Stair Calculator

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.

How to use this calculator

1. Enter the total rise — the exact floor-to-floor height in meters, measured from finished floor to finished floor (including floor finishes on both levels).
2. Optionally enter the number of steps — leave blank to let the calculator find the optimal count. Entering a number lets you check if a specific step count meets the Blondel rule.
3. Click "Calculate stairs" — the result shows riser height, tread depth, Blondel result, stair angle, horizontal run and stringer length.
4. Values outside acceptable ranges are flagged directly in the result panel.

Building codes — permitted dimensions by region

Residential stair dimensions are regulated to ensure safety. Permitted ranges vary by country:

• USA (IRC R311.7): Riser max 7¾ in (19.7 cm). Tread min 10 in (25.4 cm). Handrail required at 4+ risers. Headroom min 6 ft 8 in (2.03 m).
• UK (Part K): Riser 150–220 mm. Going (tread) 220–300 mm. Max pitch 42°.
• Australia (NCC / AS 1657): Riser 115–190 mm. Going 240–355 mm. Blondel rule 550–700 mm. Private stair: riser 165–200 mm, going 240–355 mm.
• Chile (OGUC Art. 4.1.5): Huella (tread) min 28 cm. Contrahuella (riser) max 18 cm. Blondel 60–66 cm for residential.
• General Blondel rule: 2R + G = 60–66 cm (R = riser, G = going). This is the ergonomic standard used worldwide as a comfort check.

Finished floor thickness — why it matters

The total rise must be measured to the finished floor on both levels, not the structural slab. Tile adds 10–15 mm. Hardwood or laminate adds 7–12 mm. If you measure to the concrete and then add floor finishes later, the top and bottom risers will be a different height from the rest — a serious trip hazard. Always calculate after confirming finished floor thickness on both levels.

Frequently asked questions