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Concrete Calculator
Enter your slab or column size to see how many cubic yards of concrete you need, and how many 40, 50, 60 or 80 lb bags that is. A 10 ft × 10 ft slab, 4 in thick, needs about 1.36 cubic yards with 10% extra.
Updated · Sources re-checked by
1.36 cubic yards
1.04 cubic meters · 36.67 cubic feet · with 10% waste
Ready-mix truck: order about 1.5 cubic yards (rounded up to a quarter yard).
| Bag size | Bags |
|---|---|
| 40 lb | 123 |
| 50 lb | 98 |
| 60 lb | 82 |
| 80 lb | 62 |
You can type 10, 10.5, 4 1/2 or 10 ft 6 in. Nothing you type is saved or sent anywhere. This is an estimate; order a little extra.
How the concrete calculator works
Volume is length × width × thickness. For a slab in feet and inches: length (ft) × width (ft) × thickness (in) ÷ 12 gives cubic feet, and cubic feet ÷ 27 gives cubic yards. For a round column: π × (diameter ÷ 2)² × height. The waste percentage is added on top. Bags are the total cubic feet divided by what one bag makes, rounded up to a whole bag.
| Bag size | Yield (cubic feet) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.6 | 45 |
Worked example: 10 ft × 10 ft patio, 4 in thick
- 10 × 10 × (4 ÷ 12) = 33.33 cubic feet.
- 33.33 ÷ 27 = 1.23 cubic yards.
- Add 10% for waste: 36.67 cubic feet = 1.36 cubic yards. A ready-mix order would be about 1.5 cubic yards.
- In bags: 62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb.
Common questions
How do you calculate cubic yards of concrete?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27. Thickness in inches must be divided by 12 first. A 10 ft × 10 ft slab that is 4 in thick is 33.33 cubic feet, or 1.23 cubic yards.
How many bags of concrete are in a cubic yard?
Divide 27 cubic feet by the yield of one bag: 90 bags of 40 lb, 72 bags of 50 lb, 60 bags of 60 lb, 45 bags of 80 lb. These use the QUIKRETE yields of 0.3 cubic feet, 0.375 cubic feet, 0.45 cubic feet, 0.6 cubic feet. Other brands can differ a little, so check the bag label.
How much extra concrete should I order?
Many people add about 5% to 10% for uneven ground, spills and forms that are not perfectly square. This calculator starts at 10%. Run out in the middle of a pour and you get a cold joint, so a little extra is cheaper than too little.
Should I use bags or a ready-mix truck?
Bags are practical for small jobs you can mix yourself, such as a few posts or a small pad. For bigger pours the bag count adds up fast: the 10 ft × 10 ft example above needs 62 bags of 80 lb, and each one is mixed by hand or in a mixer. For jobs like that, ask a ready-mix supplier for a quote. Suppliers often have a minimum load, so ask them.
How thick should a concrete slab be?
It depends on the use and your local rules. Many home patios and sidewalks are poured about 4 in thick, and driveways or garage floors are often thicker. Follow your plans, your engineer or your local building department rather than a general number.
How do I calculate concrete for a round column or post hole?
Use π × (diameter ÷ 2)² × height, with the diameter and height in the same unit. Choose "Round column" in the calculator, enter the diameter in inches and the height in feet, and set "How many" to the number of holes or columns.
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Sources
- QUIKRETE Concrete Mix 1101 data sheet (bag yields: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 cubic feet) (checked October 26, 2026)
This calculator gives an estimate for planning. Real jobs vary with ground, forms and mix. Check your plans, local rules and a supplier before you order.