Every fertilizer recommendation — from your soil test, your extension office, or any lawn program worth following — is written the same way: pounds of nitrogen per 1,000 square feet. But no bag sells you pure nitrogen. The bag says 10-10-10, or 24-0-6, or 30-0-4, and quietly leaves the conversion to you. Get it wrong in one direction and the lawn stays hungry; get it wrong in the other and you burn it, push surge growth that mowing can’t keep up with, and send the excess into the nearest storm drain.
This calculator does the conversion the way turf programs at Purdue and Minnesota teach it: pounds of product = target nitrogen ÷ (the bag’s first number ÷ 100). Enter the three numbers on your bag, your square footage, and what you’re feeding — established lawn, new seeding, vegetable bed, or trees — and get the exact pounds to spread, per pass and total. It also shows the phosphate and potash you’re applying along the way, flags any single application over 1 lb of nitrogen per 1,000 square feet, and warns you when a bag with a middle number doesn’t belong on soil that already tests high in phosphorus. One bag, one number, no algebra at the garden center.
Frequently Asked Questions
What do the three numbers on a fertilizer bag mean?
The N-P-K analysis: percent nitrogen (N), percent phosphate (P₂O₅), and percent potash (K₂O) by weight. A 50 lb bag of 10-10-10 contains 5 lb of nitrogen, 5 lb of phosphate, and 5 lb of potash — the remaining 35 lb is carrier material. The first number drives application math because recommendations are written in pounds of nitrogen.
How much 10-10-10 fertilizer per 1,000 square feet?
To deliver the standard 1 lb of nitrogen per 1,000 square feet, you need 10 lb of 10-10-10 (1 ÷ 0.10). For a lighter 0.5 lb nitrogen feeding, spread 5 lb per 1,000 square feet. The same logic works for any analysis: divide the nitrogen target by the bag’s first number as a decimal.
Why are fertilizer recommendations based on nitrogen and not P or K?
Nitrogen is the nutrient plants consume in the largest amounts and the only major one that leaches readily from soil, so it must be replaced on a schedule. Phosphorus and potassium bind to soil particles and accumulate over years — their rates should come from a soil test, not a calendar. That is why a lawn program says “1 lb N per 1,000 sq ft” rather than naming a product.
Can I apply more than 1 lb of nitrogen per 1,000 square feet at once?
Not with quick-release fertilizer. University turf programs cap single applications at 1 lb N per 1,000 square feet because more risks foliar burn, surge growth, and runoff. If your annual plan calls for 3–4 lb of nitrogen, split it into applications 4–6 weeks apart. Slow-release formulations can stretch the per-application cap slightly — check the label percentage of slow-release N.
Do I need a fertilizer with phosphorus?
Only if a soil test says so. Most established lawns and many garden soils already hold adequate-to-excessive phosphorus, and several states restrict phosphorus lawn fertilizer for that reason. Excess P locks out iron and zinc and contributes to algae blooms in waterways. If your test reads High or Excessive, choose a zero-middle-number product like 24-0-6.