Concrete Calculator: Cubic Yards and Bags
Pick the shape, enter its size in feet and inches or metres, add a waste allowance, and see the volume and the number of pre-mix bags. After switching units, re-enter your measurements.
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Formula
- Box shapes (slab, footing, wall):
volume (ft3) = length (ft) x width (ft) x thickness (in) / 12 - Round column:
volume (ft3) = pi / 4 x diameter (ft)^2 x height (ft) order = volume x pieces x (1 + waste %), thencubic yards = ft3 / 27andm3 = ft3 x 0.028316846592bags = round up (order in ft3 / bag yield), with yields of 0.60 (80 lb), 0.45 (60 lb) and 0.30 (40 lb) cubic feet
Worked example
A 10 ft x 10 ft slab, 4 in thick, with 10% waste:
- 10 x 10 x 4 / 12 = 33.33 ft3
- x 1.10 = 36.67 ft3 = 1.36 yd3 (1.038 m3)
- 36.67 / 0.60 = 61.1, so 62 bags of 80 lb (82 of 60 lb, 123 of 40 lb)
Assumptions
- Bag yields are typical label values; check the bag you buy.
- Metric inputs are converted with exact factors (1 ft = 0.3048 m, 1 in = 2.54 cm).
- A wall uses its height in the width field. A column ignores thickness.
- Bag counts are always rounded up to whole bags.
- Reinforcement, sub-base and formwork are not included.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length by width by thickness, all in feet, and divide by 27 for cubic yards. A 10 ft by 10 ft slab 4 in thick is 10 x 10 x (4 / 12) = 33.3 cubic feet, which is 1.23 cubic yards before waste.
How many bags of concrete do I need?
Divide the volume in cubic feet by the yield printed on the bag. An 80 lb bag yields about 0.60 cubic feet, so 36.7 cubic feet (the 10 x 10 ft slab with 10 percent extra) needs 36.7 / 0.60 = 61.1, so 62 bags. A 60 lb bag yields about 0.45 and a 40 lb bag about 0.30 cubic feet.
How much waste should I allow?
Ten percent is a common allowance for uneven sub-grade, spillage and over-excavation. Use 5 percent for a neat form on firm ground and 15 percent or more for rough ground. Ready-mix is usually ordered to the nearest quarter or half yard.
Should I buy bags or order ready-mix concrete?
Bags are practical up to about 1 cubic yard, which is roughly 45 bags of 80 lb. Above that, mixing by hand gets slow and the finish suffers if the pour pauses, so a ready-mix truck is usually better.
How do I work out a round column or pier?
Volume is pi / 4 x diameter squared x height. A column 2 ft across and 8 ft tall holds 3.1416 / 4 x 4 x 8 = 25.1 cubic feet, or 0.93 cubic yards before waste. Enter the diameter as the width and the height as the length.