Every bag of fertilizer carries three numbers, such as 10 10 10 or 24 0 6. That is how to read fertilizer numbers: they are the percentages by weight of nitrogen, phosphate and potash, in that order. To work out how much product to spread, divide the nitrogen rate you want by the nitrogen percentage as a decimal. A 1 lb nitrogen rate with a 20 5 10 fertilizer needs 5 lb of product per 1,000 square feet.
That single step is the key to using fertilizer accurately. Many recommendations are written in pounds of nutrient, not pounds of product. A bag that is 20 percent nitrogen is twice as strong as a bag that is 10 percent nitrogen, so you need half as much of it.
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How to Read Fertilizer Numbers: What NPK Means
The numbers are the guaranteed percentage, by weight, of three nutrients. N is nitrogen, which drives leafy growth and green colour. P is phosphate, which supports root development, flowering and fruiting. K is potash, which supports overall vigour, water regulation and stress tolerance. The numbers always appear in that order.
A 50 lb bag of 10 10 10 contains 5 lb of nitrogen, 5 lb of phosphate and 5 lb of potash. The remaining weight is filler and carrier material.
A grade with a zero means that nutrient is not in the bag. A 24 0 6 fertilizer has 24 percent nitrogen, no phosphate and 6 percent potash. A 46 0 0 fertilizer, which is urea, has 46 percent nitrogen and nothing else. A zero tells you that you are buying nitrogen only, which is the right choice when a soil test says phosphorus and potassium are already high enough.
Turning a Recommendation Into Product
Lawn and garden recommendations are usually given as pounds of nitrogen per 1,000 square feet. To find how much product supplies that amount, divide by the nitrogen percentage written as a decimal. Total product equals product per 1,000 square feet times area divided by 1,000.
Work through the standard example. To apply 1 lb of nitrogen per 1,000 square feet with 20 5 10, you need 1 divided by 0.20, which is 5 lb of product per 1,000 square feet. On a 5,000 square foot lawn that is 5 times 5, which is 25 lb of product. The same method works for any grade and any area.
Try a different rate. To apply 0.75 lb of nitrogen per 1,000 square feet with the same 20 5 10 fertilizer, you need 0.75 divided by 0.20, which is 3.75 lb of product per 1,000 square feet. On an 8,000 square foot lawn that is 3.75 times 8, which is 30 lb of product.
Once you know how to read fertilizer numbers, the fertilizer calculator works this out for any grade, any area and any nutrient, and also shows how much phosphate and potash you apply along the way. Use it to check your own numbers before you buy, because buying the wrong grade is an expensive mistake to fix after the fact.
Product Needed for 1 lb of Nitrogen per 1,000 Square Feet
| Fertilizer | Nitrogen | Per 1,000 sq ft | For 5,000 sq ft |
|---|---|---|---|
| 10 10 10 | 10 percent | 10 lb | 50 lb |
| 16 4 8 | 16 percent | 6.25 lb | 31.3 lb |
| 20 5 10 | 20 percent | 5 lb | 25 lb |
| 24 0 6 | 24 percent | 4.17 lb | 20.8 lb |
| 29 0 4 | 29 percent | 3.45 lb | 17.2 lb |
| 46 0 0 urea | 46 percent | 2.17 lb | 10.9 lb |
Once you know how to read fertilizer numbers, the pattern is clear: the stronger the nitrogen percentage, the less product you need. A 46 0 0 urea needs only 2.17 lb per 1,000 square feet for the same nitrogen as 10 lb of 10 10 10. That does not make urea better in every case: it supplies no phosphate or potash. A balanced grade such as 10 10 10 suits a lawn that needs all three nutrients, while a nitrogen only grade suits a lawn that already has enough phosphorus and potassium.
Check the rounding in the table. The 16 4 8 row needs 1 divided by 0.16, which is 6.25 lb per 1,000 square feet, and 6.25 times 5 is 31.25 lb, rounded to 31.3 lb. The 24 0 6 row needs 4.17 lb, and 4.17 times 5 is 20.8 lb. The 29 0 4 row needs 3.45 lb, and 3.45 times 5 is 17.2 lb. The 46 0 0 row needs 2.17 lb, and 2.17 times 5 is 10.9 lb.
What Else Comes With the Nitrogen
When you apply a blended fertilizer for nitrogen, you also apply phosphate and potash in the same pass. The table below shows what 25 lb of product supplies for three common grades, all worked out from the percentages on the bag.
| Product applied | Nitrogen | Phosphate | Potash |
|---|---|---|---|
| 25 lb of 10 10 10 | 2.5 lb | 2.5 lb | 2.5 lb |
| 25 lb of 20 5 10 | 5 lb | 1.25 lb | 2.5 lb |
| 25 lb of 24 0 6 | 6 lb | 0 lb | 1.5 lb |
| 10.9 lb of 46 0 0 urea | 5 lb | 0 lb | 0 lb |
The comparison is the point of the table, and it is why knowing how to read fertilizer numbers saves money. To get 5 lb of nitrogen you can use 50 lb of 10 10 10, 25 lb of 20 5 10, or 10.9 lb of 46 0 0 urea. The 10 10 10 route also adds 5 lb of phosphate and 5 lb of potash. The 20 5 10 route adds 1.25 lb of phosphate and 2.5 lb of potash. The urea route adds none of either. If your soil already has plenty of phosphorus, the balanced grade wastes money and adds nutrient the lawn does not need.
Work out the phosphate and potash for your own application before you spread. Multiply the total product by each percentage as a decimal. For the worked example earlier, 30 lb of 20 5 10 supplies 1.5 lb of phosphate and 3 lb of potash.
How Much Nitrogen Does a Lawn Need?
Needs depend on grass type, climate, whether clippings are returned and how green you want it. Many extension programs suggest single applications of about 0.5 to 1 lb of nitrogen per 1,000 square feet, with annual totals ranging from roughly 1 to 4 lb depending on the grass. Slow release nitrogen is gentler and lasts longer.
A lawn that gets its clippings returned needs less nitrogen than a lawn where clippings are bagged and removed, because the clippings return nitrogen to the soil as they break down. Do not chase the darkest green lawn: that colour usually comes from a high nitrogen rate, and it brings fast growth, more mowing and more disease pressure with it.
Split the annual total into two or three applications rather than putting it all down at once. A single heavy application can burn the lawn. The grass seed calculator and the sod calculator can help you plan a new lawn area before you work out its fertilizer needs.
Why a Soil Test Is Worth It
Nitrogen is needed regularly, but phosphorus and potassium build up in soil. A soil test shows whether you need them at all. Many established lawns do not need phosphorus, and some areas restrict it to protect waterways.
A soil test also shows soil pH, which affects how well the grass can use the nutrients you apply. A test costs less than a bag of the wrong fertilizer, and it stops you from adding phosphate year after year to soil that already has too much.
Keep the test results with your fertilizer notes. When you know the phosphorus and potassium levels and how to read fertilizer numbers, you can pick a grade that fills only the gaps. A lawn that is high in phosphorus and low in potassium needs a grade such as 24 0 6, not a balanced 10 10 10. The soil calculator can help you work out soil volumes for a new bed, but the soil test is what tells you which nutrients that soil needs.
Liquid and Water Soluble Fertilizers
The same numbers apply to liquids and soluble powders, and how to read fertilizer numbers works the same way, but they are usually dosed by volume, such as 1 tablespoon per gallon. To scale a label rate to a watering can or sprayer, use the fertilizer dilution calculator.
Liquid fertilizers act fast because the nutrients are already in solution, but they also wash through the soil faster than granules. That makes them useful for a quick correction, and less useful as the only fertilizer for a whole season. Granules release more slowly and are easier to spread evenly over a large lawn.
Always mix liquids exactly as the label says. Doubling the dose does not double the benefit: it doubles the risk of leaf burn. Mix in a clean container, stir until the powder is fully dissolved, and apply on a cool, calm day.
Applying Fertilizer Evenly
Calibrate your spreader on a measured area before treating the whole lawn. Mark out 1,000 square feet, weigh out the product for that area, and spread it. If the spreader runs out early or has product left over, adjust the setting and try again.
Spread half the product in one direction and half at right angles to avoid stripes. Walk at a steady pace and keep the spreader moving when it is open. Sweep granules from hard surfaces back onto the lawn, because fertilizer left on paths and driveways washes into drains when it rains. Water in lightly if the label says so, and never apply before heavy rain.
Always follow the product label and any local regulations, because they take priority over general guidance.
Frequently Asked Questions
Q: What do the three numbers on a fertilizer bag mean?
A: That is how to read fertilizer numbers: they are the percentages by weight of nitrogen, phosphate and potash, in that order. A 50 lb bag of 10 10 10 contains 5 lb of each nutrient.
Q: How much fertilizer do I need per 1,000 square feet?
A: Divide the nitrogen rate you want by the nitrogen percentage as a decimal. For 1 lb of nitrogen with 20 5 10, you need 5 lb of product per 1,000 square feet. With 10 10 10 you need 10 lb for the same nitrogen.
Q: What does 10 10 10 fertilizer mean?
A: It means the bag is 10 percent nitrogen, 10 percent phosphate and 10 percent potash by weight. A 50 lb bag contains 5 lb of each nutrient.
Q: How much nitrogen does a lawn need each year?
A: Many extension programs suggest annual totals of roughly 1 to 4 lb of nitrogen per 1,000 square feet depending on the grass, split into single applications of about 0.5 to 1 lb.
Q: Should I use a fertilizer with no phosphorus?
A: Yes, if a soil test says phosphorus is already high enough, or if local rules restrict it. Many established lawns do not need phosphorus at all. A grade such as 24 0 6 or 46 0 0 supplies nitrogen without adding phosphate.
Q: Can I mix liquid fertilizer to an exact nitrogen rate?
A: Yes. The same numbers apply to liquids, but they are dosed by volume, such as 1 tablespoon per gallon. Use the fertilizer dilution calculator on this site to scale a label rate to your watering can or sprayer, and always mix exactly as the label says.
Read the three numbers, divide by the nitrogen percentage, and spread half in each direction. That is how to read fertilizer numbers in practice. A soil test tells you which grade to buy, and the label tells you how to apply it.
