Working out the volume is the easy part. This goes on to what you actually order in the UK: ready-mix rounded up to the next quarter m³, with a warning when you are under the usual 3 to 4m³ minimum load; 25kg or 20kg bags; or cement, ballast and water to mix on site, with mixer loads. It names the BS 8500 mix to ask for, prices the job at 2026 rates and shows the working. Slabs, footings, post holes and steps, in metric or feet and inches. Free, no signup.