The Blondel Comfort Rule for Stairs

A 350-year-old rule that still holds

In 1675 the French architect Nicolas-François Blondel proposed that stair comfort depends on a fixed relationship between riser and tread, tied to the length of a human stride on the level. The rule has survived because it works: stairs that obey it feel natural, and stairs that break it feel either cramped or exhausting.

The formula

2 × riser + tread ≈ 24–25 in (550–700 mm)

The logic is that climbing consumes more effort vertically than horizontally — roughly twice as much — so the riser is doubled in the sum. As risers get taller, treads must get shorter to keep the total in range, and vice versa.

The sweet spot

A 7-inch riser with an 11-inch tread gives 2×7 + 11 = 25, the textbook ideal, and a pitch around 32 degrees. A 7.5-inch riser pairs comfortably with a 10-inch tread (25). Combinations that drift far outside 24 to 25 feel wrong underfoot even when they pass code.

Comfort vs code

Code sets the legal envelope; Blondel sets the comfortable one inside it. A stair can be fully code-compliant yet uncomfortable if it ignores Blondel. Aim for both. Explore how different rise and tread pairs score on the rise and run combinations pages.

The remaining gap between code and comfort is where good stair design lives. A staircase can pass every prescriptive limit and still feel punishing if its riser-tread pairing drifts from the Blondel range, which is why experienced builders check the comfort number even when the code box is already ticked.

Reading the comfort number

Compute 2×riser + tread and compare it to the 24 to 25-inch target. A result well below 24 means short risers with deep treads — a shallow, space-hungry stair that makes you break stride. A result well above 25 means tall risers with shallow treads — a steep, tiring climb that feels unsafe descending. The closer to 25, the more natural the rhythm.

Common comfortable pairings

7" riser × 11" tread → 2(7) + 11 = 25 (textbook ideal, ≈32°)
7.5" riser × 10" tread → 25 (slightly steeper, space-efficient)
6.5" riser × 12" tread → 25 (gentle, generous, needs more run)
7.75" riser × 10" tread → 25.5 (at the code limit, acceptable)

When you can't hit the ideal

Tight floor area sometimes forces a pairing outside the sweet spot. If you must compromise, err toward the taller-riser / shallower-tread side only as far as code allows, since a slightly steep stair is more usable than one so shallow it forces an unnatural stride. Better still, a turn or winder can buy back the run needed to reach a comfortable pairing without lengthening the footprint.