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Lawn Seed and Fertilizer Calculator
Computes the grass seed needed for a new lawn or overseeding from the area and the species' seeding rate (Penn State and UF/IFAS extension rates in lb per 1,000 ft²), and the fertilizer needed to apply a target amount of nitrogen from the N percentage on the bag.
When to use
You are seeding or overseeding a lawn of known size and need to know how many pounds or kilograms of seed and of a given N-P-K fertilizer to buy.
Do not use when: You are laying sod or turf rolls, need a soil-test-based lime or phosphorus recommendation, or want spacing for individual plants (use plant-spacing).
Formula
area_ft2 = area (× 10.7639 if m²); seed_lb = rate_lb_per_1000ft2 × (0.5 if overseed) × area_ft2 / 1000; fertilizer_lb = target_n_lb_per_1000ft2 × 100 / fertilizer_n_percent × area_ft2 / 1000; kg = lb × 0.45359237; g/m² = lb/1000 ft² × 4.882
New-lawn rates are the midpoints of the extension ranges (Penn State for cool-season species, UF/IFAS for warm-season species); overseeding uses half the rate as a rule of thumb, although some services seed thin lawns at the full rate. Nitrogen should not exceed about 1 lb per 1,000 ft² per application (Penn State starter guidance 0.5–1 lb), so larger seasonal totals are split into several applications.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
area | number | yes | Area to seed, in the chosen unit. Range: > 0, ≤ 10000000 | |
area_unit | enum: ft2 | m2 | default ft2 | Unit of area. | |
grass_type | enum: kentucky_bluegrass | tall_fescue | perennial_ryegrass | fine_fescue | mix_cool_season | bermuda | bahia | centipede | yes | Grass species or mixture; sets the seeding rate for a new lawn. | |
seeding | enum: new_lawn | overseed | default new_lawn | New lawn on bare soil, or overseeding into existing grass at half the rate. | |
fertilizer_n_percent | number | % | default 24 | First number of the N-P-K analysis on the bag (24 for a 24-0-4 fertilizer). Range: > 0, ≤ 100 |
target_n_lb_per_1000ft2 | number | lb N/1000 ft² | default 1 | Nitrogen to apply per 1,000 ft² in this application; starter applications use 0.5–1 lb. Range: > 0, ≤ 3 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
area_ft2 | number | ft² | Lawn area in square feet. |
area_m2 | number | m² | Lawn area in square metres. |
seed_rate_lb_per_1000ft2 | number | lb/1000 ft² | Rate applied (half the new-lawn rate for overseeding). |
seed_rate_g_per_m2 | number | g/m² | Same rate in grams per square metre. |
seed_lb | number | lb | seed_rate × area_ft2 / 1000. |
seed_kg | number | kg | Seed in kilograms. |
fertilizer_rate_lb_per_1000ft2 | number | lb/1000 ft² | target_n × 100 / fertilizer_n_percent. |
fertilizer_rate_g_per_m2 | number | g/m² | Same rate in grams per square metre. |
fertilizer_lb | number | lb | fertilizer_rate × area_ft2 / 1000. |
fertilizer_kg | number | kg | Fertilizer in kilograms. |
nitrogen_lb | number | lb | target_n × area_ft2 / 1000. |
bags_note | string | Quantities to buy and the application limit. |
Example
5,000 ft² new tall fescue lawn, 24-0-4 fertilizer: {"area":5000,"area_unit":"ft2","grass_type":"tall_fescue","seeding":"new_lawn","fertilizer_n_percent":24,"target_n_lb_per_1000ft2":1} → {"area_ft2":5000,"seed_rate_lb_per_1000ft2":7,"seed_lb":35,"seed_kg":15.88,"fertilizer_rate_lb_per_1000ft2":4.17,"fertilizer_lb":20.8,"fertilizer_kg":9.45,"nitrogen_lb":5}
400 m² bluegrass overseeding, 20-5-10 at 0.75 lb N: {"area":400,"area_unit":"m2","grass_type":"kentucky_bluegrass","seeding":"overseed","fertilizer_n_percent":20,"target_n_lb_per_1000ft2":0.75} → {"area_ft2":4306,"seed_rate_lb_per_1000ft2":1.25,"seed_lb":5.4,"seed_kg":2.44,"fertilizer_rate_lb_per_1000ft2":3.75,"fertilizer_lb":16.1,"fertilizer_kg":7.32}
GET https://tttkmbb.com/api/v1/calculate/lawn-seed-fertilizer?area=5000&area_unit=ft2&grass_type=tall_fescue&seeding=new_lawn&fertilizer_n_percent=24&target_n_lb_per_1000ft2=1
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/lawn-seed-fertilizer(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/lawn-seed-fertilizer · Markdown: https://tttkmbb.com/everyday/lawn-seed-fertilizer.md · JSON definition: https://tttkmbb.com/everyday/lawn-seed-fertilizer.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="lawn-seed-fertilizer" - OpenAPI operationId:
estimate_lawn_seed_and_fertilizer - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
FAQ
Why is the seeding rate so different between species?
Seed size: Kentucky bluegrass has about 2 million seeds per pound, tall fescue about 230,000, so fescue needs far more weight to reach the same 15–20 seeds per square inch.
How do I convert a 20-5-10 bag?
The first number is the percentage of nitrogen by weight: 20 % means 5 lb of product per pound of nitrogen, so 1 lb N per 1,000 ft² needs 5 lb of that fertilizer per 1,000 ft².
Should new seed get fertilizer at all?
Cool-season lawns benefit from a starter fertilizer at 0.5–1 lb N per 1,000 ft² worked into the seedbed; Florida guidance for warm-season grass is to wait until the lawn is fully covered before adding nitrogen.
Related calculators
- Area Converter — Convert the lawn area between square feet and square metres.
- Plant Spacing Calculator — Plant counts for beds and borders.
- Gravel Calculator — Aggregate for paths around the lawn.