Concrete Calculator
for Bags and Cubic Yards
Enter the size of your slab, footing or column. See how many bags to buy, or how many yards of ready-mix to order, with waste included.
Slabs, footings and columns · 40, 60 and 80 lb bags · Works on your phone
Estimates only. Your numbers stay in your browser and aren't saved or sent anywhere.
How to calculate concrete
Work out the volume in cubic feet, with every measurement in feet, then convert it to whatever you're ordering:
Slab = length × width × (thickness ÷ 12)
Footing = length × (width ÷ 12) × (depth ÷ 12)
Round column = 3.14 × (diameter ÷ 24)² × height
Cubic yards = cubic feet ÷ 27
For example, a 20' by 10' slab at 4" thick is 66.67 ft³. Adding 10% waste makes it 73.33 ft³, which is 2.72 yd³ of ready-mix or 123 80 lb bags.
How much a bag of concrete covers
| Bag | Yield | Covers at 4" thick | Covers at 6" thick |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 0.9 sq ft | 0.6 sq ft |
| 60 lb | 0.45 ft³ | 1.35 sq ft | 0.9 sq ft |
| 80 lb | 0.60 ft³ | 1.8 sq ft | 1.2 sq ft |
Yields vary a little by brand, so check the bag.
Bags or ready-mix?
Bags work well for small jobs like post footings and pads, but they're slow to mix and the cost adds up. As the job grows, a ready-mix truck usually makes more sense. Many suppliers charge a short-load fee for small orders, so ask yours before you decide.
How much extra to order
The default is 10%, because running short in the middle of a pour is much worse than having some left over. Allow more if your sub-base is uneven or your forms might bow.
Questions
- About 45. A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.6 cubic feet.
- About 60. A 60 lb bag yields about 0.45 cubic feet.
- 4" is typical for sidewalks, patios and garage floors with normal loads. Driveways and heavier loads are often 5" or 6". Check your local code and your soil conditions.
- Common residential choices are around 2,500 psi for footings and 3,000 to 4,000 psi for slabs and driveways. Your local code or engineer sets the requirement.
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