Concrete Calculator

How It Works

Concrete is ordered by volume, and the maths is length × width × depth. The calculator converts the depth from inches to feet, multiplies the three dimensions to get cubic feet, then divides by 27 — the number of cubic feet in a cubic yard — because ready-mix concrete is sold by the cubic yard. For bagged concrete it divides the cubic feet by the yield of each bag size and rounds up, since you cannot buy part of a bag.

The bag yields used here are the standard approximate figures: a 60-lb bag yields about 0.45 cubic feet and an 80-lb bag about 0.6 cubic feet. Yields vary slightly by brand, so check the yield printed on the bag you buy.

Worked Example

A 10 ft × 10 ft slab, 4 inches deep: the area is 100 sq ft and the depth is 4 ÷ 12 = 0.333 ft, so the volume is 100 × 0.333 = 33.33 cubic feet. That is 33.33 ÷ 27 = 1.23 cubic yards. In bags: 33.33 ÷ 0.45 = 74.07, so 75 sixty-pound bags; or 33.33 ÷ 0.6 = 55.56, so 56 eighty-pound bags.

Assumptions

• The slab has a flat, even depth throughout — uneven ground uses more concrete.
• Bag yields are approximate: 0.45 cu ft per 60-lb bag, 0.6 cu ft per 80-lb bag.
• No extra allowance for waste or spillage is added; round up or add 5–10% yourself for uneven forms.

Concrete for a 10 ft × 10 ft slab at different depths:

How to Use This Calculator

Enter the length and width in feet and the depth in inches. Results update live, showing cubic feet, cubic yards and bag counts. For an irregular shape, split it into rectangles, run each one separately, and add the volumes.

Tips and Practical Advice

For anything over about a cubic yard, ready-mix delivered by truck is usually cheaper and far easier than mixing dozens of bags by hand. Add a small buffer — ground is rarely perfectly level, and a slab that comes up short mid-pour leaves a weak cold joint. And check the depth with a tape measure inside the forms before the concrete arrives.