Riser and tread: the basic vocabulary
A riser is the vertical face of a single step, and riser height is the vertical distance climbed from one tread to the next; a straight staircase's total rise (the full floor-to-floor or floor-to-landing height) divided evenly among all its risers gives the height of each individual riser. A tread is the horizontal surface a foot lands on, and tread depth — also called the 'going' — is the horizontal distance from the front (nosing) of one tread to the front of the next, measured along the direction of travel.
For safety, every riser in a single flight of stairs must be equal in height, and every tread must be equal in depth; even a small inconsistency between one step and the next is a well-documented tripping hazard, which is why stair calculations start by dividing the total rise evenly across a whole number of risers rather than using a fixed riser height that might leave an uneven final step.
Blondel's rule for stair comfort
Blondel's rule is a stair-comfort formula dating to 17th-century French architecture, expressed as 2 × riser height + tread depth ≈ 630 mm (approximately 24.5 inches). It captures the practical observation that a comfortable stair balances riser height against tread depth — a taller riser should be paired with a shallower tread and a shorter riser with a deeper tread — because the total distance a foot travels per step, in a combined vertical-and-horizontal sense, tends to feel comfortable within a fairly narrow band around this value.
Blondel's rule remains widely cited today as a useful starting point for choosing tread depth once a riser height has been set, but it is a historical rule of thumb rather than a building-code requirement, and it does not by itself guarantee a stair meets any specific jurisdiction's minimum or maximum riser and tread limits.
Common residential comfort ranges
Commonly cited residential stair-comfort guidance places riser height in a range of roughly 150 to 190 mm and tread depth (going) at 250 mm or more, figures that are frequently referenced in residential stair design discussion as a comfortable, walkable balance for everyday stairs. A riser at the shorter end of this range paired with a deeper tread tends to feel gentler and safer for a wide range of users, while a riser near the taller end with a shallower tread makes for a steeper, more compact stair that uses less floor space but demands a larger stride.
These figures are advisory ranges, not binding limits: the applicable local building code sets the actual minimum and maximum riser height, minimum tread depth (going), headroom clearance, handrail and guard requirements for a given jurisdiction and application, and those code limits can differ from the commonly cited comfort range described here, particularly for exterior stairs, commercial egress stairs, or stairs serving a specific occupancy type.
Worked example: a 2,700 mm total rise
Consider a staircase covering a 2,700 mm total rise, with a preferred riser height of 180 mm as a starting point. The number of risers is the total rise divided by the preferred riser height, rounded to the nearest whole number: 2,700 ÷ 180 = 15 exactly, so 15 risers are used, and the actual riser height is recalculated as 2,700 ÷ 15 = 180 mm — in this case matching the preferred value exactly because the total rise divides evenly.
A straight flight has one fewer tread than riser, so this stair has 15 − 1 = 14 treads. Applying Blondel's rule, tread depth is 630 − 2 × 180 = 270 mm, giving a total run (the total horizontal distance the stair occupies) of 14 × 270 = 3,780 mm. The stringer — the diagonal structural member the treads and risers are cut into — spans the hypotenuse of the total rise and total run: √(2,700² + 3,780²) ≈ 4,645 mm.
| Step | Calculation | Result |
|---|---|---|
| Number of risers | round(2,700 mm ÷ 180 mm) | 15 |
| Actual riser height | 2,700 mm ÷ 15 | 180 mm |
| Number of treads | 15 − 1 | 14 |
| Tread depth (Blondel's rule) | 630 − 2 × 180 | 270 mm |
| Total run | 14 × 270 mm | 3,780 mm |
| Stringer length | √(2,700² + 3,780²) | ≈4,645 mm |
Checking the result against comfort guidance
The worked example's 180 mm riser falls within the commonly cited 150-190 mm residential comfort range, and its 270 mm tread depth exceeds the commonly cited 250 mm minimum, so this particular geometry sits within general residential comfort guidance. Changing the preferred riser height changes every downstream figure: a shorter preferred riser increases the number of risers and, through Blondel's rule, increases tread depth and therefore total run, producing a gentler but longer stair for the same total rise.
When a calculated riser or tread falls outside the commonly cited comfort range, the usual adjustments are to change the preferred riser height (which changes the whole-number rounding of risers) or, where the design allows, to change the total rise available for the stair — but any such adjustment should still be checked against the applicable local building code's actual riser, tread, headroom and handrail requirements, not only against the advisory comfort range described here.
Codes govern final design
This calculation produces a geometric, comfort-guideline estimate only — it does not address headroom clearance above the stair, handrail and guard requirements, winder or landing rules for stairs that change direction, or the specific minimum and maximum riser and tread limits set by the applicable local building code, all of which vary by jurisdiction and by whether the stair is interior, exterior, residential or serves as a required means of egress.
Any staircase design intended for construction should be reviewed against the applicable local building code and, for anything beyond a simple straight residential flight, by a qualified professional such as an architect, structural engineer or building inspector before the stringers are cut.
Questions fréquentes
What is Blondel's rule for stairs?
Blondel's rule is a historical stair-comfort formula, 2 × riser height + tread depth ≈ 630 mm, dating to 17th-century French architecture. It is widely used today as a starting point for choosing a comfortable tread depth once riser height is set, but it is a rule of thumb, not a building-code requirement.
What is a comfortable riser height for residential stairs?
Commonly cited residential comfort guidance places riser height around 150 to 190 mm, with tread depth (going) of 250 mm or more. These are advisory ranges frequently referenced in stair design discussion; the applicable local building code sets the actual binding minimums and maximums.
How do I calculate the number of risers for a staircase?
Divide the total rise by a preferred riser height and round to the nearest whole number, then recalculate the actual riser height as total rise divided by that number of risers, so every riser in the flight is exactly equal. A 2,700 mm total rise with a 180 mm preferred riser gives exactly 15 equal risers.
What is the difference between a riser and a tread?
A riser is the vertical face of a step, and riser height is the vertical distance between one step and the next. A tread is the horizontal surface a foot lands on, and tread depth (going) is the horizontal distance from the front of one tread to the front of the next.
Does Blondel's rule guarantee a stair meets building code?
No. Blondel's rule is a historical comfort guideline, not a code requirement. The applicable local building code sets the actual binding riser height, tread depth, headroom, handrail and guard requirements, and any stair design should be checked against that code and reviewed by a qualified professional before construction.
Références
- International Code Council (ICC) — International Residential Code (IRC), stairway provisions governing riser height, tread depth, headroom and handrail requirements (general reference; exact limits vary by jurisdiction and code edition adopted).
- American Wood Council (AWC) — general residential stair framing and stringer terminology conventions.
- Blondel F. Cours d'architecture. Paris, 17th century — origin of the classic 2R + T stair-comfort proportion still cited in stair design discussion today.
- Standard Pythagorean relationship (rise/run right-triangle geometry) underlying stringer length calculation.