How to use the grass seed calculator
- Choose the unit you measured in and add each lawn area as a rectangle, triangle or circle. Mark patios, beds and paths as Subtract.
- Pick New lawn or Overseeding, then choose the grass type. Select the low end, the middle or the high end of the published range, or enter your own rate from the seed bag.
- Set the bag size under More options to match the bag you plan to buy. Add extra seed if you expect uneven spreading.
- Read the pounds of seed and the number of bags, then copy the shopping list or add a price per bag.
Formula
Net area = added areas - subtracted areas, in square feet
Rate = low + (high - low) x position in the range, or your own rate, in lb per 1,000 sq ft
Overseeding rate = new-lawn rate x overseeding share
Seed (lb) = net area / 1,000 x rate x (1 + extra percent / 100)
Bags = seed (lb) / bag size (lb), rounded up
Seeding rates are always quoted per 1,000 square feet, so the area is divided by 1,000 first. The overseeding share is an input, and half of the new-lawn rate is only a common starting point.
Worked example: a 60 by 40 foot back lawn with a patio
The lawn is 60 x 40 = 2,400 sq ft. A patio of 12 x 15 = 180 sq ft is subtracted, which leaves 2,220 sq ft, or 2.22 thousand square feet.
Turf-type tall fescue is listed at 6 to 8 lb per 1,000 sq ft, so the middle of the range is 7 lb. Seed for a new lawn is 2.22 x 7 = 15.54 lb. Across the whole range the need is 13.32 to 17.76 lb.
With 5 lb bags, 15.54 / 5 = 3.108, which rounds up to 4 bags (20 lb).
Overseeding the same lawn at half the new rate uses 3.5 lb per 1,000 sq ft, so 2.22 x 3.5 = 7.77 lb, or 2 bags.
Where the rates come from
The built-in new-lawn ranges follow Penn State Extension's publication on turfgrass seed and seed mixtures, which lists seeding rates in pounds per 1,000 square feet for 100 percent stands in sunny sites: Kentucky bluegrass 2 to 3, turf-type perennial ryegrass 4 to 5, turf-type tall fescue 6 to 8 and fine fescues 4 to 5. The publication notes that its rates assume the seed meets minimum purity and germination levels.
The publication does not give overseeding rates, so the overseeding share is a number you control. Many lawn guides suggest about half of the new rate, and some local extension offices publish their own figure. Use the one for your region when you can find it.
Why more seed is not better
Putting down far more seed than the rate calls for does not make a thicker lawn. The seedlings compete with each other for water and light, and crowded stands can be thin and weak. Spread the seed evenly at the chosen rate, split it into two passes at right angles, and keep the top inch of soil moist until the grass is established.
Reading the seed label
Every bag lists the species or blend, the percent purity, the germination rate and often a recommended rate. When the label rate differs from this tool, follow the label and use the custom rate option. A blend or a coated seed can cover a different amount per pound than straight seed, so the label is the better guide for that product.
Assumptions and limits
- Rates are for a 100 percent stand of one species in a sunny site. Shade mixes, blends and warm-season grasses use different rates.
- The overseeding share defaults to 50 percent as a rule of thumb that you can edit. It is not taken from the extension publication.
- Bag size is an example of 5 lb that you can edit. Seed is sold in many bag weights.
- Seed purity, germination, seed coatings and mulch are not modeled.
- Planting dates, soil preparation, fertilizer and watering are outside this tool. Ask your local extension office for timing in your area.
Frequently asked questions
How much grass seed do I need per square foot?
Rates are quoted per 1,000 square feet, not per square foot. For example, a range of 6 to 8 pounds per 1,000 square feet is about 0.006 to 0.008 pounds per square foot.
How much grass seed do I need for 1,000 square feet?
It depends on the grass. Penn State Extension lists 2 to 3 lb for Kentucky bluegrass, 4 to 5 lb for turf-type perennial ryegrass and 6 to 8 lb for turf-type tall fescue on a new lawn.
How much seed do I need to overseed an existing lawn?
Overseeding generally uses less seed than a new lawn because some grass is already growing. The tool uses a share of the new-lawn rate that you can edit, starting at half.
Can I mix seed types in one calculation?
Run the tool once for each species and add the pounds. For a bagged blend, use the custom rate and enter the rate printed on that bag.
Why do I need to subtract the patio and flower beds?
Seed is only needed where grass will grow. Subtracting paved areas and planting beds keeps you from buying seed you cannot use.