Concrete Calculator

It works out the volume of concrete you need for a slab, footing or round column in cubic yards (m³), adds a waste allowance, and then estimates what to buy — ready-mix to order, the number of premix bags, or cement, sand and stone to mix on site.

For a slab or footing it is length × width × thickness, with the thickness converted from inches to feet. For example a 10 × 12 ft slab at 4 in is 10 × 12 × 0.33 ≈ 40 ft³ = 1.5 yd³ (1.1 m³). For a round column it is π × radius² × height. The calculator does the conversions and totals multiple identical pours for you.

Allow 5–10% for waste — spillage, over-excavation and an uneven sub-grade all eat into the volume, and running short mid-pour creates a weak cold joint. It is much cheaper to have a little left over than to stop and mix more, so the calculator adds your waste percentage on top.

It depends on the bag size. As a rough guide an 80 lb (40 kg) premix bag yields about 0.6 ft³, so roughly 45 bags make a cubic yard (27 ft³); a 40 lb (20 kg) bag is about half that. Pick your bag size and the calculator shows the number of bags for your volume, including waste.

A 1:2:4 (cement:sand:stone) mix suits most general work like slabs and paths; 1:1.5:3 is a stronger structural mix for columns and beams; 1:3:6 is a leaner mix for footings and mass fill. Choose a ratio and the calculator estimates the cement bags, sand and stone, using a 1.54 bulking factor for the dry materials.

For anything above roughly a cubic yard (m³), ready-mix delivery is usually cheaper, faster and pours in one go — avoiding cold joints. Bags and on-site mixing suit small jobs and hard-to-reach spots. The calculator gives you all three figures so you can compare.