Stair Concrete Volume Calculator

How is stair concrete calculated?

A straight reinforced concrete flight has three parts: a sloping waist slab underneath, the step triangles on top and the landing that connects the flights. The concrete volume is the sum of the three.

• Waist slab: first find the slope length. The diagonal of one step is √(riser² + going²) and the slope length is that times the number of steps. The slab volume is slope length × waist thickness × flight width.
• Steps: each step has a right triangle section. The volume is number of steps × (riser × going / 2) × flight width.
• Landing: plan area × thickness.

Example

A flight of 10 steps with a 15 cm riser and 20 cm going, 1 m wide, has a step diagonal of 25 cm and a slope length of 2.5 m. With a 12 cm waist the slab is 2.5 × 0.12 × 1 = 0.30 m³, the steps are 10 × (0.15 × 0.20 / 2) × 1 = 0.15 m³ and the total is 0.45 m³.

A common mistake

Multiplying the waist thickness by the horizontal length underestimates the volume. The slab slopes, so the slope length is used. Forgetting the step triangles lowers the total by roughly a third.

Limits

The tool is for straight flights on a waist slab. Winding and helical stairs have tapered steps, so this formula does not apply. Take the waist thickness and reinforcement from the structural design.