Fertilizer Calculator

The three numbers on a fertilizer bag – such as 20-5-10 – tell you the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Lawn and garden recommendations, on the other hand, are usually given as an amount of nitrogen per area, such as “1 lb of nitrogen per 1,000 sq ft”. This fertilizer calculator bridges the two: enter the bag analysis and the nitrogen rate you want, and it tells you how much product to spread and how much of each nutrient you are applying.

How to use this calculator

Choose imperial (lb per 1,000 sq ft) or metric (g per m²).
Enter the area to be fertilized by rectangle, circle or known area. Leave out patios, paths and buildings.
Type the three N-P-K numbers from the front of the fertilizer bag.
Enter the nitrogen rate you want to apply. About 0.5–1 lb N per 1,000 sq ft (2.5–5 g N per m²) per feeding is a common lawn rate.
Read the total amount of product and the rate per area, then set your spreader to match or divide the product into two passes.

The fertilizer formula

Product per area = target nitrogen ÷ (N% ÷ 100). With a 20-5-10 fertilizer and a target of 1 lb N per 1,000 sq ft, you need 1 ÷ 0.20 = 5 lb of product per 1,000 sq ft. Multiply by your area in thousands of square feet to get the total: a 5,000 sq ft lawn needs 25 lb. The same product also delivers 25 × 0.05 = 1.25 lb of phosphate and 25 × 0.10 = 2.5 lb of potash. In metric, a target of 5 g N per m² with 10-10-10 means 5 ÷ 0.10 = 50 g of product per m².

Why calculate by nitrogen?

Nitrogen is the nutrient that drives leafy growth and the one most likely to cause problems when overapplied: too much at once can scorch grass, encourage disease and wash into streams and groundwater. Basing the rate on nitrogen keeps each application safe, whatever the product. Slow-release fertilizers (look for “slow release” or a percentage of water-insoluble nitrogen on the label) can be applied at the higher end of the range; quick-release products are best kept to about 0.5–0.75 lb N per 1,000 sq ft per application.

How much nitrogen does a lawn need per year?

As a general guide, low-maintenance lawns get by on 1–2 lb N per 1,000 sq ft per year, while high-quality cool-season lawns of Kentucky bluegrass or perennial ryegrass may receive 3–4 lb, split into several feedings with the largest in fall. Warm-season grasses such as Bermuda are fed during their summer growing season, while centipede and Bahia need much less. Leaving grass clippings on the lawn returns roughly a quarter or more of the nitrogen the lawn needs.

What about phosphorus and potassium?

Established lawns and gardens often have enough phosphorus, and several US states restrict phosphorus fertilizer on lawns unless a soil test shows a need or a new lawn is being seeded. A soil test from your local extension service or a reputable lab is the best way to know whether you need P or K at all, and it costs far less than a season of the wrong fertilizer.

Spreading tips

Calibrate or check your spreader setting against the rate per 1,000 sq ft the calculator shows. For even coverage, spread half the product walking in one direction and the other half at right angles. Sweep any granules off paving back onto the lawn, and water lightly afterwards unless the label says otherwise.

Frequently asked questions

What do the three numbers on fertilizer mean?

They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 20-5-10 contains 10 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash; the rest is carrier and other ingredients.

How much 10-10-10 per 1,000 square feet?

To apply 1 lb of nitrogen per 1,000 sq ft you need 10 lb of 10-10-10. For half a pound of nitrogen, use 5 lb.

How much fertilizer for a vegetable garden?

A common pre-planting rate is about 1–2 lb N per 1,000 sq ft (roughly 5–10 g N per m²), adjusted by a soil test. With 10-10-10 that is 10–20 lb per 1,000 sq ft, or about 1–2 lb per 100 sq ft.

Can I use the calculator for organic fertilizers?

Yes. Enter the N-P-K on the label, for example 5-3-4. Organic products have lower percentages, so you need more product to deliver the same nitrogen, and it is released more slowly.

How do I convert lb per 1,000 sq ft to g per m²?

Multiply by about 4.88. One pound of nitrogen per 1,000 sq ft equals roughly 4.9 g of nitrogen per square meter.