Material takeoff
How much fertilizer do I need?
Enter your target nitrogen rate and the first number on the bag. The result is pounds of product to actually put in the spreader.
How the math works
pounds of product = (square feet ÷ 1,000 × target lb N) ÷ (nitrogen % ÷ 100)
An example. You want 1 pound of nitrogen per thousand square feet on a 5,000 square foot lawn, using a 24-0-4 bag.
Nitrogen needed: 5 × 1 = 5 pounds.
Product needed: 5 ÷ 0.24 = 20.8 pounds of the bag.
Put down 5 pounds of that bag and you applied a fifth of the intended rate. Put down 5 pounds of a 46-0-0 urea bag on the same lawn and you applied nearly half a pound of N per thousand — a very different outcome.
Reading the bag
The three numbers are percentages by weight of nitrogen, phosphorus (as P₂O₅), and potassium (as K₂O). A 50 pound bag of 24-0-4 contains 12 pounds of actual nitrogen.
One pound of nitrogen per thousand square feet per application is a common general rate for established lawns, with total annual nitrogen varying by grass type and region.
Slow-release nitrogen — look for "slowly available" or a coated product on the label — can go down at higher single rates with less burn risk than fast-release urea. If you are establishing a new lawn from seed, a starter fertilizer high in phosphorus is one of the few exceptions to the standard nitrogen-focused approach.
Many states restrict phosphorus on established lawns, and several restrict application timing near water or before rain. Your county extension office publishes the rates and rules for your specific area, and they are the authority here rather than a national rule of thumb.
Where people get it wrong
- Confusing pounds of nitrogen with pounds of bag
- These differ by a factor of two to five. The calculation above is the whole point.
- Fertilizing dormant or stressed grass
- Feeding a lawn in summer heat or drought stress does harm, not good.
- Not calibrating the spreader
- Spreader settings on the bag are approximations. Weigh what you put in and check what you have left.
- Fertilizing right before heavy rain
- Runoff carries it into storm drains and waterways, which wastes money and is restricted in some jurisdictions.
Background
Lawn fertilizer rates are given in pounds of nitrogen per thousand square feet, but bags are sold by total weight at some percentage of nitrogen. Those are different numbers, and mixing them up is how lawns get burned.
What you need for the job
Common questions
How much fertilizer do I need for 5,000 square feet?
For 1 pound of nitrogen per thousand square feet using a 24-0-4 product, about 21 pounds of product. The bag analysis changes the answer substantially.
What do the three numbers on fertilizer mean?
Percentage by weight of nitrogen, phosphate, and potash. A 50 pound bag of 10-10-10 contains 5 pounds of each.
How much nitrogen does a lawn need per year?
It varies by grass species, region, and whether clippings are returned. Your county extension service publishes local recommendations, which are more reliable than any national figure.
Can you put down too much fertilizer?
Yes. Excess nitrogen burns turf, pushes weak top growth at the expense of roots, and runs off into waterways. Calculate rather than estimate.
Should I fertilize before or after mowing?
After mowing is generally preferred, so the granules reach the soil surface rather than sitting on leaf blades that are about to be cut off.
Measuring by hand is where estimates go wrong
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