppm N / 10,000 / (N percent / 100).
A 150 ppm nitrogen feed from a 20 percent nitrogen fertilizer is therefore 150 / 10,000 / 0.20 = 0.075 percent by weight, which is 0.75 g per litre of water. A 150 ppm nitrogen feed from a 5 percent nitrogen liquid fertilizer would need four times as much product by weight because the nitrogen concentration is one quarter as strong.
Start with the total product weight, not the amount you plan to scoop. If the label gives pounds, enter pounds and read the per-bag result in pounds. If the label gives kilograms or grams, enter that unit consistently and read the result in the same unit. The percentage calculation is unit-neutral; 5 pounds x 10 percent and 5 kilograms x 10 percent both return 0.5 of the same unit.
Then enter the three label numbers. For a product labeled 7-9-5, use 7 for nitrogen, 9 for phosphorus or phosphate, and 5 for potassium or potash. If the product is sold in the United States or Canada, assume the middle and final numbers are P2O5 and K2O unless the label explicitly says it is reporting elemental P and elemental K. Clemson Extension explains that the fertilizer grade indicates percentages by weight of nitrogen, phosphate, and potash in its guide to reading a fertilizer label.
Do not enter the ratio after simplifying it. A 20-20-20 and a 1-1-1 have the same ratio, but they are not the same strength. The calculator needs the actual label percentages because dose depends on concentration. A 20-20-20 soluble powder is four times as concentrated in nitrogen as a 5-5-5 product.
The target ppm field is not a universal prescription. It is a way to translate plant tolerance into a measurable feed strength. A practical indoor range is often low to moderate: weak feeding for sensitive plants, moderate feeding for actively growing foliage plants, and stronger feeding only for heavy feeders in bright conditions. Iowa State University Extension recommends fertilizing houseplants only when they are actively growing and mixing general all-purpose fertilizers at half or quarter label strength in its houseplant fertilizing guidance, which is a useful caution even though the page is not a fertilizer calculator.
For most foliage houseplants, 100 to 200 ppm nitrogen is a reasonable calculator range when the plant is actively growing, the pot drains well, and the root system is healthy; Maryland Extension stresses that large amounts of fertilizer are unnecessary for most indoor plants and that indoor plants should not be fertilized during low-growth winter conditions in its guide to fertilizer for indoor plants. Use the lower end for calathea, maranta, ferns, recently repotted plants, or plants under modest indoor light. Use the higher end only when the plant is growing strongly, receiving bright indirect light or grow-light support, and drying the pot on a predictable schedule.
Heavy feeders need context. Hibiscus is a good example: an Extension expert response describes hibiscus as heavy feeders and suggests balanced fertilizer such as 20-20-20 or 10-10-10 at half label strength outdoors, with less frequent indoor feeding in spring and summer in its hardy hibiscus response. That does not mean every houseplant should receive hibiscus-level feeding. It means a vigorous, light-hungry flowering plant can use more nutrients than a slow-growing understory plant.
Imagine you are choosing between a 1 kg box of 20-20-20 soluble fertilizer and a 1 kg bag of 10-10-10 granular fertilizer. The ratios look identical because both are balanced, but the nutrient concentration is not identical.
The 20-20-20 product contains 200 g nitrogen, 200 g phosphate equivalent, and 200 g potash equivalent per kilogram. Converted to elemental forms, the phosphorus is about 87.2 g and the potassium is about 166 g. The 10-10-10 product contains half as much of each label-standard nutrient per kilogram: 100 g N, 100 g P2O5, and 100 g K2O, or about 43.6 g elemental P and 83 g elemental K.
For a 150 ppm nitrogen liquid feed, the 20 percent nitrogen product needs 0.75 g per litre. The 10 percent nitrogen product needs 1.5 g per litre. If the 10-10-10 product is granular and not meant to dissolve fully, that comparison is a product-form warning rather than a mixing instruction. Use soluble products for liquid feeding unless the label says otherwise. N.C. Cooperative Extension notes that many houseplants benefit from soluble fertilizers in spring and summer in its guide to fertilizing house plants.
This is the main value of the calculator: it separates ratio from concentration. A balanced ratio tells you the nutrients move together. It does not tell you how much product to use.
Say you have a 20-20-20 water-soluble fertilizer and you want a moderate 150 ppm nitrogen feed for a healthy monstera in active growth. Enter 20 for nitrogen and 150 for target ppm. The calculator returns 0.75 g per litre.
If you mix 2 litres of water, multiply the dose by 2: 1.5 g total fertilizer. If you mix 4 litres, use 3 g total fertilizer. Keep the arithmetic linear and avoid rounding upward just because the amount looks small. Concentrated soluble fertilizers are designed to work at small weights.
Now change the target to 75 ppm for a sensitive plant or a plant under lower light. The dose becomes 0.375 g per litre. That may be below the precision of a kitchen spoon, which is why a small digital scale is useful for soluble powders. If you do not have a scale, mix a larger batch, use the portion you need, and discard or store only if the product label allows storage after mixing.
Now compare a fish emulsion labeled 5-1-1. A 1 litre bottle is not the same as a 1 kg bag, and liquids can have different densities, so label directions still matter. But the percentage logic explains why low-analysis products need larger measured volumes to supply the same nitrogen.
To reach 150 ppm nitrogen by weight from a 5 percent nitrogen product, the formula gives 150 / 10,000 / 0.05 = 0.30 percent, or about 3 g product per litre if you are treating the liquid as roughly water-like. That is four times the product weight required from a 20 percent nitrogen powder. The nutrient ratio is also different: 5-1-1 is nitrogen-forward, not balanced.
Organic fertilizers can be useful, but their nutrient release is often less immediate than a fully soluble mineral fertilizer. Nebraska Extension notes that organic fertilizers are typically slow-release because soil microbes must break them down before nutrients become available, a process that may take days or weeks, in its guide to natural vs. synthetic fertilizers. That is why the calculator is best for reading the label and comparing nutrient concentration, not for predicting exact release timing from compost, manure, castings, or meals.
Read the per-bag result as a concentration audit. It tells you how much guaranteed nutrient is present according to the label convention. It helps you compare two products, estimate cost per unit nutrient, and avoid assuming that all balanced fertilizers are equally strong.
Read the grams-per-litre result as a mixing target, not a command. A healthy plant in active growth may use a moderate feed well. A stressed plant may not. Fertilizer does not repair root rot, low light, compacted potting mix, pest damage, or chronic overwatering. Maryland Extension states that fertilizers will not help plants injured by insects, diseases, site problems, or weather problems unless soil nutrients are deficient in its garden fertilizer basics.
The calculator also cannot know your water chemistry. Alkalinity, dissolved salts, calcium, magnesium, sodium, and bicarbonates can change how plants respond to a feeding program. If you grow sensitive plants, use semi-hydroponics, or see persistent tip burn despite conservative feeding, water quality may be part of the problem.
The first mistake is entering the simplified ratio instead of the label percentages. A 20-20-20 product and a 5-5-5 product are both 1-1-1 ratios, but one is four times as concentrated. Always enter the label analysis exactly as printed.
The second mistake is treating P and K as elemental without checking the label convention. For casual houseplant feeding, this usually does not change the dose because nitrogen is the dosing anchor. For nutrient comparison, it matters a lot. A product labeled 10 percent P2O5 is about 4.4 percent elemental P, not 10 percent elemental phosphorus.
The third mistake is fertilizing a dry, wilted plant. If roots are dry or damaged, fertilizer can intensify stress. N.C. Cooperative Extension advises not applying fertilizer to dry potting mix and watering first if the medium is not already moist in its house-plant fertilizing rules. That principle is useful beyond one plant group: hydrate first, then feed gently.
The fourth mistake is using fertilizer as a response to every yellow leaf. Nitrogen shortage can yellow older leaves, but so can low light, root stress, underwatering, overwatering, pests, cold exposure, and normal leaf aging. If the plant is declining, use related LeafyPixels tools such as the /tools/plant-watering-calculator/, /tools/water-amount-calculator/, and /tools/fertilizer-schedule-calculator/ before increasing strength.
Foliage plants, flowering plants, succulents, seedlings, and outdoor containers do not respond the same way. The calculator uses nitrogen because it is the easiest common denominator, but the right target still depends on growth rate, light, pot size, root health, and season.
For slow growers, succulents, cacti, and plants in cool or dim rooms, use a weak target and longer intervals. These plants may need nutrients, but they do not need frequent high-nitrogen feeding when growth is slow. Too much fertilizer can push soft growth that the environment cannot support.
For vigorous foliage plants such as monstera, pothos, philodendron, and bird of paradise, a moderate feed during active growth is usually more sensible than sporadic high-strength feeding. For bloom-focused plants, nutrient balance matters as much as nitrogen concentration. RHS explains that nitrogen, phosphorus, and potassium support leaf and shoot growth, flowering, and cropping in its guide to fertilisers, so a flowering plant may call for a different ratio even when the nitrogen dose is moderate.
Indoor plants do not follow a calendar as cleanly as outdoor crops. They respond to actual light, temperature, root-zone moisture, and growth. A plant under strong grow lights in January may be actively growing. The same plant near a cold, dim window may barely use nutrients in March.
Use the calculator more aggressively when growth is visible and conditions support it. New leaves, expanding stems, active roots, and faster drying all suggest the plant can use nutrients. Use it conservatively when growth has slowed, when the plant was recently shipped, when the roots were disturbed, or when leaves show stress.
Containers also lose nutrients differently from garden soil. Frequent watering can leach nutrients from potting mix, which is one reason soluble fertilizers are commonly used in containers. Nebraska Extension notes that houseplant fertilizer needs depend on plant type, growth rate, available light, soil media, watering frequency, and fertilizer type in its houseplant fertilization guidance. The practical takeaway is not “feed constantly”; it is “match feeding to growth and water movement.”
Use this calculator when the hard part is reading the fertilizer label or converting a target nitrogen dose. Use the /tools/fertilizer-dilution-calculator/ when you already know the product and need a dilution plan for a specific concentrate. Use the /tools/fertilizer-schedule-calculator/ when timing is the bigger problem than math.
If fertilizer is tangled with watering, solve watering first. A plant in soggy mix cannot use nutrients normally, and adding fertilizer to a struggling root zone can make diagnosis harder. The /tools/water-amount-calculator/ and /tools/plant-watering-calculator/ are better first steps when the pot stays wet too long or dries unpredictably.
If you are growing outdoors, use soil-test guidance whenever possible. The NPK calculator can tell you how much nutrient is in the bag, but a soil test tells you whether the soil needs that nutrient. This distinction matters most for phosphorus, where unnecessary application can create environmental problems and may be restricted by local rules.
Fertilizer burn is not a mysterious reaction. It is often the result of too much soluble salt around roots, fertilizer applied to a dry or weak root system, or a dose that exceeds what the plant can use under current light and growth conditions. The calculator reduces arithmetic mistakes, but it cannot make an aggressive target safe.
Start lower when in doubt. Half-strength feeding is not a sign of poor care; it is often the right indoor strategy. If the plant responds with steady new growth and no tip burn, you can repeat the same strength or move up gradually. If leaf tips brown, the pot crusts with residue, or growth becomes soft and weak, reduce strength, increase plain-water flushes where appropriate, and recheck watering.
Never mix products casually. Combining a complete fertilizer, calcium-magnesium supplement, micronutrient product, and pH adjuster can create a stronger solution than intended even if each item seems mild by itself. When feeding food crops, lawns, or outdoor beds, follow local extension recommendations, label directions, and any applicable fertilizer laws.
The NPK Fertilizer Calculator is most useful when you treat it as a label translator and dose-checker. Enter the real bag weight, use the fertilizer analysis exactly as printed, remember that P and K are usually oxide equivalents, and let nitrogen drive the liquid-feed math. The result gives you a clean starting point: how concentrated the product is, how different products compare, and how much fertilizer is needed for a target ppm.
The final decision still belongs to the plant and the growing conditions. Feed actively growing plants more confidently, feed stressed or slow plants cautiously, and do not use fertilizer to cover up watering, light, pest, or root problems. When the numbers and the plant disagree, slow down, check the inputs, and choose the lower-risk dose.
This NPK Fertilizer Calculator was researched and written by . Logic, safety notes, and result copy for NPK Fertilizer are reviewed against LeafyPixels plant-care data, extension references, and veterinary toxicity sources where pet safety is involved.
We prioritize sources that hold up under scrutiny:
The LeafyPixels editorial team then reviews the draft for clarity, step-by-step usefulness, and fit with real apartment and home conditions-not ideal greenhouse setups. When guidance changes materially, we update the page and note the revision date.
Elemental weight per nutrient = bag weight x percentage / 100. For converting between elemental and oxide forms (relevant for P and K), elemental P = P2O5 x 0.436 and elemental K = K2O x 0.830 (per the AAPFCO standard and most extension fertilizer guides). Target N in ppm to grams-per-litre = target_ppm / 10,000 / (N_percent / 100). Standard houseplant target N rate is 100 to 200 ppm for most tropicals, 50 to 100 ppm for sensitive plants (calathea, fern), and 200 to 300 ppm for heavy feeders (monstera, hibiscus) during active growth. The calculator returns elemental weight per bag plus a recommended dose for a target N rate.
The long-form review for this page covers NPK Fertilizer Calculator. Its bottom source list includes 11 external citations pulled from the long-form guide, then deduplicated with the tool’s frontmatter sources.
The three numbers on an N-P-K fertilizer (for example 20-20-20 or 10-10-10) are percentages by weight, not absolute weights. A 1 kg bag of 20-20-20 contains 200 g of nitrogen, 200 g of phosphorus (as P2O5 equivalent), and 200 g of potassium (as K2O equivalent). The calculator takes the bag weight and the three percentages and returns the actual elemental weight of each nutrient so you can compare products fairly and avoid over-applying.
A standard liquid-feed target for most houseplants is 100 to 200 ppm nitrogen, which works out to roughly 0.5 to 1.0 g of a 20-20-20 powder per litre (or 1 to 2 ml of a typical 10-10-10 liquid concentrate). For sensitive plants (calathea, fern, prayer plant) drop to the lower half of this range; for robust plants (monstera, bird of paradise) the upper half is fine. The calculator converts your target N rate directly into grams of fertilizer per litre based on the percentage on the bag.
A balanced fertilizer (equal N, P, K such as 10-10-10 or 20-20-20) suits most foliage houseplants during the active growing season. For flowering houseplants (African violets, begonias, hibiscus), shift to a bloom-booster ratio with higher phosphorus (middle number) - something like 7-9-5 or 15-30-15 - during the bud-setting phase. For leafy greens and herbs, a higher-nitrogen ratio (3-1-2 or similar) pushes vegetative growth. Succulents and cacti do well on low-nitrogen, balanced ratios such as 2-7-7 diluted to half strength.
20-20-20 at full label strength is usually too concentrated for houseplants - it can burn sensitive roots and accumulate salts in the potting mix. The standard houseplant practice is to use 20-20-20 at half or quarter strength (about 0.5 g per litre), which delivers 100 ppm nitrogen and is well within the safe range for tropicals, ferns, and most foliage. The calculator’s target-N feature takes the math out of this so you do not have to convert percentages in your head.
Divide your target ppm by 10,000, then divide by the nitrogen percentage (as a decimal) on the bag. For example, 150 ppm N from a 20 percent N fertilizer: 150 / 10,000 / 0.20 = 0.075 percent, which equals 0.75 g per litre. The calculator does this conversion in both directions - you can enter a target N rate in ppm and get the grams per litre back, or enter a dose and see what ppm it delivers.
The middle and right numbers on a fertilizer bag (P and K) are traditionally expressed as the oxide equivalents (P2O5 and K2O), not the elemental phosphorus and potassium. To convert: elemental P = P2O5 x 0.436; elemental K = K2O x 0.830. Nitrogen is always reported as elemental N. Most modern fertilizer calculators (including this one) work in elemental weight because that is what plants actually absorb.
The math works for any product with an N-P-K label, including fish emulsion (typically 5-1-1), kelp meal (1-0-2), and worm castings (1-0-0). For untested organic amendments (compost, manure), the N-P-K is variable, so the calculation gives a rough estimate at best. Slow-release organic fertilizers also release nutrients over weeks to months, so a one-shot application rate is a less useful number than for water-soluble feeds.