What is the maximum riser height allowed?

The IRC caps residential risers at 7.75 inches and requires a minimum tread depth of 10 inches. It also requires that the tallest and shortest riser in a flight differ by no more than 3/8 inch. Local codes can be stricter.

Why is there one fewer tread than riser?

Because the upper floor is the surface you step onto at the top, so it acts as the final tread. A flight with 14 risers has 13 physical treads.

What is the 2R + T rule?

Blondel's comfort rule: twice the riser height plus the tread depth should land between 24 and 25 inches. It reflects the natural length of a human stride and is a good sanity check on a layout that meets code but feels awkward.

Stair Rise and Run Calculator

Work out riser height, tread count and total run from a total rise, with IRC code checks.

Draft — figures checked, awaiting an editorial read. Not indexed.

Number of risers— risers

Riser heightEvery riser must be within 3/8 in of the others.: — in
Number of treadsAlways one fewer than risers.: — treads
Total runHorizontal floor space the stair occupies.: — in
Minimum stringer length: — in
IRC check: —
Comfort rule (2R + T): —

Formula: Risers = round(total rise ÷ preferred riser). Riser height = total rise ÷ risers. Treads = risers − 1. Total run = treads × tread depth.

Stair geometry is life-safety construction and is regulated. The IRC checks here are a planning aid, not a code compliance review — confirm riser, tread, headroom and handrail requirements with your local building department before building.

What this calculates

This lays out a straight flight of stairs from the one dimension you cannot change: the total rise, floor to floor. Everything else follows from it. The calculator finds the riser count that lands closest to your preferred riser height, divides the total rise evenly across those risers, and reports the tread count, the total horizontal run and the stringer length. It also checks the layout against the IRC limits and against the Blondel comfort rule, because a stair can satisfy code and still feel wrong underfoot.

How it works

Divide the total rise by your preferred riser height and round to the nearest whole number — that is the riser count, and it must be a whole number because every riser in a flight has to be the same height. Dividing the total rise back by that count gives the actual riser height. There is always exactly one fewer tread than riser, since the upper floor itself serves as the last surface you step onto. Total run is the tread count times the tread depth, and the stringer length is the hypotenuse of the rise and run triangle.

Worked example

A total rise of 108 inches — a typical 9 foot floor-to-floor — with a preferred riser of 7.5 inches. 108 ÷ 7.5 = 14.4, which rounds to 14 risers. The actual riser height is then 108 ÷ 14 = 7.714 inches, comfortably under the 7.75 inch IRC maximum. That gives 13 treads, and at 10.5 inches each the stair occupies 136.5 inches of floor — well over eleven feet, which is the number that surprises people. The stringer must span √(108² + 136.5²) = 174 inches.

Common mistakes

Measuring the total rise from subfloor to subfloor rather than finished floor to finished floor. If one level gets tile and the other gets carpet, the difference lands entirely in the top or bottom riser, and an uneven riser at the end of a flight is both a code violation and a genuine trip hazard. Second, underestimating the run — nearly everyone is surprised by how much floor space a stair needs. Third, forgetting that tread depth is measured nosing to nosing, not the width of the board.

Common questions

What is the maximum riser height allowed?: The IRC caps residential risers at 7.75 inches and requires a minimum tread depth of 10 inches. It also requires that the tallest and shortest riser in a flight differ by no more than 3/8 inch. Local codes can be stricter.
Why is there one fewer tread than riser?: Because the upper floor is the surface you step onto at the top, so it acts as the final tread. A flight with 14 risers has 13 physical treads.
What is the 2R + T rule?: Blondel's comfort rule: twice the riser height plus the tread depth should land between 24 and 25 inches. It reflects the natural length of a human stride and is a good sanity check on a layout that meets code but feels awkward.