Slabs and footings

Concrete Calculator

Estimate ready-mix concrete volume and an approximate number of 80-pound bags. Enter the job dimensions below, then review the guide before ordering.

Concrete calculation formula

length x width x (thickness in inches / 12) / 27 = cubic yards

Worked example: A 12 ft by 10 ft slab at 4 inches thick is 40 cu ft, or 1.48 cu yd. A 10% allowance brings the calculation to 1.63 cu yd, commonly rounded up to a supplier's available order increment.

Measuring a concrete pour

Measure the inside dimensions of the forms. For multiple pads or footings, calculate each section separately and add the volumes. Thickness has a large effect on volume, so verify excavation and form elevation in several places.

Why concrete needs an allowance

Subgrade variation, form movement, spillage, and measurements that are slightly out of square can all increase demand. A small rectangular pour often uses a 5% to 10% allowance. Irregular excavation may require more.

Ready-mix or bags?

Bagged concrete can suit small repairs and posts. Larger placements are usually more manageable with ready-mix. Bag yield varies by product, so the calculator's bag count is only a general comparison. Use the yield printed on the selected bag for purchasing.

Before placing an order

Recheck field measurements, confirm the selected product's actual package coverage or density, and ask the supplier about order increments, minimum quantities, delivery limits, and return rules. Keep unlike products and assemblies in separate calculations.