✓Convert slab thickness from inches to feet before calculating cubic feet.
✓There are 27 cubic feet in one cubic yard.
✓An ordering allowance can help cover grade variation, spillage and small measurement differences, but it should not hide poor measurements.
Thickness in feet = thickness in inches ÷ 12Cubic feet = length × width × thickness in feetCubic yards = cubic feet ÷ 27Order volume = cubic yards × (1 + waste allowance) 1. Measure the actual pour dimensions
Measure the length and width of the planned concrete area and the intended thickness. For forms or excavations that vary in depth, a single uniform thickness may only be an approximation.
Footings, piers, steps and irregular slabs use different geometry. Use a calculator designed for the actual shape instead of forcing every pour into a rectangular slab formula.
2. Keep every dimension in compatible units
If length and width are in feet but thickness is in inches, convert thickness to feet before multiplying. This one conversion prevents a very large error in the final volume.
Metric projects work the same way: use metres for all three dimensions to calculate cubic metres.
3. Convert to the supplier's ordering unit
Ready-mix concrete in the US is commonly discussed in cubic yards, so cubic feet are divided by 27. Bagged concrete is different: bag quantity depends on the yield printed on the bag.
Do not convert a ready-mix estimate into bag count using a generic assumption when the bag yield is available.
4. Treat the estimate as an ordering plan, not a structural specification
The calculator estimates volume only. It does not determine slab thickness, reinforcement, subbase, joints, concrete strength or code requirements.
For structural or code-sensitive work, confirm the design requirements separately before ordering material.
Example: 12 ft × 12 ft slab at 4 in thick
- Length
- 12 ft
- Width
- 12 ft
- Thickness
- 4 in
- Waste allowance
- 5%
The slab volume is 48 cubic feet, or about 1.78 cubic yards. With a 5% allowance, the planning quantity is about 1.87 cubic yards.
Run your own numbers in the Concrete Calculator →What to avoid
- Multiplying feet by inches without conversion
- Confusing cubic feet with cubic yards
- Using a slab formula for round columns or footings
- Treating volume calculation as structural design advice
Before you order
- Measure length and width
- Confirm thickness
- Convert units
- Calculate cubic volume
- Convert to ordering unit
- Add project-appropriate allowance