Water Soluble Fertilizer Calculator

Background.

A grower wants 250 ppm of nitrogen at every watering, has a bag of 15-16-17 and a 1:200 injector, and needs to know how much powder goes into a one-gallon stock bucket. The answer is 1,261.8039 g, about 44½ ounces. Get the injector arithmetic wrong and the plants receive either a tiny fraction of that feed or a dose strong enough to burn roots.

Soluble feed is specified by concentration, not by area. A granular lawn product goes down at so many pounds of nitrogen per 1,000 square feet; a soluble one is dissolved to a strength in parts per million and applied with the water, so the size of the bench or bed never enters the sum. Writing the strength as ppm of nitrogen also means a single recommendation, such as 200 ppm N, works for any analysis: the arithmetic absorbs whether the bag says 20-20-20 or 15-5-15.

The injector is where most mistakes happen. A 1:200 injector draws one part of concentrated stock into 199 parts of water, so one gallon of stock becomes 200 gallons of feed and the stock must be 200 times stronger than the feed. The ratio says nothing about ppm; UMass Extension warns specifically that a 1:100 setting does not mean 100 ppm. With no injector, enter a dilution factor of 1 and the volume is simply the can or tank you are filling.

The second and third label numbers are phosphate (P₂O₅) and potash (K₂O), oxides rather than elements, so the results include elemental phosphorus and potassium as well, for comparison with a tissue test or a recommendation written in P and K.

What is water soluble fertilizer calculator?

A water-soluble fertiliser is a dry N-P-K product made to dissolve completely and be applied through irrigation. Here ppm means milligrams of nutrient per litre of the finished, dilute feed, the solution that actually reaches the roots. A stock solution, or concentrate, is the strong mix in the bucket or barrel; an injector meters it into the water line at a fixed ratio, and the dilution factor is the larger number of that ratio. Constant liquid feed, applying a dilute solution at every watering, is the practice most ppm recommendations are written for.

How to use this calculator.

1. 01Choose whether you are working out the weight to dissolve or checking the concentration of a batch you have already mixed.
2. 02Copy the three analysis numbers from the bag in order: nitrogen, phosphate, potash. Nitrogen has to be above zero, since every result is scaled from it.
3. 03Enter the target ppm of nitrogen, or the amount you dissolved in ounces or grams, depending on the mode.
4. 04Enter the volume you are mixing in US gallons or litres. With an injector this is the stock bucket; without one it is the finished feed in the can or tank.
5. 05Enter the injector's dilution factor, the larger number of its ratio (1:200 means 200), or 1 for no injector. Weigh the result on a scale, put the powder in the container first and then fill to the mark.

The formula.

The finished volume comes first. The volume entered is converted to litres (one US gallon is exactly 3.785411784 L) and multiplied by the dilution factor D, giving V_d, the litres of dilute feed the batch will make.

To find the weight, m = ppm × V_d ÷ (10 × %N) grams. The 10 is UMass's grams-per-litre constant: 1,000 mg in a gram divided by 100 to turn the label percentage into a fraction, with no approximation in it. To find the concentration of a known weight, the same relation is turned round: ppm = m × %N × 10 ÷ V_d, with ounces converted at exactly 28.349523125 g.

Those two exact conversions explain a small difference from printed tables. One ounce of pure nutrient in 100 US gallons is 74.8915 ppm; UMass and the University of Georgia both round that to 75. A worksheet using 75 therefore asks for about 0.145% less powder than this calculation, a gap far smaller than any kitchen scale can resolve.

Phosphate and potash ride along in fixed proportion because they are in the same powder: ppm P₂O₅ = ppm N × %P₂O₅ ÷ %N, and likewise for K₂O. Elemental phosphorus is P₂O₅ × 0.4364268 and potassium is K₂O × 0.8301513, mass fractions worked out from the CIAAW atomic weights (P 30.973761998, K 39.0983, O 15.999).

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 by this calculator. 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.