Fertilizer recommendations are written in pounds of a nutrient. Fertilizer is sold in bags of product with three numbers on the front. This calculator does the conversion between them, and — the part most calculators leave out — shows you what else the dose you have just worked out is putting on the ground.
How to use it
- Enter the area you are feeding, in square feet or square metres.
- Enter the target rate for the nutrient you are aiming at. If a source tells you "one pound of nitrogen per 1,000 square feet", that is a target rate of 1.
- Type the three numbers from the bag into the N, P and K fields exactly as printed: 10-10-10 goes in as 10, 10, 10.
- Choose which nutrient you are aiming at. Most vegetable and lawn recommendations are written as nitrogen, which is the default.
The table underneath the result shows the phosphate and potash that come along for the ride. Worth a look before you spread.
How it is calculated
lb of product = (target rate / (nutrient % / 100)) x (area / 1000)
Two steps in one line. First, divide the target rate by the nutrient's fraction to find how much product carries that much nutrient per 1,000 square feet. Then scale by your actual area.
The other two nutrients follow from the same weight of product:
lb of nutrient supplied = lb of product x (its label % / 100)
A worked example
You want one pound of nitrogen per 1,000 square feet over a 400 square foot bed, using a 10-10-10 fertilizer.
- Nitrogen fraction: 10% = 0.10
- Product per 1,000 sq ft: 1 / 0.10 = 10 pounds
- Scaled to your bed: 10 x (400 / 1000) = 4 pounds of product
- Nitrogen supplied: 4 x 0.10 = 0.4 lb
- Phosphate supplied: 4 x 0.10 = 0.4 lb
- Potash supplied: 4 x 0.10 = 0.4 lb
Now the same target with a 24-8-16 instead. Nitrogen is 24%, so you need 1 / 0.24 = 4.17 pounds per 1,000 sq ft, or 1.67 pounds over the bed. That also supplies 0.13 lb of phosphate and 0.27 lb of potash — a third of the phosphate the balanced product would have given for the same nitrogen.
Product needed for 1 lb of nitrogen per 1,000 sq ft
| Fertilizer | Nitrogen | Product per 1,000 sq ft |
|---|---|---|
| 5-10-5 | 5% | 20 lb |
| 10-10-10 | 10% | 10 lb |
| 12-4-8 | 12% | 8.3 lb |
| 24-8-16 | 24% | 4.2 lb |
| 34-0-0 | 34% | 2.9 lb |
| 46-0-0 (urea) | 46% | 2.2 lb |
| Blood meal, 12-0-0 | 12% | 8.3 lb |
Tips and common mistakes
Weigh it. A cup of one granular product is not a cup of another — densities differ enough to matter. A cheap kitchen scale removes all the guesswork here.
Split the season rather than feeding once. Most vegetables do better with a portion at planting and the rest side-dressed as they grow. A single heavy dose is more likely to be wasted, and more likely to burn.
Get a soil test before adding phosphate or potash. Nitrogen moves and leaches, so it needs topping up. Phosphate and potash accumulate, and many long-established gardens have plenty of both. A test costs little and can save you years of pointless applying.
Skip nitrogen for beans and peas. They fix their own from the air, and a nitrogen feed gives you foliage instead of pods.
Keep it away from stems and out of watercourses. Granules resting against a stem scorch it. Anything spread on a hard surface or a slope before rain ends up in a drain.