Fertilizer ppm calculator.

Worked example

Grams for a target. You want 150 ppm nitrogen in a 10 litre can with a 20-20-20 fertilizer. 150 ppm is 150 mg per litre, 1,500 mg in total; the fertilizer is 20 percent N, so 1,500 / 0.20 = 7,500 mg = 7.5 g (0.75 g per litre).

ppm from a dose. 1 g in 1 litre of 20-10-20 gives 1,000 mg/L × 20% = 200 ppm N, 1,000 × 10% × 0.4364 = 43.6 ppm P and 1,000 × 20% × 0.8301 = 166 ppm K.

How it works

1 ppm in water is 1 mg per litre. Grams = ppm × litres / 1,000 / (nutrient% / 100). For the reverse, ppm = grams × 1,000 / litres × nutrient%. Labels state phosphorus as phosphate (P₂O₅) and potassium as potash (K₂O). Elemental P is 43.64 percent of P₂O₅ and elemental K is 83.01 percent of K₂O, from atomic masses, so the tool converts them. US gallons are converted to 3.785 litres.

What this is not. It does not tell you what ppm a plant wants, which depends on crop, stage, medium and local water. Too strong a feed can burn roots, and too much runs off into waterways. Start at the label rate or lower and watch the plants. Your tap water already contains some nutrients, which this ignores. Add fertilizer slowly while stirring and do not use pesticide containers or food utensils for mixing.

Sources and basis (checked 2026-10-06): ppm is taken as mg per litre, the usual convention for dilute water solutions; the P₂O₅ and K₂O conversion factors follow from atomic masses (P₂O₅ 141.94 g/mol, K₂O 94.20 g/mol).

FAQ

How do I convert ppm to grams per litre?

Divide ppm by 1,000 for grams of the element per litre, then divide by the nutrient fraction in your fertilizer. 150 ppm N from a 20 percent N product is 0.15 / 0.20 = 0.75 g per litre.

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

Labels give phosphate (P2O5) and potash (K2O), which include oxygen. Elemental P is 43.64 percent of P2O5 and elemental K is 83.01 percent of K2O, so a 20 percent P2O5 product is only about 8.7 percent P.

How much fertilizer per gallon is that?

The calculator shows grams per US gallon as well as per litre, since the grams per litre figure is multiplied by 3.785.

Is more ppm better for plants?

No. Too much salt burns roots and wastes fertilizer. Start at the label rate, or lower for seedlings, and adjust based on plant response and a soil or leaf test.