How to Calculate a Staircase: Steps, Riser, Tread & the Blondel Rule (2026 Guide)
A staircase is both a structural and an architectural element: a small error in the riser height or the tread width turns a comfortable stair into a daily source of tripping and fatigue — and may violate the code and fail inspection. Calculating a staircase accurately guarantees that every step is equal, comfortable, and safe, and that the flight fits the available space without an awkward odd last step. In this guide you will learn step by step: how to compute the number of steps from the floor height, how to distribute it into a comfortable riser and tread, what the Blondel rule (2R+T) that links them is, how the stair slope angle is measured, and the limits of the Saudi SBC 1001 code and the international codes (IBC, BS, Eurocode). Every value here matches exactly the free Staircase Calculator on “Site Engineer” — verify it yourself in seconds.
🧮 The Four Staircase Formulas
- Number of steps: n = round(floor height ÷ preferred riser) — the preferred riser is first clamped within the code limits.
- Riser height: R = floor height ÷ n
- Tread width (going): T = horizontal run ÷ (n − 1)
- Blondel rule: 2R + T (comfortable range 60–65 cm in the Saudi code)
- Slope angle: θ = arctan(height ÷ run) in degrees
Why divide by (n − 1) for the tread? Because the number of treads is always one less than the number of risers — the top step reaches the upper floor level and has no tread of its own. The Blondel rule is the comfort metric: twice the riser plus the tread must fall within a narrow range so the climbing step feels natural — neither exhausting nor dangerous.
📋 Steps to Calculate a Staircase
- Determine floor height and available run: Measure the clear floor-to-floor height from one floor level to the one above, and the available horizontal run for the stair, in centimeters.
- Choose the code and preferred riser: Select the engineering code (SBC / IBC / BS / Eurocode) and set the preferred riser height (17 cm is a comfortable standard value); it is automatically clamped within the code limits.
- Calculate the number of steps: Divide the floor height by the preferred riser and round to the nearest integer: n = round(height ÷ riser).
- Calculate the actual riser and tread: Riser = height ÷ n, and tread = run ÷ (n − 1). Remember that the number of treads is one less than the number of risers.
- Check the Blondel rule and the angle: Compute 2×riser + tread and confirm it is within 60–65 cm, and compute the slope angle θ = arctan(height ÷ run), confirming it is within 25°–40° for the Saudi code.
- Compare with code limits and finalize: Verify that the riser, tread, Blondel value, and angle are all within the chosen code limits; an “excellent” rating means all four match together. Adjust the run or the preferred riser if any value falls outside.
✅ Worked Example
An internal stair with a floor height of 300 cm (3.0m) and a horizontal run of 480 cm (4.80m), preferred riser 17cm, under the SBC 1001 code:
The riser (15–19), tread (25–30), Blondel (60–65), and angle (25°–40°) are all within the SBC 1001 limits → four passing checks = “Excellent.” Enter the same inputs in the Staircase Calculator and you'll get the exact same result — these values come from the calculator's own formulas.
📊 Stair Dimension Limits Across the Four Codes (cm / degrees)
| Code | Riser (cm) | Tread (cm) | Blondel (cm) | Angle | Min width (cm) |
|---|---|---|---|---|---|
| SBC 1001 | 15–19 | 25–30 | 60–65 | 25°–40° | 100 |
| IBC 2021 | 10.2–19.4 | 25.4–35.6 | 58–68 | 20°–42° | 91.4 |
| BS 5395 | 15–22 | 22–32 | 60–66 | 25°–38° | 100 |
| Eurocode | 14–21 | 24–35 | 60–66 | 20°–40° | 100 |
🛡️ The Saudi Angle: SBC 1001 Stair Dimensions
The Saudi SBC 1001 code (architectural building and means-of-egress requirements) sets the dimensions of a safe, comfortable stair: riser 15–19 cm, tread 25–30 cm, Blondel rule 60–65 cm, and a minimum stair width of 100 cm for main circulation paths, within a comfortable slope of 25°–40°. The most comfortable standard values are a riser of about 17cm and a tread of about 28cm.
Execution tips: keep every step exactly equal (do not absorb the height difference in the last step alone); provide a vertical headroom of at least 200–210 cm above the nosing line; add a protruding nosing of 2–3 cm to increase the effective tread depth without extending the run; and install a handrail at 90–100 cm height. For long flights, add a landing every 12–16 steps for comfort and safety.
🚫 Common Staircase Calculation Mistakes
- Unequal steps: leaving the height difference to the last step violates safety — distribute the height equally across all steps.
- Confusing riser count with tread count: the number of treads = number of risers − 1, since the top step has no tread. Use (n − 1) for the tread.
- Ignoring the Blondel rule: a riser and tread each within limits does not guarantee comfort; 2R+T together must fall within 60–65 cm.
- Neglecting headroom: a dimensionally correct stair can still hit your head on the ceiling; provide 200–210 cm above the nosing line.
- Insufficient stair width: less than 100cm obstructs movement and egress on main paths per the Saudi code.