Water Soluble Fertilizer Calculator - PPM & Dosage

Calculate exactly how much water-soluble fertilizer to add to reach a target PPM nitrogen — whether mixing a one-time batch or feeding through an injector.

Formula

oz per 100 gallons = (PPM × 100) ÷ (% N × 75)

For injector stock: multiply by the injector ratio (e.g. 1:100 means stock concentration is 100× the final feed).

Common Fertilizer Programs

Crop Stage	PPM N
Seedling / Plug	50-75
Light Feeders (poinsettia)	150-200
Heavy Feeders (tomato, geranium)	250-300
Hydroponic Lettuce	120-150

Important Reminders

Always dissolve fertilizer in warm water before topping up the stock tank.
Check EC (electrical conductivity) after mixing — typical target is 1.5–2.5 mS/cm.
Calibrate injectors monthly. A 1:100 injector that drifts to 1:90 increases EC by 11%.
Adjust pH to 5.8–6.2 with acid or potassium bicarbonate as needed.