Fertilizer Calculator

How much lawn fertilizer to spread for a rate of nitrogen, phosphorus or potassium, from the N-P-K on the bag, with the spreader rate and bags.

The three numbers on a fertilizer bag, such as 24-0-6, are the percentages of nitrogen, phosphorus and potassium by weight. To put down a set amount of nitrogen, divide it by the nitrogen share. For 1 lb of nitrogen per 1,000 ft² with 24-0-6, spread 1 ÷ 0.24 = 4.17 lb of product per 1,000 ft², so a 5,000 ft² lawn needs 20.8 lb. In metric, 5 g of nitrogen per m² from a 20-5-10 feed takes 25 g of product per m², or 2.5 kg for 100 m². This calculator works out the product for a rate of nitrogen, phosphorus or potassium, the rate to set on your spreader, the bags to buy, and how much of each nutrient you'll apply.

Units
ft²

For example 24-0-6

lb/1,000 ft²
lb
Fertilizer needed
20.83 lb

9.45 kg

Spread at
4.17 lb per 1,000 ft²

set the spreader for this

Bags
2

1.04 bags of 20 lb

Nutrients applied
5 lb N

0 lb P · 1.25 lb K

How it was worked out
  1. N is 24% of the bag, so 1 lb per 1,000 ft² of N takes 1 lb per 1,000 ft² ÷ 0.24 = 4.17 lb per 1,000 ft² of fertilizer
  2. 4.17 lb per 1,000 ft² × 5,000 ft² = 20.83 lb
  3. 20.83 lb ÷ 20 lb bags = 1.04 bags
  4. That spreads 5 lb N, 0 lb P and 1.25 lb K in total

The three numbers are the percentages of nitrogen, phosphorus and potassium by weight. US and UK bags give phosphorus and potassium as P₂O₅ and K₂O, while Australian bags usually give the elements themselves; the calculator uses whichever your bag shows. Many university lawn guides suggest no more than about 1 lb of nitrogen per 1,000 ft² (about 5 g/m²) in one feed. Follow the rate on the bag, and keep fertilizer off paths and out of drains.

Common use cases

  • Work out how much lawn feed to buy for your lawn
  • Turn a recommendation of pounds of nitrogen per 1,000 ft² into pounds of product
  • Compare fertilizers with different N-P-K numbers
  • Find the rate to set on a broadcast or drop spreader

How to use

  1. Choose feet and pounds or metres and grams.
  2. Enter the lawn area and the three numbers from the bag.
  3. Pick the nutrient your rate is for, usually nitrogen, and enter the rate.
  4. Enter the bag size.
  5. Read the fertilizer needed, the spreading rate and the bags to buy.

FAQ

What do the numbers on a fertilizer bag mean?

They're the percentages of nitrogen (N), phosphorus (P) and potassium (K) by weight. A 50 lb bag of 24-0-6 contains 12 lb of nitrogen, no phosphorus and 3 lb of potassium. US and UK bags give phosphorus and potassium as phosphate (P₂O₅) and potash (K₂O); Australian bags usually give the elements.

How much nitrogen should I put on my lawn?

Follow the bag and local advice for your grass and climate. Many university lawn guides suggest no more than about 1 lb of nitrogen per 1,000 ft² in a single feed, which is about 5 g per m².

How much 10-10-10 do I need for 1 lb of nitrogen per 1,000 ft²?

1 ÷ 0.10 = 10 lb of 10-10-10 per 1,000 ft². That also puts down 1 lb each of phosphate and potash.

How do I convert pounds per 1,000 ft² to grams per m²?

Multiply by 4.88. 1 lb per 1,000 ft² is 4.88 g per m², and 4 lb of product per 1,000 ft² is about 20 g per m².

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Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work.