🧱 Concrete Calculator
Ordering concrete is a volume problem: length × width × thickness gives cubic feet, divide by 27 for cubic yards — the unit ready-mix is sold in. For bagged concrete, the calculator converts the same volume into bag counts.
Always add a waste margin (5–10%); forms leak, subgrades are uneven, and running short mid-pour is far worse than having one bag left over.
How to use this calculator
Enter the length, width and thickness of your slab or footing and the calculator returns the concrete volume in cubic yards plus the number of 80, 60 and 40 lb bags you need.
- Enter the Length in feet and the Width in feet of the area to pour.
- Enter the Thickness in inches, for example 4 for a standard patio slab.
- Leave the Add 10% waste margin box checked unless you have a reason to remove it.
- Read the cubic yards for ready-mix orders, or the bag counts if you are mixing yourself.
Frequently asked questions
What does the Concrete Calculator compute?
It converts your slab dimensions into concrete volume, shown first in cubic yards, the unit ready-mix is sold in. It also converts the same volume into bag counts for 80 lb, 60 lb and 40 lb bags, with the 10 percent waste margin applied by default.
What formula does it use?
Volume is length times width times thickness converted to feet, divided by 27 to get cubic yards. Bag counts come from standard yields: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3.
Can you give a worked example?
A 10 ft by 10 ft slab at 4 inches thick gives 33.33 cubic feet. With the 10 percent waste margin that becomes 36.67 cubic feet, or 1.36 cubic yards, which needs about 61 eighty-pound bags (order a little extra).
Is the 10 percent waste margin really necessary?
Yes, for almost every real pour. Forms leak, subgrades are uneven and a little concrete always sticks to the mixer, so running short mid-pour costs far more than one leftover bag. Keep the box checked unless your measurements are professional-grade precise.