Fertilizer Calculator

Fertilizer Calculator — guide & how to use

Guide only. Follow your soil test and the product label, and never exceed the label's maximum single-application rate. Over-applying fast-release nitrogen can burn turf and pollute waterways. All math runs privately in your browser.

What it does

This is the real agronomic fertilizer calculation, not a guess. Lawn fertilizer is dosed by nitrogen, not by bag weight — so this tool takes your fertilizer's N-P-K analysis and your target nitrogen rate (lb N per 1,000 sq ft) and tells you the exact pounds of product per 1,000 sq ft, the total for your whole lawn, the nitrogen you'll actually apply, and how many bags to buy. It warns you if your target exceeds the safe single-application max.

How to use it

1. Enter your fertilizer's N-P-K numbers (or tap a common analysis like 28-0-3).
2. Set your target nitrogen rate — 0.5–1.0 lb N / 1,000 sq ft is typical per application.
3. Enter your lawn area (direct sq ft, rectangle, or circle).
4. Pick your bag size.
5. Read the lbs of product per 1,000 sq ft — that's your spreader rate — plus the lawn total and bag count.

How the math works

1. Nitrogen fraction = N% ÷ 100 (e.g. 28% → 0.28).
2. Product per 1,000 sq ft = target N rate ÷ nitrogen fraction.
3. Total product = product per 1,000 × area ÷ 1,000.
4. Actual N applied = total product × nitrogen fraction (rounds back to your target).
5. Bags = total product ÷ bag size, rounded up.
6. Bag coverage = bag size ÷ product-per-1,000 × 1,000 sq ft.

Worked example

A 28-0-3 fertilizer (28% N) at a 1.0 lb N / 1,000 sq ft target, on a 5,000 sq ft lawn, 50-lb bag:

• Product per 1,000 = 1.0 ÷ 0.28 = 3.57 lb / 1,000 sq ft (your spreader setting target).
• Total product = 3.57 × 5,000 ÷ 1,000 = 17.86 lb.
• Actual N applied = 17.86 × 0.28 = 5.0 lb of nitrogen across the lawn (= 1.0 lb / 1,000).
• Bags = 17.86 ÷ 50 → 1 bag; a 50-lb bag covers ~50 ÷ 3.57 × 1,000 ≈ 14,000 sq ft.

Safe nitrogen rates

Always defer to your local extension recommendation, your soil test, and the bag label's maximum single rate.

Accuracy notes & assumptions

• This is the correct formula (product = target N ÷ N-fraction). The uncertainty is in your area measurement and spreader calibration, not the math.
• Rate is by nitrogen. Two bags with the same N% apply the same per-1,000 even if P and K differ.
• Don't exceed ~1 lb of fast-release N per application — it can burn the lawn. Split the season's total.
• Calibrate your spreader to actually put down the per-1,000 rate this tool gives you.

Common mistakes this avoids

• Dosing by bag weight instead of by nitrogen — the #1 error thin competitors make.
• Forgetting to divide the target N by the N fraction (applying way too little or too much).
• Putting down a full year's nitrogen in one pass and burning the turf.
• Treating P or K as the rate driver for routine lawn feeding.

How to embed this tool on your website

Frequently asked questions

Related tools

Grass Seed Calculator · Sod Calculator · Raised Garden Bed Soil Calculator · Mulch Calculator · Square Footage Calculator · Lawn Care Pricing Calculator