Ordering concrete by guesswork often means paying for too much or running short halfway through a pour. This concrete slab calculator gives you a clear volume for a rectangular slab, pad, patio, floor or footing, so you know how much to order before the truck arrives.
Enter the length and width in feet and the thickness in inches. If you are pouring more than one identical slab, enter the count. You can also add a percentage for spills and uneven ground. The result shows the volume in both cubic yards and cubic feet.
How to use the concrete slab calculator
- Measure the length of the slab in feet. If you have a measurement in feet and inches, convert the inches to a decimal of a foot first.
- Measure the width of the slab in feet, using the same method.
- Enter the thickness in inches, as planned for the finished slab.
- If you are pouring several identical slabs, enter how many. Leave this at 1 for a single slab.
- Optionally, enter an extra percentage to cover spills and uneven ground.
- Read the volume in cubic yards for ordering, and check the cubic feet figure against your own notes.
The formula
- Cubic feet = length × width × thickness × how many, with lengths in feet (inches ÷ 12)
- Cubic yards = cubic feet ÷ 27 (a cubic yard is 3 ft × 3 ft × 3 ft)
- With the extra: multiply by (1 + extra % ÷ 100)
Worked example
The figures below are made up to show the maths. They are not current prices, rates or product figures.
| Length | 20 ft |
|---|---|
| Width | 10 ft |
| Thickness | 4 in |
| How many slabs | 1 |
| Extra for spills and uneven ground | 5% |
- Volume 2.59 cubic yards
- Volume 70.00 cubic feet
Typical slab thicknesses
- Garage floor or driveway: 4 to 6 inches
- Patio, walkway or shed pad: 4 inches
- Heavy-use or load-bearing slab: follow your engineer or local code
Slab example: a 20 ft by 20 ft garage floor
A 20 ft by 20 ft slab that is 4 inches thick has an area of 400 sq ft. Volume = 400 × (4 ÷ 12) = 133.3 cubic feet, which is 133.3 ÷ 27 = 4.94 cubic yards. Adding 10 percent for spills and uneven ground gives about 5.43 cubic yards to order from a ready-mix truck.
If you mix from bags, an 80 lb bag yields roughly 0.6 cubic feet (check the label for your product). 146.7 cubic feet with the 10 percent extra is about 245 bags. For a slab this size, ready-mix is usually the practical choice.
These figures are examples only. Need footings, walls or round columns as well? Use the concrete calculator.
Tips for an accurate answer
- Measure the slab at its widest and longest points, and check that the sides are square before you enter the numbers.
- Thickness is easy to mistype. Double-check that you have entered inches, not feet, in the thickness field.
- Ground that is not level or compacted can hold more concrete than planned, so a larger extra percentage is a sensible safety margin.
- Check the example on the page against your own numbers to confirm you are reading the results correctly.
- Keep a note of the inputs you used so you can recalculate quickly if the plan changes.
Questions people ask
Why does the calculator show two volume figures?
Both describe the same volume.
What does the extra percentage do?
It increases the calculated volume to allow for spills, waste and ground that is not perfectly level.
Can I use this for a footing or a floor?
Yes, as long as the shape is a rectangle. Enter the footing or floor length, width and thickness, and the calculator will give the volume for that shape.
Does this calculator work for irregular shapes?
No. It is built for rectangular slabs only. For an L-shape or a curved edge, split the area into rectangles, calculate each one separately, and add the results together.
Should I trust the result without checking it?
Treat the result as a planning figure. Confirm your measurements on site, and talk to your concrete supplier before ordering, since they can advise on how to round up the volume for delivery.
Sources
- 1 foot = 0.3048 metre exactly, so 1 inch (a twelfth of a foot) = 2.54 cm. Source: NIST, Revised unit conversion factors (the international foot).
- 1 yard = 0.9144 metre (3 feet). Source: NIST, NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically.
An estimate from the numbers you enter, worked out with the formula shown. Check quantities with your supplier before you order. Formulas and examples reviewed 24 Oct 2026. Read the disclaimer