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Concrete Calculator
Pouring a slab, a footing or a round column? Enter the dimensions and this calculator works out the volume in cubic feet, cubic yards and cubic metres, adds a waste allowance for spillage and uneven ground, and tells you how many bags of premixed concrete to buy — or how many cubic yards to order from a truck.
Bag yields are the manufacturers' nominal figures (0.30 / 0.45 / 0.60 cu ft); real yield varies a little with mixing water. Order a little extra for truck deliveries — a short load can cost more than the extra concrete. This is an estimate for planning, not a structural specification: footings and load-bearing work need local code and an engineer.
How it works
Slab: volume = length × width × thickness, with thickness converted from inches to feet (divide by 12). Round column: volume = π × radius² × depth, where radius is half the diameter, all in feet, multiplied by the number of columns. Cubic yards = cubic feet ÷ 27 (there are 27 cu ft in a cu yd); cubic metres = cubic feet × 0.0283168. The waste allowance is applied on top: adjusted volume = exact volume × (1 + waste%). Bags = adjusted cubic feet ÷ the bag yield (0.30 cu ft for 40 lb, 0.45 for 60 lb, 0.60 for 80 lb), rounded up — you cannot buy part of a bag. Worked example: a 10 ft × 12 ft slab, 4 in thick: 10 × 12 × (4 ÷ 12) = 40.00 cu ft = 1.48 cu yd. With the recommended 10% waste: 40 × 1.10 = 44.00 cu ft = 1.63 cu yd. In 80 lb bags: 44.00 ÷ 0.60 = 73.3, rounded up to 74 bags.
Frequently asked questions
How much waste should I allow?
10% is the usual rule for a slab on reasonably level, well-compacted ground — it covers spillage, an uneven sub-base and formwork bulge. Use 15% on rough or sloping ground, and 5% only for a precisely formed, flat pour such as a sonotube.
When should I order a truck instead of bags?
Roughly above 1 cubic yard (about 45 eighty-pound bags) a ready-mix truck is usually cheaper and far less labour — and above about 74 bags the mixing alone becomes a full day. Many suppliers charge a short-load fee below 3–4 cubic yards, so check that before ordering.
How thick should my slab be?
A footpath or shed base is commonly 4 inches; a driveway carrying cars is usually 4–6 inches with reinforcement, and heavier vehicles need more. Thickness is a structural decision — follow local code, not this calculator, for anything load-bearing.
Why are my bags not quite enough?
Bag yields are nominal: adding extra water, spilling mix, or an unlevel base all eat into the yield. The waste allowance exists for exactly this reason — and it is always cheaper to have one bag left over than to stop a pour halfway.
Can I use this for a footing or a wall?
Yes for a rectangular footing (use the slab shape with the footing's length, width and depth). For a wall, enter its length and height as length and width, and its thickness as the thickness — the volume math is identical.