Hong Life CalculatorsUnderstanding the concrete calculator — concept and principle
This calculator gives the volume of concrete for a rectangular slab or footing, or for a round column, and converts it into a ready-mix order and a number of premix bags. Use it for small DIY pours such as shed bases, paths, fence post footings and pier columns.
How it is calculated
- Slab or footing: V = length × width × depth.
- Round column: V = π × (diameter ÷ 2)² × height.
- Ready-mix to order = V × 1.05, a 5% allowance for spillage, uneven ground and formwork bulging.
- Bags of premix = V × 1.05 ÷ (0.0107 m³ × bag mass ÷ 25), rounded up. A 25 kg bag of dry premix is assumed to yield about 0.0107 m³ (10.7 L) of placed concrete.
All dimensions are in metres; enter a 100 mm slab as 0.1.
The principle
Concrete is ordered and mixed by volume, so the first step is geometry. Bag yield follows from density: fresh concrete weighs roughly 2,300–2,400 kg per m³ including its water, and 25 kg ÷ 0.0107 m³ corresponds to about 2,340 kg/m³. Larger or smaller bags scale linearly. The extra 5% exists because subgrade is never perfectly flat; a 10 mm dip across a 100 mm slab alone means 10% more concrete in that area.
Worked example
- Shape: slab, 3 m × 3 m × 0.1 m deep, 25 kg bags.
- Volume = 3 × 3 × 0.1 = 0.9 m³.
- Ready-mix with 5% extra = 0.9 × 1.05 = 0.945 m³.
- Bag yield = 0.0107 m³ per 25 kg bag.
- Bags = 0.945 ÷ 0.0107 = 88.3, rounded up to 89 bags (about 2.2 tonnes of material).
| Length or diameter (m) | Volume (m³) |
|---|---|
| 0.75 | 0.225 |
| 1.5 | 0.45 |
| 2.25 | 0.675 |
| 3 | 0.9 |
| 3.75 | 1.13 |
| 4.5 | 1.35 |
| 6 | 1.8 |
How to read the result
Around 1 m³ or more, ready-mix is usually far easier than bags: 89 bags must be carried and mixed consistently within the working time. Many suppliers deliver in steps (for example 0.1 or 0.5 m³) and may charge for small loads, so ask before ordering. Typical depths are about 75–100 mm for garden paths and shed bases, more for driveways, while footings follow the structural design.
Common mistakes
- Entering depth in millimetres or centimetres (100 instead of 0.1).
- Using diameter as the radius for a column, which quadruples the volume.
- Ignoring the compacted sub-base and reinforcement cover, which affect slab thickness.
- Mixing too much water into premix, which weakens the concrete.
Frequently asked questions
How many 20 kg bags make one cubic metre?
Under this calculator's yield, one 20 kg bag gives about 0.00856 m³, so roughly 117 bags per m³ before any extra allowance.
How long before I can walk on a slab?
Often about a day for light foot traffic, but strength keeps developing for weeks; keep it moist and follow the product guidance.
Can I pour a column with premix bags?
For small piers, yes, but pour in layers and compact each layer to avoid voids.
Sources: EN 206: Concrete, Specification, performance, production and conformity, CEN · A. M. Neville, Properties of Concrete, 5th ed., Pearson, 2011