Stair calculator

How the stair calculator works

The stair calculator needs only two measurements, the total rise and the tread depth you want; everything else is arithmetic. It works in five steps:

1. Measure the total rise from the finished lower floor to the finished upper floor, plumb, with a level and tape. Include flooring that is not down yet.
2. Count the risers. Divide the rise by the code's maximum riser and round up. 8′ 9″ ÷ 7-3/4 in = 13.55, so 14 risers.
3. Size each riser. Divide the rise by that count: 105 ÷ 14 = 7-1/2″. Every riser in the flight must match within 3/8 in under the IRC^[1].
4. Count the treads. A straight flight has one tread fewer than risers, because the upper floor is the last step: 13 treads.
5. Find the run. Treads × tread depth: 13 × 10-1/2 in = 11′ 4-1/2″ of floor space, face of the first riser to face of the last.

Under More options you can also enter the floor space you have, to check the run fits, and the nosing, to get the depth of each tread board. If the run is too long for the room, try a stair rise and run calculator that starts from the space you have, or split the climb with a stair calculator with landing. For outdoor steps off a deck, the deck stair calculator builds the treads from deck boards.

Stair code limits in the US, UK, Canada and Australia

The riser maximum, tread minimum and any rise-to-going rule decide how many steps a stair needs. These are the dwelling (private) stair limits the calculator checks:

The IRC is a model code that states and cities adopt, often with amendments^[9]. The 2024 IRC moved stairways from R311.7 to R318.7 without changing these numbers^[2]. Check which edition your building department enforces.

Stair calculator rise and run: a worked example in metric

For a 2,700 mm floor-to-floor height in Australia, the maximum riser is 190 mm, so 2,700 ÷ 190 = 14.2 rounds up to 15 risers of 180 mm. With a 250 mm going, 2R + G = 610 mm, inside the 550–700 mm band^[6]. The 14 treads take 3,500 mm of floor, and 15 risers stay under the 18-riser limit for one flight.

The same height under the IRC gives 2,700 ÷ 196.85 = 13.7, so 14 risers of 193 mm. Different codes often give different counts for the same opening, which is why the calculator lets you pick the code.

Stair calculator with diagram: what the drawing shows

The stair calculator with diagram draws your flight to scale as you type. The wood shape is one cut stringer on the board width you chose, with the treads laid on top. Two details are easy to miss:

• The drop. The first riser on the stringer is cut short by the tread thickness, so every finished step comes out equal once the treads are on.
• The throat. The wood left under the notches. A 2x12 with 7-1/2″ × 10 in steps keeps 5-1/4″; deeper steps eat more. The stair stringer calculator gives the layout marks.

Headroom and the stairwell opening

The opening in the floor above must be long enough that nobody hits their head: the IRC wants 6 ft 8 in measured plumb from the sloped line through the tread nosings^[1]. Add the upper floor thickness (joist, subfloor and ceiling) under More options and the calculator gives the opening length. For 14 risers of 7-1/2″, 10 in treads and an 11 in floor, it is about 10 ft. England and Australia require 2.0 m, Canada 1,950 mm for a dwelling stair^[4][5][7].

Picking a comfortable tread depth

A comfortable stair keeps twice the riser plus the tread near 25 in (63 cm). The 1979 NBS Guidelines for Stair Safety cites 2R + T = 63 cm as a good formula and R + T of 17 to 18 in as an older rule^[8]. England and Australia write a band of 550–700 mm into the code^[4][6]. With 7-1/2″ risers, a 10 in tread gives 25 in; a 9 ft rise (14 risers of 7-11/16″) is just as easy with a 10 in tread.

Common stair calculation mistakes

Most stair errors come from the total rise or the bottom step, not from the arithmetic:

• Measuring before the flooring. Total rise is finished floor to finished floor. A 3/4 in floor added later changes the top or bottom step by more than the 3/8 in variation the 2021 IRC (R311.7.5.1) allows^[1].
• Rounding the riser count down. 105 in ÷ 13 risers is 8.08 in, over the 7-3/4 in limit. Always round the count up.
• Counting a tread for every riser. The upper floor is the last step, so a straight flight has one tread fewer than risers.
• Forgetting the drop. Cut the tread thickness off the bottom of the stringer or the first step is too tall.
• Skipping headroom. Any beam or floor edge must clear 6 ft 8 in above the nosing line (R311.7.2; R318.7.2 in 2024).
• Using nominal sizes. A 2x12 stringer is 11-1/4 in wide, not 12 in, which matters for the throat.
• Open sides without a guard. Where the stair is more than 30 in above the floor, the open side needs a guard whose openings stop a 4-3/8 in sphere, sized so a small child cannot slip through^[1]; see the baluster spacing calculator.

Parts of a stair

A stair is risers, treads and nosings carried on stringers, with headroom and a handrail measured from the line through the nosings. The diagram labels each part with the 2021 IRC limit^[1]: