Fertilizer label calculator
Decode the N-P-K numbers on a fertilizer bag: how much product to spread for your lawn or garden, how many bags, and the real cost per pound of nitrogen.
The three numbers on a fertilizer bag (N-P-K) are percentages by weight. A 20 lb bag of 24-0-6 contains 4.8 lb of nitrogen; the rest is the other nutrients and filler. This turns the label into how much product to spread, and shows what you’re really paying per pound of nutrient.
Reading a fertilizer label
- N (nitrogen) drives green leafy growth. It’s the nutrient lawns need most, and it doesn’t stay in soil long, which is why it’s applied regularly.
- P (phosphate, P₂O₅) supports roots and establishment. The label shows phosphate, not pure phosphorus: elemental P is about 44% of the P₂O₅ number.
- K (potash, K₂O) helps with stress tolerance. Elemental K is about 83% of the K₂O number.
- Slow-release or organic products feed over weeks rather than days. The arithmetic is the same; they’re just gentler.
More is not better. Extra nitrogen means more mowing, weaker roots and nitrate washing into waterways. A soil test tells you whether you need phosphate or potash at all.