Concrete Calculator

Concrete is sold by volume, so every project comes down to length × width × depth. Measure each dimension, pick its unit, and the calculator converts everything to cubic feet before reporting cubic yards (the unit ready-mix suppliers quote in the US), cubic meters, and the number of premixed bags. A 5 ft × 2.5 ft pad 5 inches thick holds about 5.21 cubic feet, or 0.19 cubic yards: nine 80-lb bags or twelve 60-lb bags.

Round and hollow shapes use the area of a circle, π × d² ÷ 4. A tube or ring subtracts the inner circle from the outer one. Curbs combine two rectangles, the upright curb and the flat gutter flag, and stairs add up a stack of blocks, one per step, plus the landing at the top.

Order a little more than the calculated volume. Forms bow, subgrade is rarely perfectly flat, and some concrete is always lost in the truck chute or the mixing tub, so 5 to 10 percent extra is normal. Running short in the middle of a pour leaves a cold joint, a weak seam where fresh concrete meets material that has already started to set.

For small jobs, bagged mix is practical: an 80-lb bag yields roughly 0.6 cubic feet and a 60-lb bag about 0.45. Beyond about one cubic yard, around 45 of the larger bags, a ready-mix delivery usually costs less and produces more consistent concrete. Many suppliers charge a short-load fee for small orders, so compare both options.

Concrete is a mix of portland cement, water and aggregate (sand and gravel or crushed stone). It hardens through hydration, a chemical reaction between cement and water, not by drying out, which is why freshly placed slabs are kept moist for the first week. Concrete typically reaches most of its design strength in about 28 days.

Typical residential sidewalks and patios are about 4 inches thick, while driveways that carry vehicles are often 4 to 6 inches. Footing size depends on the load and the soil, so check local building codes before you pour structural work.

Frequently asked questions