By ToolzHive · Published
Measure three dimensions
Measure the slab’s length, width and depth. Use consistent units before multiplying. Depth is often measured in inches while length and width are in feet; divide inches by 12 to convert depth to feet. An uneven excavation needs additional measurements because a single depth may understate the required volume.
Use the volume formula
For a rectangular slab, volume = length × width × depth. With all measurements in feet, the result is cubic feet. Divide cubic feet by 27 to get cubic yards. With all measurements in meters, the result is cubic meters. Square footage alone is not enough to determine concrete volume; you also need thickness.
Worked example: 20 ft by 10 ft by 4 inches
The depth is 4 ÷ 12 = 0.3333 ft. The slab volume is 20 × 10 × (4 ÷ 12) = 66.67 cubic feet. Divide by 27 for approximately 2.47 cubic yards. If you choose a 10% allowance, multiply by 1.10 for approximately 2.72 cubic yards. The allowance is a planning assumption, not a structural design recommendation.
Estimate bags using the product yield
Bag weight alone is not enough to determine coverage; read the mixed-concrete yield on the product packaging. If a bag’s stated yield is 0.60 cubic feet, the example with a 10% allowance needs 73.33 ÷ 0.60 = 122.22 bags, rounded up to 123. Use the yield for the actual product you plan to buy.
Check before ordering
Use Concrete Calculator to compare the dimensions, project shape and allowance. Confirm excavation measurements and supplier ordering increments. This estimates quantity; it does not determine the correct slab thickness, reinforcement, concrete strength or suitability for a structure. Use the project specification and appropriate professional advice for those decisions.
Put this guide into practice
- Concrete Calculator — Calculate the concrete needed for slabs, footings, columns and steps, plus bags required.
- Cement Calculator — Work out cement, sand and aggregate quantities for a concrete mix ratio.
- Cubic Yard Calculator — Convert length, width and depth into cubic yards, feet and meters.