Concrete Calculator

How this calculator works

Concrete is sold by volume, so the calculator multiplies length × width × thickness after converting every measurement to feet. Thickness and depth are usually measured in inches, so they are divided by 12 first.

For round columns and post holes the volume is π × radius² × depth.

The result is divided by 27 to convert cubic feet to cubic yards, the unit ready-mix suppliers use. Bag counts use standard yields: 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb.

Example: a 10 × 10 ft patio, 4 inches thick

1. Convert the thickness: 4 in ÷ 12 = 0.333 ft.
2. Volume: 10 × 10 × 0.333 = 33.3 cu ft.
3. Add 10% for waste: 33.3 × 1.10 = 36.7 cu ft.
4. In cubic yards: 36.7 ÷ 27 = 1.36 cu yd.
5. In 80 lb bags: 36.7 ÷ 0.60 = 61.1, so buy 62 bags.

Tips before you buy

• Above about 1 cubic yard, ordering ready-mix is usually cheaper and much faster than mixing 45+ bags by hand.
• Measure the depth of the excavated area in several spots and use the average. Ground is rarely perfectly flat.
• Ready-mix plants often have a minimum order or charge a short-load fee for small quantities, so ask before you book.