A step-by-step guide to calculating stair layout, from measuring total rise to cutting your first stringer.

Step 1: Measure the Total Rise

The total rise is the vertical distance from the finished surface of the lower floor to the finished surface of the upper floor. This is the single most important measurement — every other stair dimension derives from it.

Measure carefully and in multiple spots. Floors are rarely perfectly level, so take the measurement at the actual stair location. Include any finish flooring that hasn't been installed yet (hardwood, tile, carpet pad).

Step 2: Calculate the Number of Risers

Divide the total rise by your target riser height. Most carpenters target 7" to 7-1/2"for comfort. The IRC maximum is 7-3/4".

Example: Total rise = 108"

108 ÷ 7.5 = 14.4 → round to 15 risers

Actual riser height = 108 ÷ 15 = 7.2" (7-3/16")

Always round to a whole number — you can't have a fraction of a riser. Then divide the total rise evenly by that number to get the actual riser height. Every riser must be the same height (within 3/8" per IRC).

Step 3: Determine the Number of Treads

For standard mount stairs (the most common type), the number of treads is one less than the number of risers. The upper floor itself acts as the final step — you don't need a tread board at the top.

15 risers → 14 treads

Step 4: Calculate the Total Run

The total runis the horizontal distance the stair covers. Multiply the number of treads by the tread depth. The IRC minimum tread depth is 10".

14 treads × 10" = 140" total run (11'-8")

Step 5: Check the Comfort Rule

The rise + run rulesays the riser height plus the tread depth should total 17" to 18". This isn't code, but it's a widely trusted guideline for comfortable stairs.

7.2" + 10" = 17.2" &check; (within 17-18 range)

Step 6: Calculate the Stringer Length

The stringer is the diagonal board that supports the treads and risers. Its length is the hypotenuse of the rise and run triangle:

Stringer = √(rise² + run²) = √(108² + 140²) = 176.8" (14'-9")

Use a 2x12 for the stringer. After cutting the notches, you need at least 3-1/2" of solid wood remaining at the narrowest point.

Step 7: Account for the First Step

The first step height is different from all other risers because of the tread thickness. The bottom of the stringer sits on the floor, but the first tread board adds thickness on top. To compensate, you subtract one tread thickness from the bottom of the stringer(this is called "dropping the stringer").

Typical riser = 7.2"

Tread thickness = 1.5" (for a 2x tread board)

First step height = 7.2" - 1.5" = 5.7" (5-11/16")

Drop the stringer 1.5" so the first step equals all others after adding the tread.

Use the Calculator

The stair stringer calculator does all of this math automatically and shows a real-time diagram of your stringer profile. Enter your total rise and it calculates everything — risers, treads, stringer length, stair angle, stairwell opening, and code checks.

How to Calculate Stairs