Grass seed calculator

The rate is the whole question

Everything else is multiplication. The rate depends on the species, on how large the seed is, and above all on whether you are covering bare ground or thickening a lawn that already exists, where roughly half as much is usual.

Plenty of pages will quote you a number. The bag in your hand knows your seed and they do not, which is why this page asks for it and then does the arithmetic properly, including what one bag covers so you can see whether the bigger bag is worth it.

More is not better

Over-sowing feels like insurance and behaves like the opposite. Seedlings that germinate on top of each other compete for the same light and water, and the stand that comes through is thin and disease prone. The rate on the bag already allows for the proportion that will not make it.

Keep the surface damp until it is up, which usually means a light watering more than once a day for the first fortnight. That is the one time the deep and infrequent rule does not apply, because the seed is sitting in the top centimetre with nowhere else to draw from.

Questions people ask

How much grass seed do I need?

Area times the sowing rate on the bag. At 35 grams a square metre, 200 square metres takes 7 kilograms. The rate is the part that varies, so it comes off the bag in front of you rather than from a page like this one.

Why does this ask for the rate instead of knowing it?

Sowing rates differ by species and by seed size, and bare ground takes roughly double what thickening an existing lawn does. A single number here would be wrong for most bags, and the bag would be the one that is right.

What happens if I sow too much seed?

Seedlings compete with each other for light, water and root room, and a crowded stand is weaker and more prone to disease than a correctly sown one. More seed does not make a thicker lawn, it makes a thinner one later.

How do I spread it evenly?

Halve the seed and make two passes at right angles, rather than one pass trying to be perfect. Uneven sowing is invisible on the day and obvious a fortnight later.

When should I sow?

When soil temperature suits the species, which for cool season grasses means spring and early autumn, and for warm season grasses means late spring into summer. Sowing into soil that is too cold wastes the bag.