YardCompute / Project Guides / Concrete Slab Calculation
How to Calculate Concrete for a Slab
Estimating concrete for a rectangular slab starts with three measurements: length, width, and thickness. Once those measurements are in the same unit, multiply them to get the slab volume. For ready-mix concrete, the result is commonly converted to cubic yards.
1. Measure the slab
Measure the slab length and width across the planned area. Measure the actual concrete thickness rather than assuming the thickness from a drawing or general rule. If the slab has thickened edges, steps, footings, or other irregular sections, calculate those areas separately.
2. Convert measurements to compatible units
The length, width, and thickness must use compatible units before multiplication. For example, if length and width are in feet and thickness is in inches, convert the thickness to feet by dividing inches by 12.
For a 10 ft by 10 ft slab that is 4 inches thick:
4 inches ÷ 12 = 0.3333 ft
3. Calculate cubic feet
Use this basic formula:
Concrete volume = length × width × thickness
For the 10 ft × 10 ft × 4 in example:
10 × 10 × 0.3333 = about 33.33 cubic feet
4. Convert cubic feet to cubic yards
Ready-mix concrete is commonly ordered by cubic yard. One cubic yard contains 27 cubic feet, so divide cubic feet by 27.
33.33 ÷ 27 = about 1.23 cubic yards
If you include a 10% planning allowance:
1.23 × 1.10 = about 1.36 cubic yards
The allowance is not a guarantee of the amount a supplier will recommend. Actual requirements can change because of grade variation, forming, spillage, waste, and other site conditions.
5. Calculate irregular areas separately
If a project contains more than one rectangular section, calculate each section and add the volumes. For example, a patio may have a main slab plus a small walkway. Keeping the sections separate makes measurement errors easier to spot.
Common mistakes
- Mixing inches and feet without converting.
- Forgetting that 1 cubic yard equals 27 cubic feet.
- Using an assumed thickness instead of the planned thickness.
- Ignoring thickened edges, steps, footings, or other separate sections.
- Ordering exactly the calculated volume without considering site variation or supplier guidance.
Quick example
A 12 ft × 20 ft slab at 4 inches thick is:
12 × 20 × (4 ÷ 12) = 80 cubic feet
That is 80 ÷ 27 = 2.96 cubic yards. With a 10% planning allowance, the estimate is about 3.26 cubic yards.
Use the Concrete Calculator
For a faster estimate, use the YardCompute Concrete Calculator. It calculates cubic feet, applies an allowance, and converts the result to cubic yards.
FAQ
How many cubic feet are in a cubic yard?
One cubic yard equals 27 cubic feet.
How do I convert slab thickness from inches to feet?
Divide the thickness in inches by 12.
Should I add extra concrete?
A planning allowance can help account for measurement variation, waste, and site conditions, but the appropriate order quantity should be confirmed with your concrete supplier or qualified professional.
Can I use this method for a footing?
The same volume principle applies, but footings should be measured according to their actual shape and dimensions. Separate irregular sections when needed.
What if my slab is not rectangular?
Break the area into simpler sections, calculate each section, and add the volumes together.
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Planning estimate: Estimates are for planning purposes only. Actual material requirements may vary based on site conditions, material specifications, installation methods, waste, and local requirements. Always verify quantities with your supplier or qualified professional before purchasing materials.