Fertilizer Calculator

Fertilizer bags list three numbers (N-P-K) representing nitrogen, phosphorus, and potassium percentages — but knowing how much to buy for your lawn takes some math. Enter your lawn area, target nitrogen, phosphorus, and potassium rates (from your soil test), and your fertilizer's N-P-K percentages and bag weight into the Fertilizer Calculator to get the total fertilizer needed in pounds and the number of bags to purchase. Secondary outputs show the actual nitrogen, phosphorus, and potassium applied per 1,000 sq ft so you can confirm the rates match your soil recommendations.

sq ft
lbs/1000 sq ft

Based on your soil test recommendation

lbs/1000 sq ft
lbs/1000 sq ft
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First number on fertilizer bag

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Second number on fertilizer bag

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Third number on fertilizer bag

lbs

Results

Total Fertilizer Needed

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Bags Needed

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Nitrogen Applied

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Phosphorus Applied

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Potassium Applied

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Frequently Asked Questions

What do the N-P-K numbers on fertilizer bags mean?

N-P-K represents the percentage by weight of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) in the fertilizer. For example, a 15-5-10 fertilizer contains 15% nitrogen, 5% phosphorus, and 10% potassium.

How do I know what application rate to use?

Application rates should be based on a recent soil test report, which will recommend specific amounts of each nutrient needed for your lawn type and soil conditions. Without a soil test, follow general guidelines for your grass type.

Can I use a different fertilizer than what my soil test recommends?

Yes, this calculator helps you determine how much of any available fertilizer to apply to meet your target nutrient rates. You may need to apply more or less depending on the N-P-K analysis of your available fertilizer.

What's the ideal fertilizer ratio for lawns?

A 3-1-2 ratio of N-P-K is generally recommended for established lawns (like 15-5-10). However, established lawns often don't need additional phosphorus, so nitrogen-only fertilizers may be appropriate if soil phosphorus levels are adequate.

How accurate does my lawn area measurement need to be?

Measure as accurately as possible since fertilizer application is calculated per square foot. For irregular shapes, break your lawn into rectangles, triangles, or circles and add the areas together for the most accurate calculation.

What happens if I apply too much fertilizer?

Over-fertilizing can burn your grass, promote excessive growth requiring more mowing, increase disease susceptibility, and contribute to environmental pollution through runoff. Always follow recommended application rates.

When should I apply fertilizer to my lawn?

Timing depends on your grass type and location. Cool-season grasses typically benefit from spring and fall applications, while warm-season grasses are best fertilized during their active growing season in late spring and summer.

Do I need to water after applying fertilizer?

Most fertilizers should be watered in lightly after application to help dissolve the nutrients and prevent burning. Check the product label for specific watering instructions, as some slow-release fertilizers may have different requirements.