Fertilizer Calculator
Calculate pounds of fertilizer needed by lawn area, N-P-K analysis and application rate (lbs N per 1000 sq ft).
The fertilizer calculator returns pounds of lawn fertilizer needed for any application rate, scaled to your lawn area and the N-P-K analysis printed on the bag. The standard target for cool-season lawns is 1 lb of actual nitrogen per 1000 sq ft per application; warm-season lawns can take 1-1.5 lbs. The math: divide your target N rate by the nitrogen percentage to get pounds of fertilizer per 1000 sq ft, then multiply by your lawn area. A bag of 20-5-10 fertilizer (20% N) applied at 1 lb N per 1000 sq ft means 5 lbs of product per 1000 sq ft - so a 5000 sq ft lawn needs 25 lbs per application. Total annual feeding usually splits this across 4 applications (early spring, late spring, late summer, fall).
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Frequently asked questions
Per application at 1 lb N per 1000 sq ft with a 20-5-10 fertilizer (20% N): 5 lbs of product per 1000 sq ft x 5 = 25 lbs per application. With 4 applications per year = 100 lbs total. That is 2.5 bags of 40 lb fertilizer per year. Slow-release fertilizers can be applied at 0.75 lb N per 1000 sq ft because the nitrogen releases over weeks.
N-P-K is the analysis of nitrogen (N), phosphorus (P) and potassium (K) by weight percentage. A 20-5-10 bag is 20% nitrogen, 5% phosphorus and 10% potassium - the remaining 65% is filler. Nitrogen drives green growth, phosphorus promotes root development (mostly for new lawns), potassium hardens the plant against stress. Established lawns rarely need much P.
Cool-season lawns (fescue, KBG, ryegrass): 4 applications - mid-spring, late spring, late summer (Labor Day window), late fall (winterizer). Warm-season lawns (Bermuda, zoysia): 3-4 applications all during the active growing season (April-September). Skip fertilizing during heat stress and during dormancy. The most important application for cool-season grass is the fall feeding.
Yes - over-fertilizing burns the grass (visible as yellow or brown streaks following the spreader path), promotes thatch and flushes nutrients into groundwater. Never apply more than 1 lb of fast-release N per 1000 sq ft in a single application. Slow-release products can go to 1.5 lbs N safely. Always sweep granules off hardscape back into the lawn and never fertilize before heavy rain.
Slow-release (urea-coated or sulfur-coated nitrogen) for most home lawns - it releases over 6-8 weeks, reduces burn risk and matches the lawn's actual demand. Fast-release (cheap urea or ammonium sulfate) for quick green-up when the lawn looks pale, but it requires more frequent applications and increases burn risk. Most commercial bagged fertilizers are blends of fast and slow release.