Concrete calculator

Enter the size and thickness of your slab or footing. You'll get the cubic yards to order from a ready-mix plant, or the number of bags to mix yourself.

For a round footing, choose Circle and enter the hole's diameter and depth.

Shape of the area

Length (feet)

Width (feet)

Diameter (feet)

Thickness or depth (inches)

How many of these

–cubic yards

Cubic feet: –
With 10% extra (cu yd): –
80 lb bags: –
60 lb bags: –
40 lb bags: –

Bag counts don't include extra. Add 5 to 10% for uneven ground and spills.

How thick should concrete be?

Project	Thickness
Walkways and patios	4 in
Shed or hot tub pad	4–6 in
Driveway	4–6 in, thicker for heavy vehicles
Post and deck footings	As deep as local code requires

How the math works

Area times thickness in feet gives cubic feet, and dividing by 27 gives cubic yards. A 10 by 10 foot slab, 4 inches thick, is about 33 cubic feet, or 1.23 cubic yards. Mixing that yourself would take about 56 bags of 80 pounds, since one 80 lb bag makes about 0.6 cubic feet.

Bags or ready-mix?

Bags make sense for small jobs like setting posts or pouring a pad under an air conditioner. Past about one cubic yard, mixing by hand gets slow and exhausting, and a ready-mix truck is usually worth it. Many plants have a minimum order, so ask about short-load fees.

Common questions

How much does one bag of concrete make?

About 0.3 cubic feet for a 40 lb bag, 0.45 cubic feet for 60 lb and 0.6 cubic feet for 80 lb. Check the bag, since yields vary slightly by brand.

Why order extra?

The ground under a slab is never perfectly flat, forms bow slightly, and some concrete stays in the mixer or wheelbarrow. Running short in the middle of a pour leaves a weak seam, so a small surplus is cheap insurance.

How do I calculate a round footing?

Choose Circle, enter the hole's diameter and depth, and the number of holes. For a 12 inch wide hole that is 36 inches deep, enter a diameter of 1 foot and a depth of 36 inches.