Fertilizer N-P-K Product Calculator
Figure out how much fertilizer product to apply to hit a target nitrogen rate, based on the bag's N percentage.
Fertilizer needed
How this calculator works
A fertilizer's N-P-K numbers (like 10-10-10) are percentages by weight — a 10% nitrogen bag is only one-tenth actual nitrogen by weight, the rest is filler and other nutrients. To deliver a target amount of actual nitrogen, you need to divide that target by the fertilizer's nitrogen percentage, not just apply the target weight directly.
Where these numbers come from
This is a standard fertilizer-math calculation used across extension lawn and garden guidance. Target nitrogen rates (commonly around 1 lb N per 1,000 sq ft per application for lawns) come from state extension turf and garden fertility guides — check your specific soil test or crop's recommendation rather than assuming 1 lb is universally correct.
Frequently asked questions
What do the three numbers on a fertilizer bag mean?
They're the percentage by weight of nitrogen (N), phosphorus as P2O5 (P), and potassium as K2O (K) — a 10-10-10 bag is 10% each, with the remaining 70% being filler and other components.
Why can't I just apply the target weight directly?
Because the target is for actual nutrient content, not product weight — a 10% N fertilizer needs 10x its target nitrogen weight applied as product to deliver that much actual nitrogen.
How much nitrogen does my lawn or garden actually need?
It depends heavily on the crop, soil, and season — a soil test from your local extension office gives the most accurate answer; 1 lb N per 1,000 sq ft per application is a common general lawn guideline, not a universal rule.
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