How to use the concrete calculator
- Choose units, then Rectangle for slabs and pads or Circle for round footings and column holes.
- Enter the length and width (or diameter) and the thickness.
- For several identical footings, enter how many.
- Pick your bag size, or choose Other bag and type the yield from the bag.
Bags per cubic yard
US figures use QUIKRETE’s published yields for its standard Concrete Mix:
| Bag | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.6 ft³ | 45 |
Bags per cubic metre (UK, Australia, Canada)
Metric bags are sold by weight, and yields differ between brands. These are the figures we use; the sources are listed at the bottom of the page.
| Bag | Yield per bag | Bags per m³ |
|---|---|---|
| 20 kg bag (UK) | 10 L | 100 |
| 20 kg bag (Australia) | 9.1 L | 110 |
| 30 kg bag (Canada) | 14 L | 72 |
The formula
volume = area × thickness × number × (1 + extra %)
bags = volume ÷ yield per bag (rounded up)
Worked examples
Imperial: a 10 ft × 10 ft slab, 4 in thick: 100 × (4 ÷ 12) = 33.3 ft³; plus 10% = 36.7 ft³ → 62 bags of 80 lb (82 bags of 60 lb).
Metric: a 3 m × 2 m pad, 10 cm thick: 0.6 m³; plus 10% = 0.66 m³ → 66 bags of 20 kg in the UK, or 73 bags of 20 kg in Australia. For other bags, enter the yield from the label.
Bags or ready-mix?
Bagged concrete suits small jobs: posts, footings, steps and small pads. As the bag count climbs, mixing by hand becomes slow and hard work, and a ready-mix delivery may be easier. Use the volume figure from the calculator to get a ready-mix quote and compare it with the cost of the bags.
Before you pour
- Measure inside the forms — that's the volume you need to fill.
- Check the ground. Soft spots and dips use more concrete; increase the extra % if the base is uneven.
- Follow the bag instructions for water, mixing and curing, and local codes for anything structural.
Common mistakes to avoid
- Mixing inches and feet. A 4-inch slab is 0.33 ft thick, not 4 ft — the calculator handles this if you pick the right units.
- Assuming every bag yields the same. Yields differ between products; use “Other bag” with the label figure if you're not using the listed sizes.
- No allowance for an uneven base. Dips in the ground fill with concrete too.
Measuring an awkward shape? See how to measure an irregular area.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. QUIKRETE lists an 80 lb bag of Concrete Mix as yielding about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags (before any waste allowance).
How many 60 lb bags of concrete are in a yard?
About 60, because a 60 lb bag yields about 0.45 cubic feet. For 40 lb bags it is about 90.
How many bags do I need for a 10 × 10 slab, 4 inches thick?
The slab is 33.3 cubic feet. With a 10% allowance that's 62 bags of 80 lb or 82 bags of 60 lb.
How many 20 kg bags of concrete make a cubic metre?
About 100 in the UK (retailers such as Wickes and Homebase list one 20 kg bag as about 0.01 m³) and about 110 in Australia (Cement Australia’s figure for its 20 kg Concrete Mix). In Canada, a 30 kg bag yields about 0.014 m³, so about 72 bags per m³. Pick your bag in the calculator.
My bag isn’t listed (for example 25 kg) — can I use this calculator?
Yes. Choose “Other bag” and enter the yield printed on the bag (litres or cubic feet). Yields differ between brands and mixes, so the label is always the best source.
Should I order extra concrete?
Yes. Ground is rarely perfectly flat, forms can bow, and some mix is always lost. The calculator adds 10% by default.
Sources
- QUIKRETE — Concrete Mix No. 1101 data sheet (bag yields)
- QUIKRETE — Concrete calculator
- Wickes — Blue Circle Multi-Purpose Ready To Use Concrete 20 kg (yield)
- Homebase — Multipurpose Concrete 20 kg (yield)
- Cement Australia — Calculators (20 kg Concrete Mix bags per m³)
- Home Hardware Canada — Concrete Mix 30 kg (yield)
Last updated 11 November 2026. Found a mistake? Tell us.