Concrete calculator

Multiply length × width × thickness in the same units to get the volume, then add 5–10% for waste and uneven subgrade. This calculator does the conversion to cubic yards or cubic meters and the waste allowance for you.

Roughly 45 bags of 80 lb premix, 60 bags of 60 lb, or 90 bags of 40 lb make one cubic yard. For anything over about one cubic yard, ready-mix delivery is usually cheaper and far less work than mixing bags.

One part cement, two parts sand, three parts gravel by volume, with about half a part of water. It produces roughly 3000 PSI (C20) concrete, suitable for slabs, paths and most residential work.

On real jobsites 5–10% extra covers spillage, uneven ground and form deflection. For pours over rough subgrade, use 10%. Running short mid-pour creates a cold joint, which is a structural weak point.