Case 1
- Slab length
- 3m
- Slab width
- 2m
- Slab thickness
- 10cm
- Yield per bag
- 0.009m³
67bags
Open with these valuesConcrete Calculator
67bags
Result: 67 bagsLength × width × thickness is the volume; the volume divided by one bag's yield is the bag count, always rounded up to a whole bag. A 3 × 2 m slab at 10 cm holds 0.6 m³ — 67 bags at 0.009 m³ each. Past roughly half a cubic metre, ready-mix by the cubic metre beats carrying bags.
Worked examples
Case 2
- Slab length
- 4m
- Slab width
- 3m
- Slab thickness
- 15cm
- Yield per bag
- 0.011m³
164bags
Open with these valuesCase 3
- Slab length
- 2m
- Slab width
- 2m
- Slab thickness
- 5cm
- Yield per bag
- 0.009m³
23bags
Open with these valuesHow it's calculated
bags = ⌈length × width × thickness ÷ yield per bag⌉
- StepMeasure the slab: length and width in metres, thickness in centimetres.
- StepRead the yield off your own bag — a 20 kg pre-mix bag makes about 0.009 m³.
- StepLength × width × thickness gives the volume; divide it by that yield.
- ResultThe count rounds up to whole bags; order 5–10 % more for spillage.
What this number means
The volume is not the order
The count assumes a perfect rectangle and no waste. An uneven base and spillage both eat mix, so around 5 to 10 % on top is the honest order.
Bags are bought whole
A 3 × 2 m slab at 10 cm holds 0.6 m³, which is 66.7 bags at 0.009 m³ each. The formula rounds that up to 67, because half a bag is not for sale.
Past half a cubic metre, weigh up ready-mix
Bagged pre-mix suits small pours up to roughly 0.5 m³. Beyond that the bag count climbs fast and hand-mixing gets heavy, and ready-mix by the cubic metre is usually cheaper and more consistent.
Commonly misread
The slab is 0.6 m³, so 0.6 m³ of mix is what I need.
That is the volume of a perfect rectangle with nothing spilt. Spillage and an uneven base take more, and concrete sets fast enough that a second trip mid-pour is not an option.
I entered the slab thickness in metres.
Thickness is in centimetres here while length and width are in metres. Enter 10 for a 10 cm slab, not 0.1.
Every bag yields the 0.009 m³ the calculator starts with.
Yield follows the bag size and the mix, and it is printed on the bag. The data sheet behind this calculator puts a 27.2 kg bag at 12.7 litres and a 36.2 kg bag at 17 litres.
Reference table
| Length, width, thickness, yield | Volume | Bags |
|---|---|---|
| 1, 1, 10, 0.009 | 0.1 m³ | 12 |
| 2, 2, 5, 0.009 | 0.2 m³ | 23 |
| 3, 2, 10, 0.009 | 0.6 m³ | 67 |
| 4, 3, 15, 0.011 | 1.8 m³ | 164 |
| 6, 4, 10, 0.0125 | 2.4 m³ | 192 |
Questions
How much concrete do I need?
Multiply length by width by thickness in metres for the volume, then divide by the yield of one bag. A 3 × 2 m slab at 10 cm deep is 0.6 m³; at 0.009 m³ per bag that is 67 bags.
How thick should a concrete slab be?
It depends on the load. A garden path or patio is usually around 10 cm, a shed base or driveway 10–15 cm, and a load-bearing footing more again. Check the local building requirements for anything structural.
What bag yield should I enter?
A 20 kg pre-mix bag yields roughly 0.009 m³ of mixed concrete, and that is the default here. The Quikrete data sheet behind this calculator puts a 27.2 kg bag at 12.7 litres, so the figure follows the bag size and the mix. Use the volume printed on your own bag when you have it.
Should I use bags or ready-mix?
It comes down to volume. Bagged pre-mix suits small pours up to roughly 0.5 m³; past that the bag count climbs fast and mixing by hand gets heavy. Ready-mix delivered by the cubic metre is usually cheaper and more consistent for larger jobs.
Should I order a little extra?
Yes. The count already rounds up to whole bags, but it still assumes a perfect rectangle and no waste. Around 5–10 % more covers spillage and an uneven base — concrete sets fast and a second trip mid-pour is not an option.
Sources and last check
Information, not professional advice.