Lawn Fertilizer Calculator

Lawn fertilizer calculations ensure you apply the correct amount of product to deliver the nitrogen dose the grass actually needs. Under-feeding gives you a weak, yellow lawn. Over-feeding burns it and sends the surplus into the groundwater.

Core formula: Bags Needed = Lawn Area (sq ft) ÷ Coverage per Bag (sq ft)

But coverage alone is not enough. What you need is the right nutrient rate. The standard target for most cool-season grasses is 1 lb of actual nitrogen per 1,000 sq ft per application.

Actual nitrogen per bag: Actual N (lbs) = Bag Weight (lbs) × N% ÷ 100

Where:

• N%: the first number on any fertilizer bag’s NPK label (e.g. “28-0-4” = 28% nitrogen)
• Bags needed for target dose: Bags = (Lawn Area ÷ 1,000) × (1 lb N target ÷ Actual N per bag)

What each variable means:

• NPK: Nitrogen (N), Phosphorus (P), Potassium (K); the three numbers always printed on every bag
• Actual nitrogen: the weight of pure N you’re delivering; 10 lbs of a 10-0-0 fertilizer = 1 lb actual N
• Application rate: cool-season grasses (fescue, bluegrass): 1 lb N/1,000 sq ft per feeding; warm-season (Bermuda, Zoysia): up to 1.5 lbs N/1,000 sq ft

Reference: nitrogen needs by grass type per year:

• Fine fescue: 1–2 lbs N/1,000 sq ft/year
• Kentucky bluegrass: 2–4 lbs N
• Bermuda grass: 3–5 lbs N
• Zoysia grass: 2–4 lbs N
• St. Augustine: 2–4 lbs N

Worked example: Lawn area 5,000 sq ft, a 28-0-4 fertilizer, target 1 lb of actual nitrogen per 1,000 sq ft.

• Product needed per 1,000 sq ft = 1 ÷ 0.28 = 3.6 lbs of product
• Across the whole lawn = 3.6 × 5 = 17.9 lbs of product
• Check it: 17.9 × 0.28 = 5.0 lbs of actual N, spread over 5,000 sq ft, which is the 1 lb per 1,000 you asked for

Notice that the bag size never entered the arithmetic. A 40 lb bag would cover this lawn twice with some left over, but that is a shopping question, not an application question. Work out the pounds of product first, then decide how many bags to buy.

Ignore the coverage figure on the bag if you can. It assumes the manufacturer’s own preferred rate, which is frequently heavier than 1 lb N per 1,000 sq ft, and it tells you nothing about the rate you actually want. The N percentage and your target rate are the only two numbers you need.

Key safety rule: Never apply more than 1 lb of actual nitrogen per 1,000 sq ft in a single application. Excess nitrogen leaches into groundwater and will burn the lawn, and a burned lawn takes a season to come back.

Working in metric, or feeding a vegetable bed? This page thinks in pounds per 1,000 square feet, because that is how turf feed is sold and labelled in the United States, and it converts a metric area into that before doing anything.
The fertilizer calculator runs the same arithmetic in either system, so a metric gardener gets kilograms per 100 square metres out rather than pounds. It also takes a bag size and tells you how many to buy. Same formula, same answer for the same lawn: 5,000 sq ft of 28-0-4 at 1 lb N comes to 17.9 lbs on both.