Understanding the comfort guidance flag
This calculator flags an 'ok' or 'caution' result using commonly cited residential comfort guidance — not a specific code citation, since exact riser and going limits vary by jurisdiction and application (interior vs. exterior stairs, residential vs. commercial, egress stairs).
| Parameter | Commonly cited residential comfort range |
|---|---|
| Riser height | ≈150–190 mm |
| Tread depth (going) | ≥250 mm |
| Blondel comfort rule | 2 × riser + going ≈ 630 mm (historic rule of thumb, not a code limit) |
- This calculator produces a geometric, comfort-guideline estimate only. Actual stair design — minimum and maximum riser height, minimum going, headroom clearance, handrail and guard requirements, and winder or landing rules — is governed by the applicable local building code, and any staircase design should be reviewed by a qualified professional (architect, structural engineer or building inspector) before construction.
- Blondel's rule (2R + G ≈ 630 mm) is a long-standing architectural comfort guideline, not a universal or code-mandated formula; different codes and design guides cite somewhat different comfort ranges and formulas.
What does a stair calculator do?
A stair calculator works out how many risers and treads a straight staircase needs to cover a given total rise (floor-to-floor height), then applies a classic stair-comfort rule to derive a matching tread depth (going) and the resulting stringer (diagonal support) length. It starts from a preferred riser height, rounds to a whole number of equal risers, and recalculates the actual riser height so that all risers in the flight are equal, which is a basic safety requirement for stairs.
Tread depth is derived using Blondel's rule, a stair-comfort formula (2 × riser height + tread depth ≈ 630 mm) dating to 17th-century French architecture and still widely cited as a starting point for comfortable stair proportions, though it is a rule of thumb rather than a building-code requirement. This calculator flags whether the resulting riser height and tread depth fall within commonly cited residential comfort ranges, but the binding requirements for any real staircase — minimum/maximum riser height, minimum going, headroom, handrail and guard requirements — are set by the applicable local building code.
How to use this stair calculator
- Enter the total rise — the vertical floor-to-floor (or floor-to-landing) height the staircase needs to cover, in millimeters.
- Enter your preferred riser height, in millimeters — a typical residential starting point is around 175–180 mm.
- Read the calculated number of risers and the actual riser height once the total rise is divided evenly.
- Check the resulting tread depth (going), total run and stringer length, and note whether the result is flagged as within or outside common comfort guidance before finalizing the design.
The formula behind stair geometry
The number of risers equals the total rise divided by the preferred riser height, rounded to the nearest whole number (a minimum of 1). The actual riser height is then recalculated as total rise ÷ number of risers, so that every riser in the flight is exactly equal. The number of treads equals the number of risers minus 1 (a straight flight has one fewer tread than riser). Tread depth follows Blondel's rule: 630 − 2 × actual riser height. Total run equals the number of treads × tread depth, and stringer length is the hypotenuse of total rise and total run.
Worked example: a total rise of 2,700 mm with a preferred riser of 180 mm gives risers = round(2700 ÷ 180) = 15, an actual riser height of 2700 ÷ 15 = 180 mm exactly, and 14 treads. Tread depth by Blondel's rule is 630 − 2 × 180 = 270 mm, giving a total run of 14 × 270 = 3,780 mm and a stringer length of √(2700² + 3780²) ≈ 4,645 mm.
Common mistakes
- Treating this comfort-guideline check as equivalent to a local building code compliance check — always confirm the exact riser, going, headroom and handrail requirements in your jurisdiction.
- Building unequal risers within the same flight, which is a well-recognized trip hazard and a near-universal building code violation.
- Ignoring headroom clearance above the stair nosing line, which this calculator does not check and which is a separate code requirement.
- Skipping review by a qualified professional for a real staircase build, especially where the stair serves as a required means of egress.
Perguntas frequentes
How do I calculate the number of stair risers?
Divide the total rise (floor-to-floor height) by your preferred riser height and round to the nearest whole number. For a 2,700 mm total rise and a 180 mm preferred riser, that's round(2700 ÷ 180) = 15 risers, with the actual riser height then recalculated as 2700 ÷ 15 = 180 mm so every riser is equal.
What is Blondel's rule for stairs?
Blondel's rule is a historic stair-comfort formula stating that twice the riser height plus the tread depth (going) should equal roughly 630 mm. It is a design rule of thumb, not a building code requirement, and different sources cite slightly different target values.
What is a comfortable riser height and tread depth for stairs?
Commonly cited residential comfort guidance suggests a riser height of roughly 150–190 mm and a tread depth (going) of at least about 250 mm, though the specific minimum and maximum values that apply to a real staircase are set by the local building code.
Why does this calculator show a caution flag?
A caution flag means the calculated riser height or tread depth falls outside the commonly cited residential comfort range used by this calculator. It is an advisory guideline only — always confirm the final stair geometry against your local building code before construction.
Do I need a professional to design a staircase?
Yes, for any real build. This calculator provides a geometric estimate and comfort-guideline check only; final stair design, including code-compliant riser and going limits, headroom, handrails and guards, should be confirmed by a qualified professional such as an architect, structural engineer or building inspector.
Referências
- Blondel, François (17th century) — historic architectural stair-comfort formula (2 × riser + going ≈ 630 mm), still widely cited in stair design references.
- International Code Council (ICC) — International Residential Code (IRC) / International Building Code (IBC), stair provisions setting minimum/maximum riser height, minimum going, headroom and handrail requirements.
- General residential stair-design comfort guidance cited across architectural and building-trade references for riser/going ranges.