How to use it
- Choose the Shape of the part you are pouring and set the Units.
- Type its size. The fields for the other shapes are hidden.
- Set the Waste allowance and the Truck capacity of the truck your supplier sends.
- Read Concrete to order and Ready-mix truck loads, or choose a Dry-mix bag size and enter its Yield of the mix to read the number of bags.
How the volume is worked out
Each shape is a simple solid, so its volume is its cross-section multiplied by its length. The waste allowance is then added on top, and truck loads and bags are rounded up to whole units.
The stair volume is an approximation for a cast flight. It adds the triangles that form the steps to a waist slab measured at right angles to the slope; a formwork drawing or the engineer's sections give the exact figure.
slab = length × width × thickness
footing = length × width × depth
column = π × (diameter ÷ 2)² × height
wall = length × height × thickness
stairs = width × [n × rise × tread ÷ 2 + waist × n × √(rise² + tread²)]Bags of dry mix
Concrete is a composite of aggregate bound together with cement that cures to a solid, and water is added to start the reaction. A bag of dry mix therefore makes a set volume of concrete once water is added, and that volume depends on the mix.
The yield, in litres of concrete per kilogram of dry mix, is printed on the bag or in its data sheet. This page uses 0.468 litres per kilogram as an example, which makes about 11.7 litres from a 25 kg bag, 18.7 litres from a 40 kg bag and 23.4 litres from a 50 kg bag. Replace it with the figure on your bag.
bags = litres of concrete ÷ (bag size in kg × yield in litres per kg)What this calculator does not include
- A site mix of cement, sand, gravel and water. This page gives no mix ratio: the mix of a structural element depends on its strength and the exposure it faces, and an engineer sets it.
- Formwork, reinforcement, the pump hire and the waiting charge if the truck is held on site.
- Curing, the surface finish and the cover over reinforcement.
Worked example: a slab 6 m × 4 m and 12 cm thick
The slab is 6 × 4 × 0.12 = 2.88 m³. With the 5 % allowance the order is 2.88 × 1.05 = 3.024 m³, shown as 3.02 m³.
One 7 m³ truck is enough. At the example yield of 0.468 litres per kg, a 25 kg bag makes 11.7 litres: 3,024 litres ÷ 11.7 = 258.5, so 259 bags. With 50 kg bags (23.4 litres each), 3,024 ÷ 23.4 = 129.2, so 130 bags.
| Volume | Bags of 25 kg | Bags of 50 kg | |
|---|---|---|---|
| Before waste | 2.88 m³ | 247 | 124 |
| With 5 % waste | 3.02 m³ | 259 | 130 |
Frequently asked questions
How much concrete do I need for a slab?
Multiply length, width and thickness, and add a waste allowance. A 6 × 4 m slab 12 cm thick is 2.88 m³ before waste, and about 3.02 m³ with 5 % added.
How many bags of dry mix make one cubic metre?
It depends on the yield printed on the bag. At the example yield of 0.468 litres per kilogram, one cubic metre takes about 86 bags of 25 kg, 54 of 40 kg or 43 of 50 kg. Use the yield on your own bag.
When is a ready-mix truck cheaper than bags?
For more than a few cubic metres, a truck is usually simpler and cheaper, because the bags must be mixed, carried and placed by hand. Ask the supplier for the price per cubic metre and the minimum order.
Can I use a volume ratio like 1:2:4 for structural work?
No. The mix for structural concrete depends on its strength and the exposure it faces, and an engineer sets it. A ratio on a site is only a rough guide and cannot be relied on for a load-bearing element.
What waste allowance should I use?
5 % is a common planning figure for a flat slab on a prepared base. Use more when the ground is uneven, the excavation is rough, or you are pouring a shape with many corners.
Does the tool store what I enter?
No. The calculation runs in your browser and nothing you type is sent to us or stored.
Sources
Figures and text last checked on Tuesday, November 10, 2026.