Concrete calculator
How many bags of concrete, or how much ready-mix, for a slab, pad or footing. For fence and deck posts, use the post hole calculator.
How this concrete calculator works
It multiplies length × width × thickness, adds your extra allowance, then divides by what one bag makes once mixed. The result is rounded up, because a pour that runs short leaves a cold joint.
US bag yields come from the manufacturer's data sheet: an 80 lb bag makes about 0.60 cu ft and a 60 lb bag about 0.45 cu ft. Metric bags use the same mix density, so a 20 kg bag makes about 9.4 litres, roughly 107 bags per cubic metre. Some retailers quote 100 bags per cubic metre; we use the more cautious figure. If your bag lists a different yield, choose Other size and enter it.
Setting fence or deck posts? Use the post hole concrete calculator, which takes off the space the post fills and any gravel at the bottom of the hole.
How to measure
- Measure the inside of your formwork, not the area you want to cover. Forms often end up a little larger than planned.
- For thickness, measure from the top of the form to the compacted sub-base in a few places and use the deepest reading.
- For several identical pads or footings, enter one and set how many.
Bags or ready-mix?
Bags make sense for steps, small pads and footings. Once you pass about 1 cubic yard (roughly 0.75 m³), mixing by hand becomes a long, heavy job and the first batches start to set before the last ones go in. Ready-mix suppliers usually have a minimum order and charge a short-load fee for small amounts, so get a quote for both.
Typical thicknesses
Most patios and footpaths are about 4 in (100 mm). Driveways and slabs that take vehicles are usually thicker and often reinforced. Footing sizes depend on your soil, frost depth and local building rules, so check those before you dig.
Common questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. A cubic yard is 27 cu ft and an 80 lb bag makes about 0.60 cu ft, so 27 ÷ 0.60 = 45 bags before any extra allowance.
How many 20 kg bags make a cubic metre?
Roughly 100 to 110, depending on the product. This calculator assumes about 107. Check the yield printed on the bag you buy.
Should I add extra concrete?
Yes. 5–10% covers spillage and uneven ground for most small jobs. Use more if the ground is rough or the forms are long and flexible.
Reviewed by Alex Rowan, Editor. Last reviewed . How we calculate · Report a problem