Fertilizer Ratio Converter
Convert NPK fertilizer ratios and calculate application rates per square foot, acre, or gallon for nitrogen, phosphorus, and potassium blends.
Enter your fertilizer's NPK numbers and bag size to calculate nutrient content.
Fertilizer labels show an NPK ratio — three numbers representing the percentage of:
- N = Nitrogen (leaf/stem growth)
- P = Phosphorus (root/flower development)
- K = Potassium (overall plant health)
How to read NPK: A bag labeled 10-10-10 contains:
- 10% nitrogen, 10% phosphorus, 10% potassium
- The rest (70%) is filler/carrier material
Application calculation:
- Nutrient per bag = Bag weight × (% / 100)
- Example: 50 lb bag of 10-10-10 contains 5 lb of each nutrient
Common NPK ratios:
| Ratio | Best for |
|---|---|
| 10-10-10 | General purpose (balanced) |
| 20-5-10 | Lawns (high nitrogen) |
| 5-10-10 | Flowers and fruit |
| 0-20-0 | Root development (starter) |
| 15-0-15 | Winter lawn care |
Liquid fertilizer dilution:
- Rate is usually given as tablespoons per gallon or mL per liter
- 1 tablespoon = 14.79 mL
- 1 US gallon = 3.785 liters
There’s a difference between a fertilizer’s ratio and its strength that trips up gardeners. A 10-10-10 and a 20-20-20 share the same 1-1-1 ratio, but the second is twice as concentrated, so you use half as much for the same feeding. The three numbers are always percentages by weight of the actual nutrient, not the compound, which is why a 50-pound bag of 10-10-10 delivers five pounds each of nitrogen, phosphorus, and potassium, with the remaining 35 pounds as carrier.
A couple of things the label won’t tell you. Most established soils already hold plenty of phosphorus, so the high-middle-number “bloom booster” products are oversold for a lot of gardens; a soil test is the only way to know what you’re actually short on. And nitrogen is the one to respect, because it’s the number that burns. Too much, or a fast-release form on a dry lawn, scorches the grass in days. Slow-release nitrogen feeds gently over weeks and is far more forgiving, usually the better choice for anyone not measuring carefully.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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