Concrete slab calculator

How this calculator works

It multiplies length by width by thickness to get the slab volume, adds a waste allowance for an uneven base and spillage, and converts the result to what you can buy: whole cubic yards of ready-mix, or bags when the pour is 1 cu yd (0.75 m³) or less.

Bag counts use standard published yields: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag 0.45 and a 40 lb bag 0.30. Metric bags make about 12 L per 25 kg. Form boards are sized to be at least as deep as the slab.

Worked example

A 12′ × 12′ patio 4″ thick holds 12 × 12 × 4 ÷ 12 = 48 cu ft, which is 1.78 cu yd. With 10% waste that is 1.96 cu yd, so the list orders 2 cu yd of ready-mix. The forms take four 2×6 boards 12′ long and 20 stakes, one every 3 ft plus the corners. Bagged, the same pour would be 88 bags of 80 lb.

Tips before you buy

• Measure the thickness you will actually pour. A base that dips half an inch across the slab adds about 12% more concrete.
• Compact a 4″ gravel base before you form the slab; it keeps the thickness even and the slab from settling.
• Ready-mix plants often sell part yards and may charge a short-load fee on small orders. Ask when you book the truck.
• Set the forms level along the top edge and stake them on the outside every 3 ft so they do not bow under the wet concrete.

Questions

How thick should a concrete slab be?

Four inches is standard for patios, walkways and shed floors. Driveways and slabs that carry vehicles are usually 5 to 6 inches. Check your local code for garages and structural slabs.

Should I use bags or order ready-mix?

Bags make sense up to about 1 cubic yard, which is already 45 bags of 80 lb to mix by hand. Beyond that a ready-mix truck is faster and gives one consistent batch. The Buy as field lets you force either option.

How many 80 lb bags are in a cubic yard?

An 80 lb bag yields about 0.60 cubic feet, so a cubic yard (27 cu ft) takes 45 bags. With 60 lb bags it is 60 bags, and with 40 lb bags 90.

Why add a waste allowance?

Subgrades are never perfectly flat, forms flex slightly, and some concrete stays in the wheelbarrow or chute. Ten percent is a common allowance; a well-graded, compacted base can get by with five.