How to Calculate Concrete Stairs
Poured concrete stairs require calculating the volume of the wedge shaped stringer plus the rectangular treads and risers. Accurate estimation prevents ordering too little mid-pour or paying for a premium short-load of ready-mix to finish.
Stair Volume Formula
Break the stair into a rectangular slab plus triangular wedges (one per step). Total volume equals:
Vslab=W×L×Thickness
Vsteps=N×R×T2×W
Where W is stair width, L is total run, N is number of risers, R is riser height, and T is tread depth. Divide by 27 to convert cubic feet to cubic yards.
Code-Compliant Dimensions (IRC R311.7)
| Dimension | Code | Comfortable |
|---|---|---|
| Max riser height | 7-3/4 in | 7 in |
| Min tread depth | 10 in | 11 in |
| Min stair width | 36 in | 42 in |
| Min headroom | 6 ft 8 in | 7 ft |
| Nosing projection | 3/4 to 1-1/4 in | 1 in |
Uniformity Rule (3/8 in)
Within a single flight, the largest and smallest riser or tread cannot differ by more than 3/8 in. This is a life-safety requirement. Set up forms precisely and string between top and bottom to mark each riser.Rebar Requirements
- Main bars - #4 down the slope, at each edge and every 12 in across the width
- Tread bars - #3 or #4 horizontally across each step, 2 per tread minimum
- Nosing bar - one continuous #4 along the front edge of each tread
- Cover - 2 inches minimum from any exposed surface
Formwork Tips
- Use 3/4 inch plywood for riser forms with 2x lumber framing
- Bevel the bottom of each riser form 1/4 in so the trowel reaches the back of the tread
- Stake forms heavily - wet concrete exerts tremendous lateral pressure
- Oil or wax forms before pouring to prevent sticking
Pour From the Bottom Up
Start at the bottom and work upward. A stiffer mix (4 to 5 inch slump, not 6 to 8) holds its shape better on stairs. Have extra workers ready to vibrate and trowel - stairs must be finished immediately to prevent cold joints.Non-Slip Finish Required
Exterior concrete stairs must have a non-slip finish by code. Broom finish (drag a stiff broom perpendicular to travel) is standard. Rock salt or exposed aggregate finishes also work. Never leave concrete stairs smooth troweled.