Skip to content
YardMath
CalculatorsGuidesAboutSearch

Fertilizer Calculator — Bags by N-P-K

Enter your lawn area, fertilizer N-P-K numbers, and target nitrogen rate to get pounds of product and bag count. Based on university extension recommendations.

Reviewed October 2026 · by The YardMath Estimating Desk · How we calculate

Lawn & Fertilizer

First number on bag label

Middle number

Last number

1.0 lb/1k sq ft is standard (Purdue, Penn State Extension)

Fertilizer Takeoff
Product needed (lb)—
Bags needed—
Actual N applied (lb)—
lb N per 1,000 sq ft—
Total area—
Spreader calibration: Always calibrate your spreader to the rate on the bag label. Over-application burns grass and wastes money. Under-application produces uneven results. When in doubt, apply half the product in perpendicular passes.

How to Calculate Fertilizer Quantities

The formula: lb of product = (area ÷ 1,000) × target lb N ÷ (N% ÷ 100). For a 6,000 sq ft lawn using 29-0-4 at 1 lb N per 1,000 sq ft:

lb product = (6,000 ÷ 1,000) × 1.0 ÷ (29 ÷ 100)
           = 6 × 1.0 ÷ 0.29
           = 20.7 lb of 29-0-4

A 50 lb bag covers 50 × 0.29 ÷ 1.0 × 1,000 = 14,500 sq ft. So one bag handles the 6,000 sq ft lawn with product to spare — save the remainder for the next application.

Common Lawn Fertilizers

Product (N-P-K)Coverage per 50 lb bag at 1 lb N/1k sq ftTypical use
29-0-414,500 sq ftMaintenance feeding, cool-season grasses
32-0-1016,000 sq ftMaintenance with potassium
16-4-88,000 sq ftStarter and all-purpose
18-24-6 (starter)9,000 sq ftNew seeding or sodding
46-0-0 (urea)23,000 sq ftBulk nitrogen, must water in
10-10-105,000 sq ftGeneral garden use

Annual N Rates by Grass Type

Grass typelb N per 1,000 sq ft per yearSource
Kentucky Bluegrass3–5U of Minnesota Extension
Tall Fescue2–4Purdue Extension
Fine Fescue1–2Penn State Extension
Bermudagrass4–6Clemson Extension
Zoysiagrass2–3NC State Extension
St. Augustinegrass4–5UF/IFAS Extension
Centipedegrass1–2Clemson Extension

Sources

Frequently asked questions

How do I read fertilizer numbers (N-P-K)?+

The three numbers on a fertilizer bag are the percentages by weight of nitrogen (N), phosphorus pentoxide (P₂O₅), and potassium oxide (K₂O). A 29-0-4 bag is 29% nitrogen by weight. So a 50 lb bag contains 50 × 0.29 = 14.5 lb of actual nitrogen.

How much nitrogen do I need per 1,000 sq ft?+

For most lawns, 1 lb of actual nitrogen per 1,000 sq ft per application is the standard rate from university extension services. Cool-season grasses typically receive 3–5 lb N per 1,000 sq ft per year (split across fall and spring applications). Warm-season grasses receive 2–4 lb N per year.

How many bags of fertilizer do I need for my lawn?+

Use the calculator: enter your lawn area, the fertilizer N-P-K (e.g. 29-0-4), target lb N per 1,000 sq ft (default 1.0), and bag size. A 50 lb bag of 29-0-4 at 1 lb N per 1,000 sq ft covers about 14,500 sq ft.

When should I fertilize my lawn?+

Cool-season grasses: main feeding in early fall (September) when grass is actively growing, secondary feeding in late spring. Avoid summer applications that force growth during heat stress. Warm-season grasses: fertilize during the growing season (late spring through early summer). Do not fertilize within 8 weeks of first expected frost.

Can I use a spreader setting guide?+

Fertilizer bags list recommended spreader settings for common brands (Scotts, Earthway, Lesco). Use those settings for calibration — they are calibrated to the specific product's granule size. The calculator gives you the total pounds to purchase; the bag label gives you the spreader setting for application.

Guides for this calculator

What Do Fertilizer Numbers Mean? NPK Explained

The three fertilizer numbers are the NPK ratio: % nitrogen, % phosphate, % potash by weight. A 28-0-6 bag has 28% nitrogen and no phosphorus. How to read and use them.

How to Lime a Lawn: Rates, Timing, and Soil pH

Lime your lawn when soil pH is below 6.0 for most grasses. Apply up to 50 lb per 1,000 sq ft per application. Soil test first — overliming causes its own problems.

When to Fertilize Your Lawn: Cool and Warm Season Schedules

Cool-season lawns get most fertilizer in fall (Aug–Nov); warm-season lawns in late spring and summer. Schedules and nitrogen rates by grass type.

Related calculators

Grass Seed Calculator

Pounds of seed by grass species and seeding rate for new or overseeded lawns.

Sod Calculator

Square footage, rolls, and pallets of sod with waste factor.

Gravel Calculator

Cubic yards, tons, and bag count for gravel, crushed stone, and decorative rock.

Mulch Calculator

Bags or bulk cubic yards of mulch for garden beds and tree rings.