Thoughtful Calc

Lawn Fertilizer Calculator

How many pounds of fertilizer to spread, from the numbers on your bag, without going over the per-application nitrogen limits.

How do you want to measure?
For odd shapes, measure each part as a rectangle, add them up, and subtract the house, driveway, and beds.
Enter the three numbers from the front of the bag (N-P-K), like 24-0-11.
The weight is on the front of the bag, near the N-P-K.
Look on the front of the bag for “slow-release nitrogen,” often with a percentage. No such claim, or mostly quick-release? Choose quick.
Many bags print it on the front (“50% slow-release nitrogen”). If not, find “slowly available” nitrogen in the guaranteed analysis on the back and divide by total N: 12% on a 24% product is 50%. Leave it blank and we treat all of the nitrogen as possibly soluble and stay at 0.7 lb, the soluble-N cap.
For a rate from a soil test or your lawn-care schedule. Leave blank to use the target for your release type. If yours is over the per-application limit, we use the limit and tell you why.
Applies your state’s per-application limits and shows only its rules. Leave it blank and we use the strictest limits we found in any state. State law only; your county or city may be stricter.
Add your price per bag (optional; we use a typical price otherwise)

Why our numbers are more realistic

The fertilizer calculators we looked at get the arithmetic right. What they skip is the limit on how much you can put down at once. Here's what changes:

Typical calculatorThis calculatorWhy it matters
Some start at 1.0 lb of nitrogen per 1,000 sq ft (4.17 lb of 24-0-11) 0.7 lb for quick-release, 0.75 lb for slow-release University guidance recommends no more than 1.0 lb at a time, and the strictest state laws cap quick-release nitrogen at 0.7 lb per application and total nitrogen at 0.9 lb. With no state picked we stay at or under both; with a state picked we follow its limits, and never go above 1.0 lb. 0.7 lb is a ceiling, not a target: some grasses, like centipede, need less.
Warn only well above the limits, if at all (one warns above 1.25 lb) Never calculates an amount above the limit for your release type, and tells you why An amount you shouldn't spread is no use to you. If your rate is over the limit, we work out the amount at the limit and tell you which limit it was.
Most don't check the middle number on the bag Reports the phosphorus (P2O5) you'd apply and warns Eleven states restrict phosphorus on lawns for homeowners, usually with exceptions for a soil-test need and new lawns, and the strictest limit we found is 0.25 lb of P2O5 per application. Choose your state to see its rule.
No state or local rules An optional state choice that applies that state's caps and shows its phosphorus rule, dates, and setbacks The rules differ a lot: some states ban lawn fertilizer over the winter, one bans products with too little slow-release nitrogen, and some counties ban it in summer. Laws change and local rules can be stricter, so we show the date we last checked.

Rules by state

Choose your state above to apply its rules. This is a summary of state law, last checked October 4, 2026. Laws change, and counties and cities can be stricter, so check your local rules too.

  • Florida (a rule on what bag labels may direct): soluble nitrogen up to 0.7 lb per 1,000 sq ft per application. Up to 2 lb of nitrogen only in spring or summer and only if the product is at least 65% controlled-release; this calculator never goes above 1.0 lb. Phosphorus (P2O5) up to 0.25 lb per application and 0.5 lb per year unless a soil test shows a deficiency, plus one starter application after planting of up to 1.0 lb. The bag must show its maximum coverage area. Many Florida counties and cities ban nitrogen and phosphorus in summer, often June 1 to September 30.
  • Maryland: 0.9 lb total N and 0.7 lb soluble N per 1,000 sq ft per application, except for enhanced-efficiency fertilizer (state-approved products only, up to 2.5 lb N or 80% of the annual recommendation, whichever is less). At least 20% of the nitrogen must be slow-release. No lawn fertilizer from November 15 to March 1. Phosphorus only for a soil-test need or when establishing, patching, or renovating.
  • New Jersey: 0.9 lb total N and 0.7 lb water-soluble N per application, at least 20% slow-release, and 3.2 lb N per year for homeowners. No nitrogen or phosphorus before March 1 or after November 15, or on frozen or saturated ground.
  • Pennsylvania: 0.9 lb total N (unless enhanced-efficiency) and 0.7 lb plant-available N per application. University extension guidance on the law says no nitrogen or phosphorus after December 15 or before March 1.
  • Vermont: bans lawn fertilizer whose nitrogen is less than 15% slow-release, so all-quick-release products are illegal on turf. No fertilizer before April 1 or after October 15.
  • Phosphorus laws that bind homeowners: Connecticut, Maryland, Massachusetts, Michigan, Minnesota, New Jersey, New York, Pennsylvania, Vermont, Washington, and Wisconsin. Most allow phosphorus for a soil-test need or a new or repaired lawn. New York also bans lawn fertilizer from December 1 to April 1.
  • Bag or store rules only: Florida, New Hampshire, and Virginia limit what lawn fertilizer can contain, and Maine requires a store sign about phosphorus. New Hampshire also limits fertilizer near protected shorelines.
  • Paid applicators only: Illinois and Delaware.
  • Everywhere else: we found no statewide homeowner law. That doesn't mean there are no rules; cities and counties may have them.

How this works

Pounds of product = pounds of nitrogen wanted ÷ (N% ÷ 100) × lawn area ÷ 1,000. So 0.7 lb of nitrogen from a 24% product is 0.7 ÷ 0.24 = 2.92 lb of product per 1,000 sq ft.

  • Quick-release: 0.7 lb N per 1,000 sq ft, the strictest state limit on soluble nitrogen per application. It's a ceiling: centipede, for example, does best on two 0.5 lb quick-release doses a year (university guidance), so enter your own rate for low-need grasses.
  • Slow-release: default 0.75 lb N, capped at 1.0 lb, the most university guidance recommends per application.
  • Mixed products: only the soluble share counts against 0.7 lb, so the rate is held where rate × (1 − slow-release share) is 0.7 lb or less. Leave the share blank and we assume it's all soluble: 0.7 lb.
  • Bags: pounds of product ÷ bag size, rounded up. Coverage per bag = bag pounds × N% ÷ rate × 1,000.
  • Phosphorus: P2O5 per 1,000 sq ft = product per 1,000 × P% ÷ 100.
  • Your state (optional): adds that state's caps on top of the ones above. In the states that cap total nitrogen at 0.9 lb (listed under Rules by state), slow-release is held to 0.9 lb. With no state chosen, the calculator uses the strictest limits we found.

Soil testing

A soil test is the only way to know whether your lawn needs phosphorus, and in many states it's what makes phosphorus legal on an established lawn.

  • What it tells you: pH, phosphorus, potassium and organic matter, usually with lime and fertilizer advice for a lawn. Nitrogen usually isn't tested.
  • Where: your state university's extension soil lab. University guidance prefers a lab that gives lawn recommendations over a home kit. Search for your state's name plus "extension soil test".
  • Cost: free to about $30 a sample at the university labs we checked (2026).
  • How: take samples about 4 in deep from 8 to 10 spots across the lawn, mix them, and send the amount the lab asks for.
  • How often: every 2 to 4 years.
  • Phosphorus rules: in most states with a homeowner phosphorus law, a recent test showing a need is what allows phosphorus on an established lawn. "Recent" means 18 months to 3 years, depending on the state, and a new lawn is usually exempt. Florida's rule holds phosphorus to 0.25 lb P2O5 per application unless a test shows a need. No test? Choose a product with 0 in the middle; that 0 is on purpose.

Common questions

What do the three numbers on a fertilizer bag mean?

Percent nitrogen (N), phosphate (P2O5), and potash (K2O) by weight. A 24-0-11 bag is 24% nitrogen, no phosphate, and 11% potash.

When should I fertilize a new lawn?

This calculator is for feeding a lawn that's already growing; starter fertilizer at seeding or sodding is a separate product, so follow its label. Don't put nitrogen down before planting. For seed, university guidance suggests 0.5 lb of nitrogen per 1,000 sq ft, 3-5 weeks after seedlings come up. For sod, feed about a month after laying it, and again 30-60 days later. Check your state's phosphorus rules before using a starter product.

Can I use fertilizer with crabgrass preventer on new grass seed?

Not on seed you've just sown, or in the 4 months before you sow it: most weed preventers stop grass seed from sprouting too. On cool-season lawns, mesotrione is the exception: it's labeled for use at seeding on most cool-season grasses (check your grass is listed). Once new grass is up, wait until it's been mowed 2 to 4 times, depending on the product. The waits by ingredient are on the grass seed calculator page.

What this doesn't cover

  • Yearly nitrogen totals and timing by grass. They vary a lot: one university table runs from as little as 0.4 lb a year for centipede to as much as 7 lb for bermuda in the hottest areas.
  • Bags with weed killer or crabgrass preventer in them (“weed and feed”). Spread those at the rate on the label: it's set for the weed control, not the nitrogen, and the label is the law. Use this calculator for plain fertilizer.
  • Lime, and turning a soil test's results into a feeding plan.
  • Liquid fertilizers and organic products with unusual analyses.
  • Local ordinances and the full text of any law.

Sources

Related calculators

Numbers last reviewed: October 9, 2026.