IRC Stair Code Requirements
| Dimension | Minimum | Maximum |
|---|---|---|
| Rise (height) | 4" | 7¾" |
| Run (depth) | 10" | — |
| Headroom | 6'8" | — |
| Width | 36" | — |
| Handrail height | 34" | 38" |
Stair Calculator
Calculate the exact rise, run, number of steps, and stringer length for any staircase. Ensures code-compliant dimensions so your stairs are safe and meet building requirements.
Stair Design Calculator
Frequently Asked Questions
What is the comfortable stair formula?
The Blondel formula: 2 × Rise + Run = 24–25 inches. With a 7" rise, the ideal run is 10–11". A 7" rise with 11" run gives 2×7+11 = 25 — perfect. This formula originated from French architect François Blondel in 1672 and matches the average human stride when climbing stairs.
How do I calculate stringer length?
Stringer length = √(Total Rise² + Total Run²). It's the hypotenuse of a right triangle formed by the total rise and total run. This is the minimum board length needed for a stringer (the angled support board). Always buy longer boards and cut to fit, accounting for notches and landing dimensions.
Rise and run
Every staircase is defined by two measurements: the rise (vertical height of each step) and the run (horizontal depth of each tread).
Number of risers = total rise ÷ target riser height
Actual riser = total rise ÷ number of risers
Round the first result to a whole number, then divide back to get the exact riser height. Every riser in a flight must be equal — uneven risers are both a code failure and a genuine trip hazard, because people climb stairs by rhythm rather than by looking.
Worked example
A total rise of 112 inches with a target riser around 7.5 inches:
- 112 ÷ 7.5 = 14.9, so use 15 risers
- 112 ÷ 15 = 7.47 inches per riser
- 15 risers means 14 treads, since the top landing is not a tread
Comfort rules of thumb
Carpenters use several long-standing relationships to check a staircase will feel right:
- Rise + run ≈ 17 to 18 inches
- (2 × rise) + run ≈ 24 to 25 inches
- Rise × run ≈ 70 to 75
A flight that satisfies these feels natural underfoot. One that violates them feels cramped or stretched even when it meets minimum code.
Codes and headroom
Building codes set maximum riser heights, minimum tread depths, minimum headroom, handrail requirements and limits on how much risers may vary within a flight. These differ by jurisdiction and between residential and commercial work.
Stair geometry is one of the most commonly cited items in inspections, and rebuilding a non-compliant flight is expensive. Confirm your local requirements before cutting stringers.