Concrete is ordered by volume, so depth matters as much as length and width. A small depth change can make a large difference on patios, shed bases, driveways, and walkways.
For a rectangular slab, multiply length by width by depth. If you are working in feet and inches, convert the depth from inches to feet before calculating. For example, four inches is one third of a foot.
After calculating the basic volume, add a waste allowance. This helps cover uneven ground, small measuring differences, spillage, and finishing loss. A ten percent allowance is often useful for early planning, but your supplier or contractor may recommend a different allowance.
For structural concrete, do not rely on a planning calculator alone. Load, reinforcement, soil, drainage, and local requirements should be checked by a qualified professional.