Fertilizer Calculator
Feed the lawn, not the bag.
Fertilizer Calculator
Product = target nitrogen ÷ (bag N% ÷ 100), then scaled by area ÷ 1,000
Calculated instantly
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What This Calculator Does
The Fertilizer Calculator converts a target nitrogen rate into the weight of product you need, using the first number on the bag. It reports the product per 1,000 square feet, the total for your lawn, the number of bags to buy, and the actual nitrogen applied.
How It Works
A fertilizer analysis such as 10-10-10 means 10% nitrogen, 10% phosphate, and 10% potash by weight. To deliver a target weight of nitrogen, divide by the nitrogen fraction. That gives product per 1,000 square feet; multiplying by the lawn area in thousands gives the total product, and dividing by the bag weight gives the bag count.
Example
To apply 1 lb of nitrogen per 1,000 sq ft using a 10-10-10 bag, you need 10 lb of product per 1,000 sq ft. For a 5,000 sq ft lawn that is 50 lb total, exactly one 50 lb bag, delivering 5 lb of actual nitrogen across the lawn.
Formula
target N ÷ (bag N% ÷ 100)product per 1,000 × area ÷ 1,000total product ÷ bag weighttarget N × area ÷ 1,000Reading the Results
Total product needed
The weight of bagged fertilizer to spread over the whole lawn. Weigh it or calibrate the spreader; guessing at the setting is the usual reason a lawn gets too much or too little.
Total nitrogen applied
The actual nitrogen delivered to the lawn. It should match your target, and it is the number to compare when switching between products with different analyses.
Sources and References
USDA Natural Resources Conservation Service — Nutrient Management
Nitrogen application planning and runoff risk
Federal guidance on matching nutrient applications to crop and site needs.
Penn State Extension — Fertilizing Lawns
Nitrogen rates and application timing
A practical extension source for the 0.5–1 lb N per 1,000 sq ft range.
Association of American Plant Food Control Officials — Fertilizer label definitions
N-P-K analysis and guaranteed analysis
Defines what the three numbers on a fertilizer bag mean by weight.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What do the three numbers on a fertilizer bag mean?
They are the percentages of nitrogen, available phosphate, and soluble potash, always in that order. A 10-10-10 bag contains 10% of each by weight, with the rest being filler and secondary nutrients.
How much nitrogen should I apply?
For most established cool-season lawns, 0.5–1 lb of nitrogen per 1,000 sq ft per application, with four to six weeks between applications. Warm-season grasses and sandy soils need different scheduling, so check local extension guidance.
Why does the bag count come out fractional?
Because the total product rarely divides evenly by the bag size. Round up to buy whole bags, but do not necessarily spread the extra; over-application is worse for the lawn than a slightly light pass.
Can I use this for a garden or a farm field?
The arithmetic is the same, but the target rate and the area unit differ. Market gardens usually work in pounds per acre or kilograms per hectare, and soil tests should set the target rather than a generic lawn rate.