Lawn Fertilizer Calculator: How Much to Apply

A common target is 1 lb of actual nitrogen per 1000 sq ft per feeding. The bag weight needed depends on its nitrogen percent: at 26% nitrogen you apply about 3.85 lb of product per 1000 sq ft to deliver 1 lb of nitrogen.

The three numbers are the NPK analysis: percent nitrogen, phosphorus and potassium by weight. A 26-0-3 bag is 26% nitrogen, which is the only figure this calculator uses to size the application.

It first finds nitrogen needed as your area in thousands of square feet times the rate, then divides that by the nitrogen fraction. For 1 lb of nitrogen from a 20% product, that is 1 divided by 0.20, which is 5 lb of product.

Any value greater than 0 and up to 100. A percent of zero, a negative number or anything above 100 is rejected, and the tool shows a prompt to enter a percent from 1 to 100 instead of a result.

No. It only works from the first NPK number, nitrogen, because nitrogen drives lawn feeding rates. The phosphorus and potassium in the bag come along at whatever ratio the product uses.

Yes. Both the nitrogen delivered and the pounds of product are rounded to two decimal places, so 5 divided by 0.26 shows as 19.23 lb rather than a long decimal.

Yes. Over applying can burn the lawn and pollute runoff. Most lawns do best at no more than about 1 lb of nitrogen per 1000 sq ft per application, with a few weeks between feedings.

It works for any product where you know the nitrogen percent, including granular and slow release lawn fertilizers. Enter that percent and the calculator handles the rest. It does not adjust for slow release timing, only the total weight to spread.

Yes. The whole calculation runs locally in your browser and nothing is uploaded to a server, so your lawn size and inputs never leave your device.