How the stair model works
Picture the stairs as a stack of boxes, all the same width. The bottom step is one rise tall. The next is two rises tall, then three, and so on. Each is one run deep. A landing at the top is a box as tall as the full stair and as deep as you enter.
This model assumes solid concrete down to grade, with no void or fill underneath. Stairs built over compacted fill or with a sloped underside use less concrete. Check the model against a worked example before relying on it .
Worked example: 3 steps, 4 ft wide, 7 in rise, 11 in run
- Run and rise in feet: 11 ÷ 12 = 0.917 ft, and 7 ÷ 12 = 0.583 ft.
- n(n+1) ÷ 2 for 3 steps: 3 × 4 ÷ 2 = 6.
- Volume: 4 × 0.917 × 0.583 × 6 = 12.83 ft³ = 0.48 yd³.
- 80 lb bags: 12.83 ÷ 0.60 = 21.4, so 22 bags.
- Add a 36 in top landing: 4 × 3 × (3 × 0.583) = 21.0 ft³ more, for 33.83 ft³ (1.25 yd³) in total.
Typical rise and run
Residential codes often cite a maximum riser of about 7¾ in and a minimum tread of about 10 in, with uniform steps . Outdoor steps are often built gentler, such as a 6–7 in rise and a 12–14 in run, for comfort and safety in wet weather.
More stair tips
- Steps that wrap around a corner or have side walls need extra sections. Add them as walls in the main concrete calculator.
- Small stairs are usually a bag pour. The concrete bag calculator lists 40, 60, and 80 lb counts.
- A footing under the bottom step may be required in cold climates. Add it with the concrete footing calculator.
Volume only. Stair design, reinforcement, and code compliance are not checked.