Stair Calculator
Design a code-compliant staircase: rise, run, stringer length and headroom.
About Stair Calculator
The stringer is the inclined structural member supporting the treads. Its length is the hypotenuse of the right triangle formed by the total rise and total run (Pythagorean theorem). The number of risers is calculated by dividing the total floor-to-floor height by the desired riser height, then rounding to keep all risers equal — unequal risers are a trip hazard. IRC code caps riser height at 7.75 in and requires tread depth ≥ 10 in. Blondel's rule (2R + T = 24–25) is the ergonomic comfort check used by European and many North American authorities.
Formulas
Actual riser height
Riser = total rise ÷ number of risers
Stringer length
Stringer = √(total rise² + total run²)
Blondel's comfort rule
2 × riser + tread depth should be between 24 and 25 inches.
Worked example
With the default values below, Stair Calculator returns Code Compliance of ✓ Passes IRC requirements. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Calculate fromone of Total rise (let me set step dimensions), Number of steps (I'll set total rise)
- Total rise (let me set step dimensions)
- Total Rise (floor-to-floor height)minimum 6 in
- 108 in
- Desired Riser Heightaccepts 4–8.25 in
- 7 in
- Number of Risersaccepts 1–100
- 14
- Tread Depth (Run)minimum 6 in
- 11 in
- Stair Widthminimum 24 in
- 36 in
Results
- Code Compliance
- ✓ Passes IRC requirements
- Number of Risers
- 15
- Number of Treads
- 14
- Riser Height
- 7.200 in
- Total Run
- 154.0 in
- Stringer Length
- 188.10 in
- Blondel Value (2R + T)Ideal comfort range: 24–25 in.
- 25.40
Step by step
- 1Number of risers= 15
- 2Actual riser height= 7.200 in
- 3Number of treads= 14
- 4Total run= 154 in (12.83 ft)
- 5Stringer length= 188.10 in (15.67 ft)
- 6Blondel comfort rule (2R + T)= 25.40 ✗ (target 24–25)
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is the 7-11 rule for stairs?
The informal '7-11 rule' says riser height should be ≤ 7 in and tread depth ≥ 11 in. The IRC code is slightly more lenient (7.75 in riser, 10 in tread), but the 7-11 rule gives a comfortable stair that most people find ergonomically natural.
Why does tread count equal risers minus one?
The first 'step' is the finished floor itself; the last 'step' is the landing. Between two floors with n risers, there are n−1 intermediate treads.
What is Blondel's rule?
The 17th-century architect François Blondel found that a comfortable step satisfies 2R + T ≈ 24–25 in, matching average human stride. A stair with 7 in risers and 11 in treads scores 2(7) + 11 = 25 — exactly in range.
What is the minimum stair width in residential construction?
IRC requires a minimum clear width of 36 inches. Handrails may project into this width by up to 4.5 inches per side.