Lawn fertilizer calculator
How much fertilizer does your lawn really need?
Type your lawn size and the three numbers on the bag. Lawn Numbers works out the pounds to spread, the bags to buy and the setting to aim for, then shows when your grass wants feeding.
- Formulas from university extension guides
- What you type never leaves your browser
Feeding calendar
When to feed, by grass type
Cool-season grass
1–4 lb of nitrogen per 1,000 sq ft a year
At 0.75 lb a feeding, that is about 1–5 feedings a year.
Main feedings: September and November. Light or regional feedings: February, April and June.
Fall is the main season: feed in September and again around November while the grass is still green. Northern schedules add light feedings in late April and early June; the transition zone feeds in February instead. Skip feeding in summer heat. NC State's calendar adds up to 3 lb a year for tall fescue.
Typical ranges from university extension guides. Your state or county guide wins. See the sources.
Read your bag
Three numbers tell you almost everything
- N
Nitrogen
The green-up nutrient and the one lawns use most. Every result on this page is worked out from it.
- P
Phosphate
Helps new roots, so starter fertilizers are high in it. Several states limit it on established lawns unless a soil test calls for it.
- K
Potash
Helps grass handle drought, heat and wear. A soil test tells you whether your lawn is short of it.
Shortcut from NC State Extension: 100 ÷ the first number = pounds of product for 1 lb of nitrogen per 1,000 sq ft. For this bag that is 100 ÷ 29 = 3.4 lb.
Show your working
The math, out loud
These lines use the numbers in the calculator above, so you can check every step by hand.
- Pounds of product per 1,000 sq ft0.75 lb N ÷ nitrogen share = 2.59 lb
- Scale to your lawn2.59 lb × 10.0 thousand sq ft = 25.9 lb
- Round up to whole bags25.9 lb ÷ 15 lb bags = 1.72 → 2 bags
Before you spread
Four habits that keep the lawn green and the water clean
The label comes first
If the product label gives a different rate or warning, follow the label. This calculator is a planning aid.
Half rate, two directions
Set the spreader for half the target and cover the lawn twice at right angles. It hides gaps and stripes.
Sweep it back
Granules on the drive or sidewalk wash into storm drains. Sweep them back onto the grass before it rains.
Check your state
Some states limit phosphorus or set dates when lawn fertilizer cannot go down. Your extension office knows the local rules.
Questions
Lawn fertilizer, answered
Still unsure? The method page lists every formula and source behind these numbers.
How much fertilizer do I need for 5,000 sq ft?
Divide the nitrogen rate by the first bag number as a decimal, then multiply by 5. For 1 lb of nitrogen per 1,000 sq ft from a 20-0-5 bag: 1 ÷ 0.20 = 5 lb per 1,000 sq ft, so 25 lb for 5,000 sq ft.
What do the three numbers on the bag mean?
They are percentages by weight: nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 29-0-4 holds 14.5 lb of nitrogen, no phosphate and 2 lb of potash. Lawns respond most to nitrogen, so this calculator works from the first number.
How often should I fertilize my lawn?
It depends on the grass. Cool-season lawns such as tall fescue and Kentucky bluegrass are fed mostly in fall. Warm-season lawns such as bermudagrass are fed from green-up through summer. Pick your grass in the feeding calendar above to see the months.
Can I put down more to get a greener lawn faster?
No. More than about 1 lb of quick-release nitrogen per 1,000 sq ft in one feeding can burn grass and wash into storm drains. Split a bigger dose into two feedings, or use a slow-release product labeled for the higher rate.
My bag only says how many square feet it covers. Can I use that?
Yes. Divide your lawn area by the coverage on the bag and round up to get the number of bags. Use this calculator when you want a different rate than the bag assumes, or to check what rate the bag delivers.
Is it too late to fertilize this year?
For cool-season grass, a late-fall feeding while the grass is still green is common. For warm-season grass, stop by early fall so new growth is not hit by frost. The feeding calendar above shows the usual months for each grass.