Formula
How this estimate works
cubic feet = length x width x thickness in feet; cubic yards = cubic feet / 27
A 20 ft by 10 ft slab at 4 in thick with 10% waste needs about 2.72 cubic yards of concrete.
How to calculate concrete
For a slab, multiply length by width by thickness. Convert thickness from inches to feet first, then divide cubic feet by 27 to get cubic yards. Add a waste factor when ordering because forms, subgrade variation, spills, and uneven surfaces can change the actual quantity.
Concrete bag estimates
Bag counts are approximate because premix yield varies by brand and mix. This calculator uses common estimated yields: 40 lb = 0.30 cu ft, 50 lb = 0.375 cu ft, 60 lb = 0.45 cu ft, and 80 lb = 0.60 cu ft.
Assumptions used
- Normal-weight concrete is estimated at 150 lb per cubic foot.
- Premix bag yield is estimated from common 40, 50, 60, and 80 lb bag sizes.
- Waste factor is applied after the base volume is calculated.
Before you order materials
- Measure the finished thickness, not only the form height.
- Add extra for uneven subgrade, spills, and small ordering errors.
- Confirm local ready-mix minimums before choosing bags for a large pour.
Frequently asked questions
How much extra concrete should I order?
For small projects, 5% to 10% extra is common. Uneven ground, complex forms, and hand mixing can justify 10% to 15%.
When should I order ready-mix instead of bags?
If the result is dozens of 80 lb bags, ready-mix may be easier and more consistent. Compare local delivery minimums and bag costs.
Is concrete weight included?
Yes. The estimate uses 150 lb per cubic foot, which is a common rule of thumb for normal-weight concrete.