How to Calculate Stairs: Rise, Run and Number of Steps

A comfortable staircase depends on getting three numbers right: the total rise, the riser height of each step and the tread depth. Get them wrong and the stairs feel steep, awkward or may not pass inspection.

Key terms

• Total rise: the vertical height from the lower finished floor to the upper finished floor.
• Riser height: the height of each step.
• Tread depth: the horizontal depth of each step, front to back.
• Total run: the horizontal length the staircase covers.

Common code limits (US residential)

The International Residential Code (IRC), used as the basis for many US local codes, sets a maximum riser height of 7¾ in and a minimum tread depth of 10 in. Local codes can differ, and other countries have their own rules, so always check with your local building department.

Step-by-step calculation

1. Measure the total rise between finished floor levels. Example: 108 in.
2. Divide by a target riser height of about 7.5 in: 108 ÷ 7.5 = 14.4.
3. Round up to get the number of risers: 15.
4. Find the actual riser height: 108 ÷ 15 = 7.2 in. That’s under the 7¾ in limit.
5. Number of treads is usually one less than the number of risers: 14.
6. Choose a tread depth, for example 10.5 in.
7. Total run = 14 × 10.5 = 147 in (12 ft 3 in).

The comfort rule

A widely used guideline is that 2 × riser + tread should be about 24–25 in. In the example: 2 × 7.2 + 10.5 = 24.9 in, which is comfortable.

Stringer length

The stringer is the diagonal board that supports the steps. Its length is roughly the hypotenuse of the rise and run:

Stringer ≈ √(total rise² + total run²) = √(108² + 147²) ≈ 182 in (about 15 ft 2 in). Buy longer boards to allow for cuts at the top and bottom.

Common mistakes

• Measuring to the subfloor instead of the finished floor, which makes the top or bottom step a different height.
• Uneven risers. Codes typically limit the variation between the tallest and shortest riser to ⅜ in.
• Forgetting headroom. Many codes require at least 6 ft 8 in of clear headroom above the stair.

Use the calculator

The Stair Calculator works out risers, treads, total run and stringer length in feet or metres. For deck stairs, see the Deck Calculator and our guide to deck boards for a 12×12 deck.

This guide is for planning. Structural work should follow local codes and, where required, be inspected.