Stair Calculator
Calculate rise, run, stringer length. Flags violations of the 7-11 rule and code limits.
Inputs
Vertical distance from finished floor to finished floor.
Code max is 7.75 in. Aim for 6.5–7.5 in for comfort.
Horizontal depth of each tread. Code min is 10 in.
Minimum 36 in for residential (IRC).
Code Compliance
✓ Passes IRC requirements
Number of Risers
15
Number of Treads
14
Riser Height
7.200in
Total Run
154.0in
Stringer Length
188.10in
Blondel Value (2R + T)
25.40
Ideal comfort range: 24–25 in.
Step by step
Number of risers
round(108 ÷ 7)
= 15
Actual riser height
108 ÷ 15
= 7.200 in
Number of treads
15 risers − 1
= 14
Total run
14 × 11 in
= 154 in (12.83 ft)
Stringer length
√(108² + 154²)
= 188.10 in (15.67 ft)
Blondel comfort rule (2R + T)
2 × 7.20 + 11 = 25.40
= 25.40 ✗ (target 24–25)
How it works
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.
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.