The math
Concrete is sold by the cubic yard. Volume is just length × width × thickness - but with thickness usually in inches and length/width in feet, getting the units right is the only place this calculation goes wrong.
slab_yd³ = (length_ft × width_ft × thickness_in ÷ 12) ÷ 27
column_yd³ = (π × (diameter_in / 24)² × height_ft) ÷ 27
The ÷ 27 converts cubic feet to cubic yards. A cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³.
Frequently asked
How much overage should I add?
Standard is 5–10%. Add more for: rough or unlevel ground, formwork that's not perfectly square, anywhere the pour might over-fill (uneven base), and crews that can't pour quickly enough to manage a tight batch. Pros routinely order 10% over.
Bag count math
A 60 lb bag of concrete makes about 0.45 ft³ of finished concrete. An 80 lb bag makes about 0.60 ft³. To convert: bags = total_ft³ ÷ ft³_per_bag. For anything over a few yards, the bag math will look painful - and it's accurate.
What about rebar and prep?
Volume calculations ignore reinforcement steel - rebar takes up negligible space. They also don't account for base preparation (gravel, sand, vapor barrier). Plan those separately.