How much concrete you need for slabs, footings, columns and stairs — in cubic metres/yards and bags.
Leave empty to skip the cost estimate. If you work in feet, the price is still entered per cubic metre and converted for display.
Concrete needed (with waste)
How to use this tool
Choose slab, footing, column or steps, and pick metric or imperial units.
Enter the dimensions. For a slab, thickness is usually given in cm (metric) or inches (imperial).
Keep the 10% waste allowance unless your base is very even or very rough.
Read the with-waste volume, the bag counts for 40, 60 and 80 lb bags, and the cost estimate if you entered a price.
Formula & worked example
A slab 5 m long × 4 m wide × 0.15 m thick needs 5 × 4 × 0.15 = 3 m³ net. With the default 10% waste allowance that is 3.3 m³ to order, or 3.3 ÷ 0.7646 ≈ 4.32 cubic yards. At 0.017 m³ per 80 lb bag, that is 3.3 ÷ 0.017 ≈ 194.1, rounded up to 195 bags.
Limitations — read this before you rely on it
Bag yields are standard manufacturer figures and vary slightly by mix and brand — always check your bag. Steps are estimated as solid blocks, which is generous by design. This estimates volume only: it does not design the mix, reinforcement, curing or structural strength, which need a professional for load-bearing work.
Frequently asked questions
Pick the shape, enter the dimensions, and set a waste allowance (10% is the usual default). The calculator gives the net volume, the with-waste volume in m³, yd³ and ft³, and the number of 40, 60 and 80 lb bags.
Bag yields used here: 40 lb ≈ 0.0085 m³ (0.30 ft³), 60 lb ≈ 0.0127 m³ (0.45 ft³), 80 lb ≈ 0.017 m³ (0.60 ft³). Bag counts are always rounded up — you cannot buy part of a bag.
Multiply cubic metres by 1.30795 to get cubic yards, or by 35.315 to get cubic feet. So 3 m³ ≈ 3.92 yd³ ≈ 105.9 ft³.
10% covers normal spillage, uneven ground and formwork gaps. Use 15% for rough or uneven bases, and 5% only for a perfectly flat, well-formed pour.
Yes — choose the column shape, enter the diameter (round) or side (square) and the height. For stairs, enter the number of steps, tread, riser and width; steps are treated as solid blocks, which is a safe, slightly generous estimate.