How to measure your lawn area
Measure the area in square feet the same way you would for sod: length times width for a rectangle, and for an irregular lawn, break it into smaller rectangles and circles you can measure separately and add together. This calculator's preview panel scales to match what you enter, using the same math you would do by hand: length times width for a rectangle, pi times radius squared for a circle.
Why the rate differs so much by species
Grass seed size varies enormously between species, and that single fact explains most of the difference in recommended rates. A pound of tall fescue seed contains far fewer individual seeds than a pound of Kentucky bluegrass seed, because each fescue seed is much larger. To land the same number of seeds per square foot, a big-seeded species like tall fescue needs more pounds per 1000 square feet (6 to 9) than a tiny-seeded species like Kentucky bluegrass (1.5 to 3) or centipedegrass (0.25 to 1). The pound is a unit of weight, not a unit of seed count, so the rate has to compensate for how much any individual seed weighs.
New lawn vs. overseeding, and why overseed is about half
A brand-new lawn is bare soil: every seed that germinates is starting from nothing, so the rate is set high enough to establish a full stand on its own. Overseeding drops seed into an existing, living lawn that is already covering most of the ground, so roughly half the new-lawn rate is enough to thicken it up without wasting seed on space that is already occupied. That is why this calculator's overseed mode roughly halves the rate for cool-season species. Warm-season lawns are the exception: they are not typically overseeded with the same warm-season species. Instead, for winter color while the warm-season grass is dormant, they are commonly overseeded with a cool-season species like perennial or annual ryegrass, which is a different project with its own rate, not a fraction of the warm-season rate.
Why more seed is not better
It is tempting to assume a heavier rate means faster, thicker coverage, but university extension guidance (Purdue, UMass, University of Tennessee) consistently warns against exceeding the top of the published range. Seedlings compete with each other for light, water, and nutrients from the moment they germinate. Overcrowd them and every individual seedling comes up weaker, thinner, and more prone to disease than it would at a proper rate, which can end up looking worse than a correctly seeded lawn at a lower rate. If you are unsure, seed at the higher end of the range for a late-season planting or a rough seedbed, but do not go past it on the assumption that more is simply safer.
The mixture-by-weight trap
If you are blending seed types yourself, weight percentages can be deceiving. Perennial ryegrass seed is much larger than Kentucky bluegrass seed, so a mix that is 70 percent ryegrass by weight can easily end up as roughly a 50/50 stand by plant count, since a given weight of ryegrass contains far fewer individual seeds than the same weight of bluegrass. If you want bluegrass to actually dominate a mix, keep ryegrass to a small minority by weight, generally in the 5 to 15 percent range, rather than assuming the label percentages translate directly into the finished lawn's makeup.
Which grasses cannot be seeded at all
Not every lawn grass is available as seed. St. Augustinegrass does not produce enough commercially viable seed and is established from sod or plugs instead; this calculator will tell you that directly rather than inventing a pounds figure for a product that does not exist at retail. Hybrid bermudagrass varieties (bred for finer texture and density) and premium zoysia cultivars are also propagated vegetatively, from sod, sprigs, or plugs, not seed, even though common seeded bermuda and some seeded zoysia types do exist and are included in the rate table above.
New seed needs decent soil to root into, see the topsoil calculator if the area needs fill or leveling first. If sod is the faster route for part of the yard, particularly for a grass type that cannot be seeded, the sod calculator handles that math instead. Once the lawn is in, the beds around it are usually mulch, see the mulch calculator, or a gravel border, see the gravel calculator.