Most seed bags or boxes carry information on how much ground they will cover, usually expressed as square metres.

I work in old measurements so 8.5 x 3.5 gives you an area of roughly 30 square metres, I usually allow 1.5 ounces to a square yard, so if you,re going to buy your seed loose, allowing for the slight difference between yards and metres, you will need 4.5 pounds, or just over 2 kilos.

Sorry, forget that last bit, you will only need about 3pounds of seed, or just over 1 1/4 kilos