Stair Calculator
Calculate the number of steps, riser height, tread depth, stringer length and Blondel rule compliance for any floor-to-floor height.
What this calculates
Calculates the number of steps, riser height, tread depth, Blondel rule result, stair angle, horizontal run and stringer (carriage) length for a straight staircase.
Formula used
Steps = round(rise ÷ 0.175). Riser = rise ÷ steps. Tread = 0.63 − 2 × riser (Blondel formula). Horizontal run = steps × tread. Stringer = √(run² + rise²).
Worked example
For a floor-to-floor height of 2.6 m: steps = 15, riser = 17.3 cm, tread = 28.4 cm, Blondel = 63 cm (ideal). Horizontal run = 4.26 m. Stringer ≈ 5.01 m.
Code compliance notes
Most residential codes require: riser 13–20 cm (5–8 in), tread 25–35 cm (10–14 in), Blondel 60–66 cm. Commercial stairs often have stricter limits. Always verify with your local building code.
When not to use this calculator
Spiral, curved, winder or ship stairs require specialized layout calculations. This calculator is for straight-run staircases only.
Frequently asked questions
How many stairs do I need?
Total rise ÷ riser height (typically 17–18 cm), rounded to the nearest whole number. The calculator does this automatically if you leave "number of steps" blank.
What is the Blondel rule?
2 × riser + tread = 60–66 cm (ideal: 63 cm). This ensures a comfortable stride. A result outside this range means the stair will feel too steep or too shallow.
What is a standard riser height?
17–18 cm is the most common residential riser. Maximum is usually 18–20 cm depending on local code.
How is stringer length calculated?
√(horizontal run² + total rise²) — the Pythagorean theorem applied to the stair triangle.
Assumptions
- Tread calculated via Blondel: tread = 0.63 − 2 × riser.
- If steps are not specified, the calculator uses a target riser of 17.5 cm.
- Does not include nosing, landings, handrail height or structural sizing.
- Straight-run staircase only.
Stair dimensions that fall outside the permitted range are flagged in the result. Always verify against your local building code before construction.
Common mistakes
- Rounding step count incorrectly — always round to the nearest whole number, not down.
- Forgetting that the floor itself counts as the top "step".
- Not accounting for finished floor thickness when measuring total rise.