Lawn Fertilizer Calculator

The three numbers on a fertilizer bag tell you what percentage of the product is nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Enter those numbers, your lawn size and how much nitrogen you want to apply, and the calculator tells you how many pounds of product to spread, how many bags to buy and what it costs — and checks the rate against typical guidance for your grass.

What the numbers on the bag mean

Every fertilizer bag shows three numbers, such as 24-0-6 or 10-10-10. They are the guaranteed analysis: the percentage by weight of total nitrogen (N), available phosphate (P₂O₅) and soluble potash (K₂O). A 15 lb bag of 24-0-6 therefore contains 15 × 0.24 = 3.6 lb of nitrogen, no phosphate and 15 × 0.06 = 0.9 lb of potash. The rest is other nutrients, carrier material and filler that help spread the product evenly.

The formula

1. Pounds of product per 1,000 sq ft = target pounds of nutrient ÷ (nutrient % ÷ 100).
2. Total product = pounds per 1,000 sq ft × lawn area ÷ 1,000.
3. Bags = total product ÷ bag weight, rounded up.

For example, to apply 0.75 lb of nitrogen per 1,000 sq ft with a 24-0-6 fertilizer: 0.75 ÷ 0.24 = 3.13 lb of product per 1,000 sq ft.

Typical yearly nitrogen by grass

These ranges reflect common US university extension guidance for home lawns. Lower ends suit low-maintenance lawns, clippings left on the lawn and shade; higher ends suit high-traffic, irrigated lawns. A soil test and your local extension service’s recommendations are the best guide.

Worked example

A 5,000 sq ft tall fescue lawn gets a spring feed of 0.75 lb N per 1,000 sq ft using a 24-0-6 fertilizer in 15 lb bags. The calculator gives 3.13 lb of product per 1,000 sq ft and 15.6 lb in total, so two 15 lb bags. That feed also adds about 0.19 lb of potash per 1,000 sq ft. Over the year, tall fescue typically needs 2–3 lb N, so this lawn might get 0.75 lb in spring and around 1 lb in each of two autumn feeds.

When to fertilize

Spreading tips

• Calibrate your spreader or use the setting on the bag for your spreader model.
• Apply half the product in one direction and half at right angles for even coverage.
• Sweep granules off paths and driveways back onto the lawn so they don’t wash into drains.
• Water in lightly after feeding unless the label says otherwise.
• Don’t fertilize before heavy rain, on frozen ground or on dormant lawns.
• Keep children and pets off until the lawn is watered in and dry, as the label directs.

Metric users

In metric units, rates are usually given in grams of nutrient per square metre. One pound per 1,000 sq ft is about 4.9 g/m², so 0.75 lb N per 1,000 sq ft is about 3.7 g of nitrogen per square metre.

Quick-release vs slow-release nitrogen

Fertilizer labels often show how much of the nitrogen is slow-release (such as coated urea or natural organic sources). Quick-release nitrogen greens the lawn within days but can burn grass if over-applied and is more easily washed away. Slow-release nitrogen feeds over several weeks, so you can apply a little more per feed and feed less often. The calculator works with the total nitrogen on the label either way.

Phosphorus and the environment

Many established lawns already have enough phosphorus, and excess phosphorus washing into lakes and rivers feeds algae blooms. Several US states and other regions restrict phosphorus in lawn fertilizers unless a soil test shows a need or a new lawn is being established. That is why many lawn products have a middle number of zero. The calculator lets you target phosphate or potash directly if a soil test calls for it.

The printable lawn care templates

The template pack includes an Excel fertilizer planner that calculates product for each application across the year, a yearly feeding calendar for cool- and warm-season grasses, a spreader calibration worksheet, a soil test record and a lawn care log — PDF, Word and Excel.