Volume = length × width × depth
Measure all three in the same unit before multiplying.
- Metric: a slab 4 m × 3 m and 10 cm deep is 4 × 3 × 0.1 = 1.2 m³.
- Imperial: a slab 10 ft × 12 ft and 4 in deep is 10 × 12 × (4 ÷ 12) = 40 ft³. Divide by 27 for cubic yards: 1.48 yd³.
For a round column or post hole, the volume is π × radius² × depth.
Add extra
Ground is never perfectly level and some concrete is always lost, so add 5 to 10%. Running short in the middle of a pour leaves a weak joint, while a little left over is cheap.
Bags or ready-mix
An 80 lb bag of premix makes about 0.6 ft³ of concrete and a 60 lb bag about 0.45 ft³. The 40 ft³ slab above needs about 67 bags of 80 lb before the extra. At around a cubic meter or more, ready-mix from a truck is usually easier and more consistent than mixing bags by hand.
Reinforcement
Slabs are usually reinforced with mesh or a grid of rebar. The Rebar Calculator gives the number of bars each way at your spacing, their total length and weight.
Calculator
The Concrete Calculator works out slabs, footings and columns in meters or feet, with the extra you choose and the number of bags.
Post holes
A hole 300 mm across and 900 mm deep holds π × 0.15² × 0.9 = 0.064 m³. Ten such holes need about 0.64 m³, or about 0.7 m³ with extra. In feet, a 12 in hole 3 ft deep holds π × 0.5² × 3 = 2.4 ft³, about four 80 lb bags.
Bags in metric
A 25 kg bag of premixed concrete makes roughly 11 to 12 liters. A cubic meter therefore needs around 85 to 90 bags, which is a lot to mix by hand.
Common problems
- Cracks across a slab. Concrete shrinks as it cures. Cut or form control joints about 24 to 30 times the slab thickness apart, so a 10 cm slab gets a joint every 2.4 to 3 m. In US terms, 2 to 3 times the thickness in inches gives the spacing in feet: every 8 to 12 ft for a 4 in slab.
- A weak, dusty surface. Too much water makes concrete easier to pour but weaker. Add only as much as the bag says.
- It dried too fast. Concrete gains strength by curing, not drying. Keep it damp, under plastic or with a spray, for about a week, and protect it from frost.