Fertilizer Calculator
How much fertilizer to spread: from the N-P-K numbers on the bag.
Every fertilizer recommendation is written in pounds of nitrogen per 1,000 square feet. But fertilizer is sold in bags of product, and a bag is only partly nitrogen. A 10-10-10 bag is 10% nitrogen, so one pound of nitrogen needs ten pounds of product. This calculator does that conversion for you, and shows the phosphate and potash that come along for the ride.
Start with a soil test if you have one: it tells you whether you need phosphorus and potassium at all. For the soil itself, see the Raised Bed Soil Calculator, and for liquid feeds, the Liquid Fertilizer Dilution Calculator.
How this is worked out
Product per 1,000 sq ft = target lb N ÷ (N% ÷ 100). Total product = product per 1,000 sq ft × area ÷ 1,000. Nutrients applied = total product × each percentage ÷ 100. The three numbers on the bag are percentages by weight, always in the order nitrogen (N), phosphate (P2O5), potash (K2O).
Assumptions:
- The target rate is nitrogen per single application, not per year. Most programmes feed several times a season.
- A single application is usually 0.5 to 1 lb of nitrogen per 1,000 sq ft; use the lower end for quick-release fertilizer and in hot weather.
- Bag counts round up to whole bags. You buy whole bags, even if the last one is barely opened.
- The calculator cannot tell you whether you need phosphorus or potassium. Only a soil test can.
Three worked examples
1. A 5,000 sq ft lawn with 10-10-10 at 1 lb N. Product per 1,000 sq ft = 1 ÷ 0.10 = 10 lb. Total = 10 × 5 = 50 lb of product. Two 40 lb bags, with 30 lb left over.
2. The same lawn with 32-0-4. Product per 1,000 sq ft = 1 ÷ 0.32 = 3.1 lb. Total = 3.1 × 5 = 15.6 lb. A higher-nitrogen bag means far less product to carry and spread, and no phosphorus at all.
3. A 1,000 sq ft vegetable bed with 16-4-8 at 0.5 lb N. Product per 1,000 sq ft = 0.5 ÷ 0.16 = 3.1 lb. That single application also delivers 0.13 lb of phosphate and 0.25 lb of potash.
Product needed for 1 lb of nitrogen per 1,000 sq ft
| Bag analysis | Product per 1,000 sq ft | Product for 5,000 sq ft |
|---|---|---|
| 46-0-0 (urea) | 2.2 lb | 10.9 lb |
| 32-0-4 | 3.1 lb | 15.6 lb |
| 26-5-10 | 3.9 lb | 19.3 lb |
| 24-0-6 | 4.2 lb | 20.8 lb |
| 16-4-8 | 6.3 lb | 31.3 lb |
| 10-10-10 | 10.0 lb | 50.0 lb |
These figures follow Iowa State University Extension and Penn State Extension examples. The shortcut turf managers use is 100 ÷ %N: the pounds of product that deliver one pound of nitrogen.
Common fertilizer mistakes
- Reading the bag as if it were pure nutrient. Ten pounds of 10-10-10 is one pound of nitrogen, not ten. Always divide by the percentage first.
- Applying more than 1 lb of nitrogen per 1,000 sq ft in one pass. Heavy single doses risk burning plants and leaching nitrogen past the roots. Split the seasonal total into several lighter applications.
- Adding phosphorus nobody asked for. Many established lawns already have enough phosphorus, and some states restrict it unless a soil test shows a need. A bag with a middle number above zero is not automatically better.
- Feeding on a fixed calendar instead of by growth. Plants take up nitrogen when they are actively growing. Feeding dormant plants wastes fertilizer and pollutes runoff.
- Trusting the bag’s coverage claim blindly. Coverage on the bag is based on the manufacturer’s recommended rate, which may be higher or lower than your extension service suggests. Calculate from your own target rate.
What to do next
Pick your bag for its nitrogen number first, then check what phosphate and potash come with it. And whether your soil test says you need them. For liquid feeds, the Liquid Fertilizer Dilution Calculator works out the mixing ratio for your watering can.
Putting the result to work
The calculator gives you the product for one application. Turn it into a season plan by multiplying by the number of applications your extension service recommends for your crop or lawn. Typically three to four light feeds a season for lawns, and two to three for vegetable beds. And buy the season’s total at once if storage is dry. A sealed bag kept off a damp floor holds its analysis for a year or more; an open bag in a wet shed cakes and loses nitrogen.
Apply on a calm day when rain is expected within a day or two, so the granules wash into the soil instead of sitting on leaves. Water lightly after spreading if no rain comes. Never spread on frozen ground or before a downpour that will wash it into drains. That is how fertilizer becomes water pollution instead of plant food. And keep the bag: write the date and the rate you applied on it, so next season you know what the bed already received.
How to read a fertilizer bag
- The three numbers are percentages by weight, always in order: nitrogen (N), phosphate (P2O5), potash (K2O). A 10-10-10 bag is 10% of each.
- Recommendations are written in lb of nitrogen per 1,000 sq ft. Not lb of product. Convert first.
- Product per 1,000 sq ft = target lb N / (N% / 100). Example: 1 lb N from 10-10-10 needs 1 / 0.10 = 10 lb of product per 1,000 sq ft.
- Multiply by your area in thousands of square feet. A 5,000 sq ft area needs 5 x 10 = 50 lb.
- The P and K come along for the ride. The same product also applies phosphate and potash in proportion.
What is a good nitrogen rate?
A single application is usually 0.5 to 1 pound of nitrogen per 1,000 square feet. Use the lower end for quick-release fertilizer and in hot weather. Your state extension service publishes the yearly total for your grass type or crop, so treat the per-application rate as one instalment, not the whole season.
Why divide by the percentage?
Each pound of product carries only part nutrient. A 24% bag carries 0.24 pounds of nitrogen per pound, so to land 1 pound of nitrogen you need 1 divided by 0.24, which is 4.17 pounds of product. The shortcut is 100 divided by the nitrogen percentage: 100 divided by 10 is 10 pounds of 10-10-10 per pound of nitrogen.
Do I need phosphorus?
Only a soil test can say. Many established lawns and beds already have enough phosphorus, and some states restrict phosphorus on established lawns unless a soil test shows a deficiency or you are seeding a new lawn. If your test says phosphorus is sufficient, choose a bag with a middle number of zero.
What do the three numbers on a fertilizer bag mean?
They are percentages by weight of nitrogen, available phosphate, and soluble potash, always in that order. A 40-pound bag of 24-0-8 holds 40 times 0.24, or 9.6 pounds of nitrogen, no phosphate, and 3.2 pounds of potash. The order is fixed by fertiliser labelling law across the United States.
How many bags do I need?
Divide the total product by the bag weight and round up to a whole bag. A 5,000 square foot area at 1 pound of nitrogen with 10-10-10 needs 50 pounds of product, which is two 40-pound bags. The second bag will be barely opened. That is normal, and the leftover keeps for the next application if stored dry.
Formula and rates follow university extension guidance (Iowa State University Extension and Penn State Extension examples: 1 lb N per 1,000 sq ft needs 3.1 lb of 32-0-4, 3.85 lb of 26-5-10, or 10 lb of 10-10-10 per 1,000 sq ft). Rates are planning estimates. Follow your product label and local extension guidance.
