A concrete slab’s self-weight is simply its thickness multiplied by the unit weight of concrete. For reinforced concrete, taken as about 25 kN per cubic metre, a 150 mm slab weighs 0.15 times 25, which is 3.75 kN per square metre. That self-weight is the largest single piece of the floor dead load, and it is the first number you set before finishes, services and partitions. StructLoads includes a self-weight helper, but the calculation is short enough to do in your head.
The formula
Self-weight per square metre of floor is the slab thickness in metres times the unit weight of concrete in kN per cubic metre. The unit weight is a density expressed as a force per volume: 25 kN per cubic metre is about 2500 kilograms per cubic metre under gravity. So the whole calculation is one multiplication once you know the thickness and the material.
Unit weight of concrete
Use 25 kN per cubic metre for reinforced concrete and about 24 for plain concrete; the difference is the embedded steel. These are the standard design values, and published concrete properties confirm normal-weight concrete sits in this range. Lightweight concrete is lower, around 18 to 20, and you should use the value your code or supplier states rather than a generic figure when it matters. The 24 plus 1 split is written into Eurocode EN 1991-1-1 itself, as New Steel Construction’s note on the density of concrete explains: Table A.1 lists 24 kN per cubic metre for plain concrete and adds 1 for a normal percentage of reinforcement.
Self-weight by slab thickness
At 25 kN per cubic metre, the self-weight scales straight with thickness.
| Slab thickness | Self-weight at 25 kN/m3 | Self-weight at 24 kN/m3 |
|---|---|---|
| 100 mm | 2.50 kN/m2 | 2.40 kN/m2 |
| 150 mm | 3.75 kN/m2 | 3.60 kN/m2 |
| 200 mm | 5.00 kN/m2 | 4.80 kN/m2 |
| 250 mm | 6.25 kN/m2 | 6.00 kN/m2 |
A 50 mm change in thickness moves the self-weight by more than 1 kN per square metre, so the slab depth is one of the bigger levers on the dead load.
Self-weight is only the start of the dead load
The slab carries itself first, but the floor dead load includes more. Add screed and finishes, the ceiling and services below, and a partition allowance where the layout can change. A bare 150 mm slab at 3.75 kN per square metre commonly becomes about 4.5 kN per square metre once those are included, which is the figure that then enters a structural load takedown. Forgetting the additions is a common way to under-load a floor.
A worked example
Take a 180 mm reinforced slab. Self-weight is 0.18 times 25, which is 4.5 kN per square metre on its own. Add 1.0 for finishes and services and 1.0 for a partition allowance and the floor dead load is 6.5 kN per square metre. Over a 30 square metre interior column tributary area that is 195 kN of dead load per floor, before live load and before any factors, the value that then accumulates down the building in the worked takedown.
Key takeaways: slab self-weight
Self-weight is thickness times unit weight: 25 kN per cubic metre for reinforced concrete, so 3.75 kN per square metre for a 150 mm slab. It is the largest single dead load but never the whole dead load; add finishes, services and partitions before the takedown. StructLoads computes the self-weight from the thickness and carries the combined dead load down, as a preliminary figure a qualified engineer confirms.
Quick answers
How do you calculate the self-weight of a concrete slab?
Multiply the slab thickness in metres by the unit weight of concrete. Reinforced concrete is usually 25 kN per cubic metre, so a 150 mm slab weighs 0.15 times 25, which is 3.75 kN per square metre. Plain concrete is closer to 24. The result is a pressure you add to the floor dead load before finishes and services.
What is the unit weight of reinforced concrete?
Reinforced concrete is commonly taken as 25 kN per cubic metre for design, about 2500 kg per cubic metre. Plain, unreinforced concrete is nearer 24 kN per cubic metre, and the extra in reinforced concrete accounts for the steel. Use your code’s stated value; 24 to 25 covers normal-weight structural concrete.
Is self-weight enough, or what else is dead load?
Self-weight is not enough on its own. It is usually the largest single dead load, but you also add finishes, screed, ceilings, services and a partition allowance, all as dead load. A bare 150 mm slab at 3.75 kN per square metre often becomes about 4.5 kN per square metre once those are included for an office floor.
Does StructLoads add slab self-weight automatically?
StructLoads has a slab self-weight helper: enter the thickness and it returns the self-weight, which you add to the dead load for that level. You still add finishes, services and partitions yourself, because those depend on the project. The tool then carries the combined dead load through the takedown.