The formula
grams = area × rate per square metre
Where it comes from
The sum is a multiplication, and all the argument is in the rate. It varies a lot by species and by purpose: a new sowing of ryegrass wants 30 to 40 grams per square metre, tall fescue somewhat more, and overseeding an existing lawn half that. The figure is on the bag, which is why it is a field here: another variety’s rate gives either a thin lawn or wasted seed.
How to work it out by hand
- Measure the area to be sown
- Find the rate on the bag, in grams per square metre
- Multiply: those are the grams
- Divide by a thousand for the kilos
What is worth knowing
Sowing more than the rate does not give a denser lawn: it gives seedlings competing with each other that end up weak and more prone to disease. Sowing less leaves bare patches for the first weed that comes along. The rate on the bag is worked out for that variety and is worth respecting. Two other things matter more for getting a sowing to take: seed-to-soil contact — which is why you roll afterwards — and keeping the soil damp without waterlogging it through the two or three weeks of germination. A single dry day at that stage takes out a good share of what was sown.