Bhuvan PoudelHow concrete volume is calculated
Slab: length × width × thickness × quantity. Column: π × (diameter ÷ 2)² × depth × quantity. Order volume = volume × (1 + waste ÷ 100). Bags = order volume ÷ yield per bag, rounded up.
- Choose a slab or footing, or a round column or post hole.
- Enter measured dimensions and select their units. Thickness or depth has its own unit.
- Multiply the volume of one shape by the quantity, then add the waste percentage.
- Divide the volume including waste by the bag yield and round up to a whole bag.
Worked example: A 4 m × 3 m slab at 100 mm thick needs 4 × 3 × 0.1 = 1.2 m³. With 10% waste, allow 1.32 m³, or 1.7265 yd³. At a typical yield of 0.010 m³ per 20 kg bag, allow 132 bags. Approximate concrete weight including waste is 3,168 kg.
Concrete volume examples
| Shape | Length | Width | Diameter | Dimension unit | Thickness or depth | Depth unit | Quantity | Volume before waste (m³) | Volume including waste | Premix bags, rounded up |
|---|---|---|---|---|---|---|---|---|---|---|
| Slab or footing | 4 m | 3 m | 0.3 m | m | 100 mm | mm | 1 | 1.2 m³ | 1.32 m³ | 132 bags |
| Slab or footing | 10 ft | 10 ft | 0.3 ft | ft | 4 in | in | 1 | 0.9439 m³ | 1.358 yd³ | 62 bags |
| Round column or post hole | 4 mm | 3 mm | 300 mm | mm | 600 mm | mm | 4 | 0.1696 m³ | 0.1866 m³ | 19 bags |
| Slab or footing | 6 m | 0.4 m | 0.3 m | m | 0.3 m | m | 1 | 0.72 m³ | 0.792 m³ | 80 bags |
Swipe the table sideways to see the rest.
All examples include 10% waste and use typical bag yields. Check the bag before ordering.
Things to consider
- Measure the actual pour dimensions, including any thickened edges separately.
- Waste covers uneven excavation, spillage and small measurement differences. It is not a structural allowance.
- Check the stated yield on the bag. Brand, mix and water instructions can change yield.
- Approximate weight uses 2,400 kg/m³ for normal concrete and includes the waste allowance.
- Footing depths, reinforcement and slab thickness depend on loads, ground conditions, local building codes and an engineer’s design.
Frequently asked questions
How do I calculate concrete volume?
For a rectangular slab or footing, multiply length by width by thickness. For a round column, multiply π by the radius squared by depth. Use consistent units, multiply by quantity, then add waste.
How thick should a concrete slab be?
As general examples, some pedestrian slabs are around 75 to 100 mm (3 to 4 in), while some residential vehicle slabs are around 100 to 150 mm (4 to 6 in). These are typical ranges, not design recommendations. Loads, soil, reinforcement, local building codes and engineers decide the required thickness.
How many premix bags do I need?
Divide the volume including waste by the yield per bag and round up. Typical yields are 20 kg: 0.010 m³, 25 kg: 0.0125 m³, 30 kg: 0.015 m³, 40 lb: 0.30 ft³, 60 lb: 0.45 ft³ and 80 lb: 0.60 ft³. Check the bag or enter a custom yield.
Should I use bags or ready-mix concrete?
Bags can suit small pours and locations with limited access. Ready-mix is often more practical for larger pours. Compare delivery access, mixing capacity, placement time and the supplier’s minimum order.
Why add a waste allowance?
Excavations may be uneven and some concrete can be lost during mixing or placement. A 10% allowance is a planning default, not a guarantee. Choose an allowance that fits your measurements and project.
How do cubic metres compare with cubic yards?
One cubic yard is exactly 0.764554857984 cubic metres. One cubic metre is about 1.307951 cubic yards. These are volume units, not square units.