Water Soluble Fertilizer Calculator

Background.

Water-soluble fertiliser is sold as a powder with three numbers on the bag, and it is used at a concentration rather than at a rate per unit area. That single fact is why it needs its own calculator. A granular lawn product is applied at so many pounds of nitrogen per 1,000 square feet, and the lawn fertilizer calculator handles that. A soluble product is dissolved to a target concentration in parts per million and applied with every watering, and the amount of ground you cover is irrelevant to the number you have to weigh out. There is no area input on this page for exactly that reason.

The standard unit is ppm of nitrogen. UMass Extension explains why: it lets a recommendation be written once and applied to any product. Telling a grower to feed at 200 ppm N works whether they are using 20-20-20, 15-15-15 or calcium nitrate, because the calculator absorbs the analysis. Telling them to use 13½ ounces per 100 gallons only works for one specific bag. UMass reports 150 to 250 ppm N on constant feed as typical for many floricultural crops — quoted here as context, not as a recommendation for your crop, because the product label is the authority on that.

The second thing this page does is handle injectors properly. A fertiliser injector draws concentrated stock into the irrigation line at a fixed ratio. A 1:200 injector makes 200 gallons of dilute feed from one gallon of stock, which means the stock has to be 200 times stronger than the feed. It does not mean 200 ppm — that misreading is common enough that UMass calls it out explicitly and this page repeats the warning beside every result. Enter the dilution factor as the larger number of the ratio, or enter 1 if you are mixing a watering can directly.

The arithmetic is UMass's formula: amount equals the desired ppm multiplied by the dilution factor, divided by the percentage of the element in the fertiliser and by a conversion constant. This page uses the exact conversion rather than the rounded one. UMass's own constant table gives 10 for grams per litre, which is exact because ppm in a nutrient solution means milligrams per litre — one gram per litre of a 20 percent nitrogen product is 200 mg N/L and there is no unit conversion in the way. Its constants for ounces and pounds per gallon, 75 and 1200, are that same relationship pushed through the ounce and the gallon and then rounded. The exact figure is 74.8915, so a worksheet using 75 asks for about 0.145 percent less fertiliser than this page does. Both numbers are shown in the notes and the difference is stated, because a page that quietly disagrees with the textbook by a fraction of a percent is worse than one that explains why.

The same care applies to the last two numbers on the bag. Those are phosphate and potash — the oxide forms — not elemental phosphorus and potassium, and if you are comparing products or matching a tissue test you need the elements. This page converts using the molar fractions computed from the CIAAW 2021 standard atomic weights rather than a rounded table factor, which puts phosphorus at 0.4364268 of P₂O₅ and potassium at 0.8301513 of K₂O. Published tables round those differently in both directions, and the FAQ below lists all three sets.

What is water soluble fertilizer calculator?

A water-soluble fertiliser is a dry N-P-K product formulated to dissolve completely in water and be applied through irrigation. The three numbers on the bag are, in order, the percentage of elemental nitrogen, the percentage of phosphate expressed as P₂O₅, and the percentage of potash expressed as K₂O. That convention is a legacy of nineteenth-century fertiliser analysis and it persists on every label in the United States, which is why a conversion step exists at all.

Parts per million, in this context, means milligrams of nutrient per litre of the finished dilute solution. It is a concentration, not a dose. A stock solution, or concentrate, is the strong mixture kept in a bucket or barrel; the dilute solution is what actually reaches the plant. A fertiliser injector connects the two, drawing stock into the water line at a fixed ratio. The dilution factor is the larger number of that ratio: 1:100 is a dilution factor of 100.

Constant liquid feed, often shortened to CLF, is the practice of applying a dilute nutrient solution at every irrigation rather than periodic heavy feeds. It is the context in which ppm recommendations are usually written. UMass's fact sheet gives 150 to 250 ppm N as a common constant-feed range for many floricultural crops, and notes that the advantage of stating a rate in ppm is that it holds independent of the fertiliser analysis.

One detail of mixing changes the answer and is easy to miss. The volume you enter is the final volume of the solution, not the volume of water you start with. UMass is explicit: add the fertiliser to the container first and then fill to the mark. Dissolving a bag into an already-full container produces more than that volume, so the stock ends up weaker than intended and every plant fed from it is underfed by the same proportion.

How to use this calculator.

1. 01Choose what you are solving for. Pick the amount when you know the concentration you want; pick the concentration when you already put something in and want to know what you made.
2. 02Copy the three analysis numbers off the bag in order — nitrogen, phosphate, potash. Nitrogen cannot be zero, because every figure on this page is derived from it.
3. 03If you are solving for the amount, enter the nitrogen concentration you want in ppm. If you are solving for the concentration, enter the amount you dissolved and pick ounces or grams.
4. 04Enter the volume you are making and its unit. With an injector this is the volume of concentrate you are mixing; with no injector it is the finished feed.
5. 05Enter the injector dilution factor — the larger number of the ratio, so 1:200 becomes 200. Enter 1 if you are mixing a watering can or a tank directly.
6. 06Read the result in grams and ounces, and check the ppm figures for phosphate, potash and their elemental equivalents against whatever your crop recommendation or tissue test is written in.
7. 07Read the note beside the result before you mix. It repeats what the injector ratio does and does not mean, and it explains the mixing order that changes the strength of the batch.

The formula.

The finished volume comes first. The volume you enter is converted to litres — a US gallon is 3.785411784 litres exactly, from NIST's definition of the gallon as 231 cubic inches and the inch as 2.54 centimetres — and multiplied by the injector dilution factor. That product is the volume of dilute feed the batch will eventually produce, and it is the volume the ppm figure refers to.

Solving for the amount, grams of fertiliser equals the target ppm multiplied by the dilute volume in litres, divided by ten times the nitrogen percentage. The ten is UMass's grams-per-litre conversion constant and it involves no approximation: it is 1,000 milligrams per gram divided by 100 for the percentage. Solving for the concentration, the same relationship runs backwards — ppm equals grams multiplied by the nitrogen percentage and by ten, divided by the dilute volume in litres.

Ounces convert at 28.349523125 grams each, which follows from the international avoirdupois pound of 0.45359237 kilograms exactly. Combining that with the gallon gives the constant behind every published shortcut table: one ounce of a pure element dissolved in 100 US gallons is 74.8915 ppm. UMass and UGA both print that as 75.

The other nutrients scale off nitrogen, because they are all in the same powder. Phosphate ppm is nitrogen ppm multiplied by the phosphate percentage and divided by the nitrogen percentage; potash works the same way. Elemental phosphorus is phosphate multiplied by the mass fraction of phosphorus in P₂O₅, and elemental potassium is potash multiplied by the mass fraction of potassium in K₂O. Both fractions are recomputed from the CIAAW 2021 standard atomic weights on every call rather than stored as rounded numbers: with P at 30.973761998, K at 39.0983 and O at the conventional 15.999, P₂O₅ has a molar mass of 141.942523996 and is 0.4364268 phosphorus, while K₂O has a molar mass of 94.1956 and is 0.8301513 potassium.

Nothing is rounded before the final answer. The page sorts nothing into bands and rounds nothing up, so every figure is carried at full precision and rounded once on the way out.

A worked example.

Frequently asked questions.

References& sources.

1. [1]University of Massachusetts Amherst, Center for Agriculture, Food and the Environment, Greenhouse Crops and Floriculture Program, "Fertilizer Calculations for Greenhouse Crops". Source of the stock-solution formula (amount = desired ppm × dilution factor ÷ (% of element × C)), the conversion-constant table (ounces per US gallon 75, pounds per US gallon 1200, grams per litre 10), the definition of the dilution factor and the warning that a 1:100 injector does not mean 100 ppm, the 150–250 ppm N constant-feed context range, the instruction to add fertiliser first and fill to the mark, and worked Examples 1–4 including the 44½ oz and 15 lb answers reproduced in the tests. Retrieved 2026-07-31; open access.
2. [2]University of Georgia Cooperative Extension, Bulletin 931, "Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use", Pennisi, B.V., published 7 November 2024. Table 10 gives 1 oz per 100 gallons = 75 ppm, used as the independent check on the conversion constant; Table 17 gives the oxide-to-element factors (P₂O₅ × 0.4366, K₂O × 0.8301) compared against the molar values on this page. Retrieved 2026-07-31; open access.
3. [3]Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC, Standard Atomic Weights 2021, table revised 2024. Source of the atomic weights used to compute the oxide-to-element mass fractions: phosphorus 30.973 761 998(5), potassium 39.0983(1), oxygen [15.999 03, 15.999 77] with a conventional value of 15.999. Retrieved 2026-07-31; open access.
4. [4]National Institute of Standards and Technology, Handbook 44 – 2026, Appendix C, "General Tables of Units of Measurement". Units of Capacity or Volume, Liquid Measure: 4 quarts = 1 gallon = 231 cubic inches, giving 3.785411784 litres with the inch defined as 2.54 centimetres exactly. The international avoirdupois pound of 0.45359237 kg exactly gives the ounce as 28.349523125 g. Source of both unit conversions on this page. Retrieved 2026-07-31; open access PDF, downloaded and text-extracted.