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Concrete calculator
Work out how much concrete you need for a slab, footing, wall or round column, in cubic yards or cubic meters, and how many premix bags to buy.
Concrete needed
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Or premix bags
- 40 lb bags
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- 60 lb bags
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- 80 lb bags
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- 20 kg bags
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- 25 kg bags
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- 40 kg bags
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A bag makes about 0.30 cu ft (40 lb), 0.45 cu ft (60 lb) or 0.60 cu ft (80 lb).
About 0.0093 m³ per 20 kg bag, 0.0117 m³ per 25 kg and 0.0187 m³ per 40 kg.
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How to calculate concrete
- Choose your units and the shape: a slab or footing, a wall, or a round column or hole.
- Enter the measurements and how many you need. Keep the 10% waste allowance unless you have a reason to change it.
- Read the volume to order, or the number of premix bags to buy.
Questions
Are my measurements sent anywhere?
No. The calculator works in your browser on this device. Your measurements are never uploaded or stored, and they’re gone when you close the page.
How many bags of concrete do I need?
Divide the volume by what one bag makes. An 80 lb bag of premix makes about 0.6 cubic feet, a 60 lb bag 0.45 and a 40 lb bag 0.3. In metric, a 25 kg bag makes about 0.012 cubic meters. Check the bag, as mixes vary.
Why add 10% for waste?
Ground is rarely perfectly level, forms bow a little, and some concrete is always spilled or left in the mixer. Running short in the middle of a pour is far worse than having a little left over, so 5–10% extra is standard.
When should I order ready-mix instead of bags?
For more than about one cubic yard (0.75 cubic meters), a ready-mix truck is usually easier and cheaper than mixing dozens of bags. These figures are estimates: check with your supplier before you order.