How to use this concrete calculator
- Units, choose Feet and Inches or Metres. The thickness field switches to inches or centimetres to match.
- Dimensions, enter the length, width and thickness of your slab.
- Bag size, pick the pre-mix bag you plan to buy (40 kg, 60 lb or 80 lb).
- Waste, leave the allowance at 10 percent or adjust it for your site.
- Read the volume and bag count, they update as you type.
Nothing is uploaded: every figure is worked out on your device, so you can plan a pour without sharing anything.
How to calculate concrete volume
Concrete volume for a slab is simply length × width × thickness, with every measurement in the same units. Working in feet, the thickness is usually given in inches, so divide it by 12 to turn it into feet before multiplying. That gives a volume in cubic feet, which you can then convert.
To convert, divide cubic feet by 27 for cubic yards (a cubic yard is 3 ft on every side, so 27 cubic feet). For cubic metres, divide cubic feet by 35.31. Working in metres instead, multiply length × width × thickness directly, using metres throughout, to get cubic metres.
Once you know the volume, pre-mix bag yields tell you the bag count. As a rough guide an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet, and a 40 kg bag about 0.018 cubic metres (roughly 0.635 cubic feet). Divide the total volume by the yield and round up, since you cannot buy part of a bag. Always add about 10 percent for waste to cover spillage, an uneven sub-base and over-excavation.
Note: bag yields and conversions are approximate and vary slightly by brand and mix. For large pours, ready-mix delivered by the cubic yard or cubic metre is usually cheaper than dozens of bags. Treat these figures as a planning estimate and check the bag label before you buy.
Frequently asked questions
How do I calculate concrete volume for a slab?
Multiply length by width by thickness in matching units. In feet, convert thickness inches to feet by dividing by 12. Cubic feet divided by 27 gives cubic yards.
How many bags do I need?
Divide the total volume in cubic feet by the bag yield and round up. An 80 lb bag yields about 0.6 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 kg bag about 0.635 cu ft.
How much should I add for waste?
About 10 percent is normal, to cover spillage and an uneven sub-base. You can change the allowance above. Running short mid-pour costs far more than a little surplus.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in a cubic yard. One cubic metre is about 35.31 cubic feet, or roughly 1.31 cubic yards.
Is my data uploaded?
No, everything is calculated on your device and nothing is sent anywhere.