Plant feeding

Fertiliser dilution calculator

Read the three numbers on the label, choose how strong you want the feed, and get the amount to measure into each litre or gallon of water. Or enter the dose you already use to see it in parts per million (ppm).

Work out Amount for a target ppm ppm from my dose

N-P-K on the label (%)

N

P₂O₅

K₂O

The fertiliser is Powder or crystals (g) Liquid concentrate (ml)

Target nitrogen at full strength

ppm N

200 ppm N is the worked example in Virginia Tech's greenhouse guide.

Strength Full Half Quarter

More is not better. Feed that is too strong can scorch roots and leaf edges (fertiliser burn). Stay within the label's directions, and keep fertilisers and mixed feed out of reach of children and pets.

Measure out (full strength, 200 ppm N)

1.00 g per litre3.79 g per US gallon

Of a 20-10-20 powder, made up to that volume with water.

Nitrogen (N): 200 ppm
Phosphorus (P): 44 ppm
Potassium (K): 166 ppm

As elements, 20-10-20 is: 20% N, 4.36% P, 16.6% K
The sum for nitrogen: 1 g/L × 20 × 10 = 200 ppm

From the label to the watering can

The three numbers are the guaranteed percentages, by weight, of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Parts per million (ppm) means milligrams per litre, so ppm = grams per litre × percentage × 10. One gram of 20-10-20 in a litre of water carries 200 mg of nitrogen: 200 ppm N. In US units the same sum uses 75 for ounces per gallon.

Phosphorus and potassium are labelled as oxides, so the calculator converts them: P = P₂O₅ × 0.4364 and K = K₂O × 0.8301. That turns 20-10-20 into 4.36% P and 16.6% K.

The strength buttons scale the whole mix. Iowa State Extension suggests mixing general, all-purpose fertilisers at half or quarter the label strength for houseplants, because they grow more slowly than the outdoor plants the label is written for, and using houseplant fertilisers as directed.

Sources

Latimer JG. The Basics of Fertilizer Calculations for Greenhouse Crops. Virginia Cooperative Extension 430-100, 2020, reviewed 2025: the conversion constant (10 for grams per litre, 75 for ounces per US gallon), the 200 ppm N and 20-10-20 examples, and P and K labelled as oxides.
Reiter M. Agronomy Handbook, Part IX: Conversion Factors Needed for Common Fertilizer Calculations. Virginia Cooperative Extension 424-100: K₂O × 0.8301 = K and P₂O₅ × 0.4365 = P. The 0.4364 used here is the same mass share of phosphorus in P₂O₅ from the standard atomic weights, rounded to four places.
Landschoot P. How Much Phosphorus and Potassium are Really in Your Fertilizer? Penn State Extension: the rounded factors 0.44 and 0.83.
Bundy L. Fertilizer Grades: Determining Application Rates of Dry and Liquid Fertilizer Materials. University of Wisconsin–Madison: the grade as guaranteed N, P₂O₅ and K₂O by weight, and weights of 10.7–11.7 lb per gallon (1.28–1.40 g/ml) for some liquid fertilisers.
Iowa State University Extension, How often should I fertilize houseplants?: half or quarter strength, and feeding only while plants grow.

Questions

What does ppm mean for fertiliser?

Parts per million: milligrams of a nutrient in each litre of water. 200 ppm N means 200 mg of nitrogen in every litre of the mixed feed.

Why are P and K lower than the label numbers?

Labels give phosphorus and potassium as the oxides P₂O₅ and K₂O. Only 43.64% of P₂O₅ is phosphorus and 83.01% of K₂O is potassium, so the elements come out lower.

Should I feed houseplants in winter?

Iowa State Extension advises feeding only while houseplants are actively growing. Many rest during the short days of winter and need no fertiliser then.