Concrete Calculator
Find the volume of concrete for a slab, footing, wall, or post holes, and how many bags of concrete mix to buy. Enter each area's shape, size, and thickness. For loose fill such as soil or sand, use the Cubic Yard Calculator.
Add slabs or footings, or subtract openings such as a drain.
- Choose US or Metric. This also sets the bag sizes in the results: 80, 60, 50, and 40 lb bags for US, and 30 and 20 kg bags for Metric.
- Pick the shape of the pour and enter its dimensions and Thickness. For a shape that is not listed, choose Known area and enter the area directly.
- Use Add section for each extra slab or footing. Set a section to Subtract for parts that do not need concrete, such as a drain or a post set in the concrete.
- Keep the 10% waste allowance or change it. If you change it, Reset to 10% brings it back.
- If your bag size is not listed, enter the yield printed on the bag in Custom bag yield to add it to the results.
- Optionally, enter a price per bag, per custom bag, or per cubic yard, cubic foot, or cubic meter to estimate the cost.
Dimension and thickness fields accept whole numbers, decimals, fractions,
and mixed numbers, such as 12, 10.5,
3/4, or 6 1/2. Choose ft + in
as the unit to enter a length such as 12 ft 6 in in two fields.
Slabs and pads
Choose Rectangle and enter the slab's length, width, and thickness. For an L-shaped slab, add a section for each rectangle. For a round pad, choose Circle and enter the diameter.
Footings and walls
- Footing: choose Rectangle, enter the footing's length and width, and enter its depth as Thickness.
- Wall: choose Rectangle, enter the wall's length and height as Length and Width, and the wall's thickness as Thickness.
Post holes and tubes
Choose Circle, enter the diameter of the hole or form tube, and enter the depth of the hole or the height of the column as Thickness. Use Quantity for several identical holes. To leave out a post set in the concrete, add a section for the post and set it to Subtract.
Steps
For solid concrete steps, use the Concrete Steps Calculator. It works out the volume from the number of steps, the riser height, the tread depth, the width, and the depth of the top landing.
How much concrete you need
The volume of a pour of even thickness is:
Volume = area × thickness
Convert every dimension to the same unit before you multiply. With dimensions in feet, divide cubic feet by 27 to get cubic yards. The waste allowance is added after subtracting any sections: total = net volume × (1 + waste % ÷ 100).
The number of bags is:
Bags = total volume ÷ yield per bag, rounded up
The yield is the volume of concrete one bag makes after mixing with water. The calculator uses these yields, taken from the manufacturers' data sheets:
| Bag | Yield | Source |
|---|---|---|
| 80 lb | 0.60 ft³ | Quikrete Concrete Mix (No. 1101) data sheet |
| 60 lb | 0.45 ft³ | Quikrete Concrete Mix (No. 1101) data sheet |
| 50 lb | 0.375 ft³ | Quikrete Concrete Mix (No. 1101) data sheet |
| 40 lb | 0.30 ft³ | Quikrete Concrete Mix (No. 1101) data sheet |
| 30 kg | 14 L | Bomix Concrete Mix (No. 1004) technical data sheet |
| 20 kg | 9.26 L (108 bags per m³) | Boral Concrete Mix product data sheet |
These are typical values. Yields differ between brands and products, so check the label on the bag you buy, and use Custom bag yield if it is different. The 10% waste allowance follows Sakrete's advice to add 10% to an estimate for bagged concrete.
Examples
A small slab
A 10 ft × 10 ft slab, 4 in thick, is 33.33 ft³. With a 10% waste allowance, the total is 36.67 ft³, or 1.36 yd³ (1.04 m³). That is 62 bags of 80 lb (36.67 ÷ 0.60 = 61.11, rounded up), 82 bags of 60 lb, 98 bags of 50 lb, or 123 bags of 40 lb. At $6.50 per 80 lb bag, the cost is $403.00.
Post holes
Four round holes, each 12 in across and 4 ft deep, hold 12.57 ft³. With 10% added, the total is 13.82 ft³, or 0.51 yd³, which is 24 bags of 80 lb.
Metric dimensions
A 3 m × 3 m slab, 10 cm thick, is 0.9 m³. With 10% added, the total is 0.99 m³, which is 71 bags of 30 kg or 107 bags of 20 kg.
FAQ
About 45. A cubic yard is 27 ft³, and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45. With 60 lb bags (0.45 ft³), it takes 60.
You can only buy whole bags, and running short in the middle of a pour is worse than having a partial bag left over.
It depends on the size of the job and the prices near you. To compare, enter the bag price to see the cost in bags, then switch the price to per cubic yard or cubic meter and enter a ready-mix quote.
About 1.36 yd³ (36.67 ft³) for a 10 ft × 10 ft slab 4 in thick, with a 10% waste allowance. That's 62 bags of 80 lb. For a different thickness, enter it in the calculator.
No. Cement is the powder that binds concrete together, and concrete is cement mixed with sand, gravel, and water. Bagged concrete mix already contains the cement, so the calculator counts bags of concrete mix, not bags of cement.
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- Concrete Steps Calculator
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