Concrete Calculator
Enter your pour and get the ready-mix cubic yards and the bag count for slabs, footings, walls, and round columns, with a waste allowance built in.
How to figure concrete for your project
Concrete is ordered by volume, so every estimate comes down to measuring the space you're filling and converting it to cubic yards (for ready-mix delivery) or to bags (for smaller jobs you mix yourself).
Slabs, footings, and walls
These are all rectangular volumes. Multiply length by width by thickness, with every measurement in feet, to get cubic feet, then divide by 27 to reach cubic yards. The one that trips people up is thickness: a 4-inch slab is only 0.33 feet thick, so this calculator lets you enter thickness in inches and handles the conversion.
Round columns and post holes
For a cylinder (a column, a footing tube, or a deck-post hole), the volume is π × radius² × height. Enter the diameter and the calculator halves it for you.
Bags vs. ready-mix
An 80 lb bag of concrete mix yields about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Because a full cubic yard is 27 cubic feet, that's roughly 45 bags of the 80 lb mix per yard, which is why hand-mixing stops making sense much past a yard, and a ready-mix truck takes over.
Always add for waste
Subgrade is never perfectly level, footings over-dig, and some mix is lost to spillage and the wheelbarrow. A 5–10% waste allowance covers it. Running short in the middle of a pour is the expensive mistake, so round up rather than down.
Concrete calculator FAQ
How much concrete do I need for a slab?
Multiply the slab's length, width, and thickness in feet to get cubic feet, then divide by 27 to get cubic yards. A 10 ft by 10 ft slab poured 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or about 1.23 cubic yards. Add 5 to 10 percent for waste.
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of concrete mix yields about 0.6 cubic feet. A cubic yard is 27 cubic feet, so a cubic yard takes roughly 45 bags of 80 lb mix. A 60 lb bag yields about 0.45 cubic feet (60 per yard) and a 40 lb bag about 0.30 cubic feet (90 per yard).
Should I order ready-mix or use bags?
As a rule of thumb, mixing bags by hand is practical for small jobs up to roughly 1 cubic yard, which is about 45 to 50 bags of 80 lb mix. Above that, a ready-mix delivery is usually cheaper per yard and far less labor. Many suppliers have a minimum load and a short-load fee for small orders.
How much extra concrete should I order for waste?
Add 5 to 10 percent to your calculated volume. Uneven subgrade, spillage, over-excavated footings, and slab edges all consume a little more than the ideal figure. Running short mid-pour is far more costly than a small amount left over, so round up.