Garden fertilizer calculator

Recommendations are given in pounds of a nutrient (usually nitrogen) per 100 or 1,000 sq ft. Enter that rate, your product's N-P-K numbers and your garden size to get the amount of product to spread.

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How to use it

Get a nutrient recommendation, ideally from a soil test.
Enter the three numbers on the fertilizer bag.
Enter the area, then spread the amount shown evenly. Follow the product label and local rules.

The formula

Product per base area = recommended lb of nutrient ÷ (nutrient % ÷ 100) Product for your area = product per base area × your area ÷ base area Example (Iowa State): 0.15 lb N per 100 sq ft with 10-2-6 → 1.5 lb per 100 sq ft → 0.9 lb for 60 sq ft

The three numbers are the percent by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Iowa State notes that for many granular fertilizers 1 cup weighs about 0.5 lb — the cup figure uses that rule of thumb, so a kitchen scale is more accurate.

Gear for this job

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Soil test kit or lab test — [affiliate link placeholder]
Granular vegetable fertilizer — [affiliate link placeholder]
Handheld broadcast spreader — [affiliate link placeholder]
Kitchen scale — [affiliate link placeholder]

Frequently asked questions

What do the three numbers on a fertilizer bag mean?

The percent by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O). A 10-6-4 product is about 10% N, 6% phosphate and 4% potash; the rest is other nutrients and filler.

How much 10-10-10 do I need per 100 square feet?

It depends on the recommended rate. For 0.1 lb of nitrogen per 100 sq ft you need 0.1 ÷ 0.10 = 1 lb of 10-10-10.

How do I convert to cups?

Iowa State says many granular fertilizers weigh about 0.5 lb per cup, so 1 lb is roughly 2 cups. Densities differ, so weigh it when you can.

Where do I get the recommended rate?

From a soil test report or your state's extension service vegetable or lawn guide. Without a soil test you risk over- or under-applying.

Sources

Last reviewed 2026-10-22. Found an error? Tell us.