How to Calculate Concrete Weight
The weight of concrete is calculated using the formula:
Weight (kg) = Volume (m3) x Density (kg/m3)
Where volume is length x width x depth for a rectangular pour, and density depends on the mix type. Standard ready-mix concrete has a density of 2,400 kg/m3. This figure is well established and used across the construction industry in New Zealand and internationally.
For example, a 4 m x 3 m concrete slab at 100 mm (0.1 m) depth has a volume of 1.2 m3. At 2,400 kg/m3, it weighs 2,880 kg (2.88 tonnes).
Concrete Density by Mix Type
| Mix type | Density (kg/m3) | Typical uses |
|---|
| Lightweight (pumice, foamed) | 1,600 to 1,900 | Non-structural fills, insulation screeds, topping slabs |
| Lean / mass fill | 2,100 to 2,200 | Blinding layers, trench fills, bulk pours where strength is not critical |
| Standard ready-mix | 2,350 to 2,450 | Slabs, footings, driveways, paths, house foundations |
| Dense / reinforced | 2,450 to 2,550 | Structural slabs, columns, beams, retaining walls |
| Heavyweight (magnetite or barite aggregate) | 2,800 to 3,500 | Radiation shielding, counterweights, ballast structures |
For most residential and light commercial work in New Zealand, the standard figure of 2,400 kg/m3 is appropriate unless the supplier specifies otherwise.
Fresh Versus Cured Concrete
Freshly placed (wet) concrete is slightly heavier than cured concrete because it contains mix water that has not yet evaporated. Wet concrete typically weighs around 2,450 kg/m3 for a standard mix, roughly 2% more than the cured value. This matters when calculating formwork and shoring loads, because wet concrete exerts hydrostatic pressure on vertical shuttering. After curing, some water is lost through evaporation and the weight drops slightly. For structural dead-load calculations on completed buildings, use the cured density.
Why Concrete Weight Matters
Concrete weight is relevant for several practical situations:
- Structural design: Engineers calculate slab self-weight as a permanent (dead) load to size beams, columns, and foundations. A 150 mm slab weighs 360 kg/m2, which is a significant load on supporting members.
- Transport: Ready-mix trucks carry a maximum load. Knowing the volume of a pour lets you estimate how many truck loads are required.
- Crane and rigging: Precast concrete panels, columns, and beams must be lifted within the crane's rated capacity. Weight calculations are a safety requirement.
- Formwork design: The pressure of wet concrete on vertical formwork increases with pour height. Formwork must be sized to resist the hydrostatic pressure of fresh concrete before it sets.
Worked Example
You are planning a concrete driveway 4 metres long, 3 metres wide, and 100 mm thick (the typical residential standard).
- Volume = 4 x 3 x 0.1 = 1.2 m3
- Weight = 1.2 x 2,400 = 2,880 kg
- Weight in tonnes = 2,880 / 1,000 = 2.88 tonnes
- Weight per square metre = 2,400 x 0.1 = 240 kg/m2
This is the default shown in the calculator above. The ready-mix truck delivering this concrete would carry approximately 0.5 to 1.0 m3 of dead-head concrete in the drum, so you would typically order 1.2 to 1.3 m3 to allow for wastage and overfill.
Related Calculators
Method: Weight = Volume x Density, where Volume = Length x Width x Depth for rectangular sections. Standard concrete density 2,400 kg/m3 per NZS 3109 and industry convention. Fresh concrete adjustment of +2% (to 2,448 kg/m3) reflects typical mix water not yet evaporated. Densities for lightweight and heavyweight mixes are indicative industry values; confirm with your concrete supplier for structural calculations.
This calculator provides indicative estimates for standard concrete mixes. For structural design, use densities confirmed by your concrete supplier and consult a licensed structural engineer. Actual concrete weight varies with aggregate type, water-cement ratio, admixtures, and reinforcement content.
