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Stair Rise and Run Calculator: Getting the Ratio Right

A stair that feels safe and comfortable to climb comes down to two numbers β€” riser height and tread depth β€” and a formula that keeps them in proportion.

Riser height and tread depth are the two core numbers

Riser height is the vertical height of each step; tread depth (or "run") is the horizontal depth you actually step on. Divide the total vertical rise of the staircase by the number of risers to get riser height, and divide the total horizontal run by the number of treads to get tread depth.

The Blondel formula checks whether the ratio feels natural

2 Γ— riser height + tread depth should land somewhere between about 63 and 65 cm (roughly 24.8 to 25.6 inches). Below that range steps feel cramped and rushed; above it they feel like an awkward stretch with every step.

A taller riser needs a shallower tread, and vice versa

Because the Blondel range is fairly narrow, choosing a taller riser (steeper stairs) forces a shallower tread to stay comfortable, while a shorter riser allows a deeper tread β€” this tradeoff is why steep stairs in old houses often also have narrow treads.

US residential code generally caps risers around 7ΒΎ inches

The International Residential Code (adopted with local amendments across most US jurisdictions) commonly limits riser height to a maximum of about 7ΒΎ inches (196 mm) and requires a minimum tread depth of about 10 inches (254 mm) for interior stairs, though your local building department has final say.

Every step in a flight should match within a small tolerance

Codes typically require riser heights (and tread depths) within a flight to vary by no more than about 3/8 inch (9.5 mm) from step to step, because even a small inconsistency is a common tripping hazard.

Why the Blondel formula still holds up

Devised in the 17th century by French architect François Blondel, the formula is really just a proxy for stride length: a comfortable walking stride covers about 63-65 cm on level ground, and a staircase that respects that same total distance per step feels natural to climb rather than forcing you to shorten or overreach your stride.

Building codes exist because comfort and safety overlap

A ratio outside the safe range is not just uncomfortable, it is a genuine fall hazard, which is why the Blondel guideline was formalized into hard maximum-riser and minimum-tread numbers in most modern building codes rather than left purely as an architectural suggestion.

Frequently Asked Questions

Is the Blondel formula a legal requirement?

No, it is a design guideline, not a code itself. Your local building code sets the actual legal maximum riser height and minimum tread depth, and those legal limits are usually stricter and simpler to check than running the full Blondel formula.

What if my calculated riser height comes out to an odd decimal?

Divide the total rise by a different number of steps until the resulting riser height is a clean, buildable number within code limits β€” stair layouts are typically adjusted this way rather than accepting an oddly precise riser height.