🧪 Fertilizer Dilution Calculator

Enter your liquid fertilizer's nitrogen percentage (the first number on the N-P-K label) and how much you're using per gallon of water to calculate the resulting nitrogen concentration in parts per million (PPM).

Nitrogen % (first N-P-K number)

Fertilizer Dose (oz per gallon)

Where the factor of 75 comes from

The tool uses the fertigation formula extension horticulture services publish for greenhouse feed:

ppm N = 75 × %N × ounces per gallon

The 75 is not arbitrary. A US gallon of water weighs 8.345 lb, or 133.5 ounces, so one ounce of a product carrying 1% nitrogen puts 0.01 oz of N into 133.5 oz of solution: 0.01 ÷ 133.5 × 1,000,000 = 74.9 parts per million. A 20-20-20 feed at 1 oz per gallon therefore gives 75 × 20 × 1 = 1,500 ppm N — six times what most plants want, which is why the box loads at 0.15 oz per gallon instead, or 225 ppm.

Getting a sensible rate

Anything past 250 ppm is flagged. Constant liquid feeding usually runs at 100–250 ppm N. To hit 200 ppm from a 20% N product, rearrange to 200 ÷ (75 × 20) = 0.133 oz per gallon, about 3.8 grams.
Ounces here means weight, not fluid ounces. That matters for liquid concentrates, which are denser than water — measuring a fluid ounce of one will put in more nitrogen than the formula assumes. Weigh powders and crystals on a kitchen scale.
Only the first N-P-K number is used. The other two are quoted as oxides in most countries: elemental phosphorus is 0.44 × the P₂O₅ figure and potassium 0.83 × K₂O.

The rate on your product's label, and any crop-specific guidance from your local extension service, outrank this arithmetic. It converts a dose into a concentration; it has no view on what your soil already holds, and over-application runs off into groundwater and waterways.

Frequently asked questions

How do I calculate ppm nitrogen in a fertilizer solution?

Multiply the nitrogen percentage by the ounces of product per gallon, then by 75. Ten grams of a 24% N feed in a gallon — 0.35 oz — works out at about 630 ppm.

What ppm of nitrogen do vegetables need?

Commonly 100 to 250 ppm N for continuous liquid feeding, with seedlings at the low end and heavy fruiting crops at the high end. Feeding at every watering uses lower concentrations than feeding once a fortnight.

Does this work for granular fertilizer spread on soil?

No. Parts per million describes a solution, so this only applies to a product dissolved in water. Granular feed is rated by pounds of nitrogen per 1,000 square feet or per acre instead.