Fertilizer NPK Calculator
This calculator turns a target nitrogen rate into the pounds of bagged fertilizer you actually spread. Enter the area you are feeding, the pounds of nitrogen you want per 1000 square feet, and the first NPK number printed on the bag. It divides your nitrogen goal by the product's nitrogen fraction, then scales that rate across your whole lawn so you buy and apply the right amount.
How to calculate a fertilizer rate
- Enter the area you are feeding in the Area field, in square feet.
- Set Target nitrogen to the pounds of N you want per 1000 square feet, using the 0.1 step arrows if needed.
- Enter Nitrogen percent, the first of the three NPK numbers on the bag.
- Read Per 1000 sq ft for the product rate and Total product for the whole area.
- Click Copy estimate to save both figures for your shopping list.
Examples
20-5-10 lawn food, 1 lb of nitrogen over 5000 sq ft
Area 5000 sq ft, target 1 lb N per 1000, nitrogen 20%
5 lbs of product per 1000 sq ft, 25 lbs total
Urea (46-0-0) at a light 0.75 lb rate over 8000 sq ft
Area 8000 sq ft, target 0.75 lb N per 1000, nitrogen 46%
1.6304 lbs per 1000 sq ft, 13.0435 lbs total
A 30 percent nitrogen blend at 1 lb over a 2500 sq ft yard
Area 2500 sq ft, target 1 lb N per 1000, nitrogen 30%
3.3333 lbs per 1000 sq ft, 8.3333 lbs total
Frequently asked questions
What do the three NPK numbers mean?
NPK is the guaranteed percent by weight of nitrogen, phosphorus and potassium. A 10-10-10 bag is 10 percent of each nutrient. This tool only reads the first number, nitrogen, because lawn programs are almost always set by the nitrogen rate.
How is the amount of product calculated?
The product rate per 1000 square feet equals your target nitrogen divided by the nitrogen fraction, which is the percent divided by 100. The total is that rate multiplied by your area divided by 1000. So 1 pound of N from a 20 percent product needs 5 pounds of product per 1000 square feet.
How much nitrogen does a lawn actually need?
A common single feeding targets about one pound of nitrogen per 1000 square feet. Across a full season most cool and warm season grasses want two to four pounds total, split into several applications rather than dumped at once.
Can I use this for a garden bed or crop, not just turf?
Yes. The math is unit agnostic as long as you keep pounds and square feet consistent. Enter the bed area and whatever nitrogen rate your plants call for, and it returns the product weight the same way.
Why does the result show four decimal places?
The engine rounds to four decimals so precise rates like 1.6304 pounds are not lost to rounding. In practice you can weigh to the nearest tenth of a pound and be perfectly fine.
What if I enter a nitrogen percent of zero or over 100?
The calculator rejects it and shows an error, because a fertilizer cannot be zero percent or more than 100 percent nitrogen. Area and target nitrogen must also be greater than zero.
How do I avoid burning the lawn?
Spread on a dry lawn, keep passes even, and water the granules in afterward unless the label says otherwise. Slow release nitrogen sources are far more forgiving than fast release ones and let you apply a bit more per feeding.
Should I split the application?
Splitting a target into two half rate passes a few weeks apart gives steadier color and less burn risk than one heavy feeding. Never exceed the maximum single rate printed on the bag.
Is my data sent anywhere?
No. Every calculation runs in your browser and nothing you type is uploaded. You can use the tool offline once the page has loaded.
Learn more
- How to read NPK numbers and set a nitrogen rate
What the three fertilizer numbers mean, how the nitrogen percent controls how much you spread, and how to build a simple seasonal feeding plan.
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