What makes a stair comfortable? (the 2R + T rule)
A stair feels natural to climb when two risers plus one tread — 2R + T — total about 24 to 25 inches (the Blondel rule). A 7-inch riser with an 11-inch tread gives 2(7) + 11 = 25 inches, right in the range. It's a comfort rule of thumb, not code, but a stair that meets both the IRC limits and 2R + T rarely feels too steep or too shallow.
Source: Blondel rule of thumb (not a code value) · Verify against your AHJ and the adopted code edition.
Code sets the limits on risers and treads, but comfort comes from their relationship. The classic guide is the Blondel rule: two risers plus one tread should total about 24–25 inches.
2R + T ≈ 24–25″
It captures how a person’s stride shortens as they climb: a taller riser wants a shorter tread, and vice versa. Examples that land in the range:
- 7″ riser + 11″ tread → 2(7) + 11 = 25″
- 7½″ riser + 10″ tread → 2(7.5) + 10 = 25″
This is a rule of thumb, not a requirement — a stair can be code-compliant and still feel awkward if 2R + T drifts far outside the band. The sweet spot is a stair that satisfies both the IRC riser/tread limits and 2R + T.
Work it out: the stair calculator reports 2R + T alongside the riser height, tread, and pitch, and flags when it falls outside the comfortable range.