Concrete quantity toolsMeasure the concrete area and thickness
For a rectangular slab of uniform thickness, volume equals length × width × thickness. Measure the space inside the forms that concrete will fill, not the larger surrounding area. Use the slab thickness shown in the project information; this calculation does not choose it.
Before calculating, make a dimension list or sketch that labels each length, width, and thickness with its unit. Mark where one section ends and another begins. If the footprint is irregular, split it into nonoverlapping shapes; if thickness changes, separate the sections by thickness as well. Include the full concrete area without counting any part twice.
Convert inches to feet before multiplying
For a slab 12 feet long, 10 feet wide, and 4 inches thick, first convert the thickness to feet: 4 / 12 = 1/3 foot. Volume is 12 × 10 × 1/3 = 40 cubic feet. Divide by 27 to convert to cubic yards: 40 / 27 ≈ 1.481 cubic yards, before any waste allowance.
Entering 4 feet instead of 4 inches would give 480 cubic feet—twelve times the correct result. Dividing cubic feet by 3 instead of 27 is another common error. A cubic yard contains 3 × 3 × 3 = 27 cubic feet. Use the cubic-foot result for bag calculations or convert it to cubic yards when that is the supplier’s unit.
Convert feet and inches to decimal feet
Ten feet six inches equals 10 + 6/12 = 10.5 feet. It is not 10.6 feet: 0.6 foot is 7.2 inches. Convert the inches separately, then add them to the whole feet. Keep the original measurement beside the converted value so you can check it.
The example slab has a surface area of 120 square feet and a concrete volume of 40 cubic feet. Use area to estimate coatings and volume to estimate concrete. If thickness varies, calculate sections that match the actual shape. Averaging scattered depth readings does not make an irregular slab uniform.
- Check the units beside every input.
- Include each section once.
- Use cubic feet or cubic yards for concrete volume and square feet for surface area.
Work through an L-shaped slab
Suppose a uniform 4-inch slab has a 12 ft by 10 ft main rectangle and a separate 6 ft by 4 ft extension attached outside it. The main rectangle holds 40 ft³; the extension holds 6 × 4 × (4 ÷ 12) = 8 ft³. Add the two nonoverlapping sections for a measured total of 48 ft³.
If the sketch dimensions make the rectangles overlap, subtract the shared area once or redraw the split so each part appears only once. Add any allowance after the measured sections are totaled, and keep it separate from the volume needed to fill the footprint.