Concrete Calculator

How this concrete calculator works

Concrete is sold by volume, so every estimate starts with cubic feet. For a slab, wall or footing the calculator multiplies length × width × thickness, converting the thickness from inches to feet by dividing by 12. A 10 × 10 ft patio at 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft. For a wall, enter the wall height in the width box and the wall thickness in the thickness box. For a round column or post hole it uses π × radius² × depth, with the diameter converted from inches to feet.

The total is multiplied by your waste allowance and divided by 27 to get cubic yards, the unit ready-mix suppliers use. A 5 to 10 percent allowance covers uneven subgrade, form bulge, spillage and the concrete left in the mixer or wheelbarrow. On a slab poured over a subgrade that was not carefully leveled, 10 percent is the safer figure because every half inch of extra depth adds about 12 percent to a 4-inch slab.

Bag counts use the manufacturer yields for standard concrete mix: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. That works out to 45 bags of 80 lb mix per cubic yard. Counts are always rounded up because a partial bag still has to be bought.

Use this page for general pours, then switch to the dedicated slab, driveway, steps or column calculators for cost and shape-specific details. If the job is a block wall rather than poured concrete, the concrete block calculator and mortar mix calculator handle the block count and mortar bags instead.

Where these numbers come from

The formulas and default values in this calculator are based on:

• QUIKRETE Concrete Mix No. 1101 technical data sheet (yields: 40 lb 0.30 cu ft, 60 lb 0.45 cu ft, 80 lb 0.60 cu ft)
• Portland Cement Association guidance on residential slab thickness

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.