How the concrete calculation works
- Volume = length × width × thickness, with all three in the same unit (4 inches = 0.333 ft; 10 cm = 0.1 m).
- Cubic yards = cubic feet ÷ 27. Cubic metres = cubic feet × 0.0283.
- Add waste: volume × (1 + waste ÷ 100).
- Bags = volume including waste ÷ the yield of one bag, rounded up.
For example, a 10 ft × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, or 1.23 yd³. With 10% waste that is 1.36 yd³, so you would order 1.5 yd³ of ready-mix or 62 bags of 80 lb premix.
Why add a waste allowance?
Running short in the middle of a pour is far worse than having a little left over. Ground that isn't perfectly level, forms that flex outwards, spillage and concrete left in the mixer or wheelbarrow all use extra. 5–10% is a common allowance for simple slabs. Increase it if the sub-base is uneven.
Ready-mix or bags?
Premixed bags are practical for small jobs like post holes, steps and small pads. The yields used here are those printed on common US bags: about 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. Check your bag, and choose "Other" to enter a different yield. For larger slabs, ordering ready-mixed concrete delivered by truck is usually more practical than mixing dozens of bags. Suppliers often have minimum order sizes.
Rounding rules
- Ready-mix is rounded up to the next 0.25 yd³ (US) or 0.25 m³ (metric). Your supplier may use different increments.
- Bags are rounded up to a whole bag.
- The exact volume, before waste and rounding, is shown separately in cubic yards, cubic feet, cubic metres and litres.