Water Soluble Fertilizer Calculator (grams for a target ppm N, direct or injector stock)

Understanding the water-soluble fertilizer calculator — concept and principle

This calculator tells you how many grams of a water-soluble fertilizer to dissolve so that the feed water carries a chosen nitrogen concentration in ppm. It covers mixing straight into a watering can and making a stock for an injector.

One gram of a 20 % N fertilizer in one litre of water gives 200 ppm nitrogen.

How it is calculated

Grams per litre = target ppm N × ratio ÷ (%N × 10). For direct mixing the ratio is 1.
Fertilizer to weigh out = grams per litre × litres mixed (US gallons × 3.785411784).
Ounces per US gallon = target ppm × ratio ÷ (%N × 75).
Phosphorus delivered (ppm P) = ppm N × (%P₂O₅ ÷ 2.3) ÷ %N; potassium (ppm K) = ppm N × (%K₂O ÷ 1.2) ÷ %N.
Feed solution made = litres mixed × ratio.

The injector ratio is used only in stock mode; in direct mode it is ignored.

The principle

One ppm by weight in water is one milligram per litre. A fertilizer labelled 20-20-20 is 20 % nitrogen by weight, so 1 g of it holds 200 mg of N. Dissolved in 1 L, that gives 200 ppm N, which is why grams per litre = ppm ÷ (%N × 10). The US constant 75 comes from 1 oz per gallon being about 7.49 g per litre. A 1:100 injector turns 1 part of stock into 100 parts of feed, so the stock must be 100 times stronger. Labels give phosphate and potash as the oxides P₂O₅ and K₂O; phosphorus is about 43.6 % of P₂O₅ and potassium about 83 % of K₂O by mass, hence the divisors 2.3 and 1.2.

Worked example

Default inputs: 20-20-20, target 200 ppm N, mixed directly in 10 L of water.

Grams per litre = 200 × 1 ÷ (20 × 10) = 1 g/L.
Fertilizer to weigh out = 1 × 10 = 10 g, or 10 ÷ 28.35 = 0.35 oz.
Ounces per US gallon = 200 ÷ (20 × 75) = 0.133 oz/gal.
Phosphorus = 200 × 20 ÷ 2.3 ÷ 20 = 87 ppm P; potassium = 200 × 20 ÷ 1.2 ÷ 20 = 167 ppm K.
Feed solution made = 10 × 1 = 10 L.

Each point is the calculator re-run with only “Nitrogen on the label (% N, first number)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Nitrogen on the label (% N, first number)	Fertilizer to weigh out (g)
5	40
10	20
15	13.3
20	10
25	8
30	6.7
40	5

How to read the result

The gram figure is what goes on the kitchen scale. Many growers feed potted plants at roughly 100–200 ppm N when they fertilize with every watering, and use a stronger rate less often; follow the rate on your label or crop guide. If you have an EC meter, compare the reading with the EC table supplied by the fertilizer maker.

Common mistakes

Reading 20-20-20 as elemental P and K; the second and third numbers are oxides.
Mixing stock for a 1:100 injector at feed strength, which delivers only 1 % of the intended rate.

Keep in mind: The result assumes the label analysis is correct and the powder dissolves completely. Plants differ in their needs, so follow the product label and watch for leaf burn or deficiency.

Frequently asked questions

How many grams of 20-20-20 per litre give 100 ppm N?

100 ÷ (20 × 10) = 0.5 g per litre.

What does 1:100 mean on an injector?

One part of stock is mixed into a total of 100 parts of feed water, so the stock is 100 times as strong as the feed.

Why are P and K lower than the label numbers suggest?

The label uses P₂O₅ and K₂O. Converting to the elements divides by about 2.3 and 1.2.

Sources: UMass Extension Greenhouse Crops and Floriculture Program, "Fertilizer Calculations for Greenhouse Crops" · NIST Special Publication 811 (1 US gal = 3.785411784 L; 1 oz = 28.349523125 g)

Checked 9 Nov 2026. UMass Extension Greenhouse Crops & Floriculture Program, “Fertilizer Calculations for Greenhouse Crops” (amount = ppm × dilution ÷ (% element × C), C = 75 for oz/gal and 10 for g/L; %P = %P₂O₅ ÷ 2.3, %K = %K₂O ÷ 1.2). 1 US gal = 3.785411784 L, 1 oz = 28.349523125 g.

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