Concrete Calculator
Work out the volume of concrete a slab needs, in cubic metres or cubic yards, before you order from the plant.
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Formula and method
Volume is calculated as area multiplied by thickness. The waste percentage is added afterwards to cover spillage, uneven sub-base and over-dig, and the result is rounded to two or three decimals.
Why the order is bigger than the geometry
The calculated volume is the plate you designed. The delivered volume is that plate plus what stays in the pump, runs over the edge of the formwork and fills the dips in the sub-base. On a prepared and compacted base the loss is around 5 %; on a rough one it reaches 10 %, and that is the range to order, not a fixed rule of thumb.
The counter-error is just as common: adding 20 % to be safe means paying for concrete that has to be removed later, and a small pour has no way to use the surplus. Round up to the plant's delivery increment after adding the loss, not instead of it.
Joints, reinforcement and strength do not change the volume
Mesh, fibres and bar reinforcement displace very little concrete: at a typical 5 to 15 kg/m² the steel occupies a few litres per cubic metre, which disappears inside the delivery tolerance. The volume you order stays the geometric figure.
Joints are a construction decision, not a quantity: saw cuts must be made at the right time or the slab cracks where it wants. Strength class changes the price and the cement content, not the cubic metres, so it can be chosen after the volume is known.
One delivery, one batch, one record
Order a slab in a single delivery. Two loads from two mixes leave a visible joint and two different strengths in one plate. For a 6 × 4 m slab at 100 mm the figure is 2.4 m³, and the plant will send 2.5 m³ in one truck, which is exactly the 5 % allowance.
Annotate the delivery note with time, volume, water added and the cube samples taken. Water added on site is the most common reason a delivered C25 tests as C20, and the record is what decides who pays for the difference.
How to measure the inputs
- Measure the slab length and width at the widest points.
- Measure the thickness in several places and use the largest value.
- Convert everything to the same unit before entering the figures.
Worked example
Slab length 13.12 ft; Slab width 9.84 ft; Thickness 5.91 in; Waste 6 % → Concrete volume: 2.50 yd³
Reference data
| Grade | Cement, kg | Sand, kg | Aggregate, kg | Water, l |
|---|---|---|---|---|
| B20 (M250) | 320 | 750 | 1,150 | 180 |
| B22.5 (M300) | 350 | 700 | 1,150 | 175 |
| B25 (M350) | 380 | 650 | 1,150 | 170 |
Cement M400, aggregate 5–20 mm. A plasticiser cuts the water by 10–15 % at the same workability.
Practical tips
- Have everything ready before the truck arrives: concrete waits for nobody.
- Order from one plant and one batch for a consistent mix.
- Do not add extra water on site, it reduces strength.
Sources
- ГОСТ 26633-2015 — Бетоны тяжёлые и мелкозернистые
- EN 206 — Concrete
Checked and updated October 22, 2026