Stair Calculator - Risers, Treads and Stringer Length

Max riser	7-3/4 in under the IRC
Min tread	10 in under the IRC
Min clear width	36 in
Headroom	6 ft 8 in measured vertically

What is Stair Calculator?

Stairs are built from one fixed number: the total rise from finished floor to finished floor. Divide that by a comfortable riser height — around 7 to 7-1/2 in — and round to a whole number of risers, then divide the rise by that count to get the exact riser height every step must share. The IRC caps risers at 7-3/4 in and requires a minimum 10 in tread depth, and it limits the variation between the tallest and shortest riser in a flight to 3/8 in, because uneven steps are a common cause of falls. The number of treads is one fewer than the number of risers, since the upper floor acts as the last step.

Common uses for Stair Calculator

Lay out a straight run of stairs from a measured floor-to-floor height
Check a stair design against the IRC riser and tread limits
Work out the total run before you frame the floor opening
Get the stringer length so you can cut two matching stringers
Check headroom under a stair before you build a landing

Why every riser in a flight must match

All risers in a flight are the same height, and the IRC allows no more than 3/8 in of variation between the tallest and the shortest. That is not fussiness: people build a rhythm as they climb, and one short or tall step breaks it and causes falls. Measuring the total rise accurately, from finished floor to finished floor and including the flooring, is where most stair mistakes begin.

The total run is what has to fit the space

The run is the number of treads times the tread depth. Thirteen treads at 10 in gives a total run of 130 in, about 10 ft 10 in, and that is the floor space the staircase consumes. A steeper stair with 9 in treads saves roughly a foot of run but breaks the 10 in minimum, so if the run will not fit, the usual answers are a landing with a turn, a winder, or a larger opening.

Stringers, landings and headroom

Two stringers carry a typical straight flight, usually cut from 2x10 or 2x12 stock, and each board must be long enough for the sloped length plus bearing at both ends. Headroom of 6 ft 8 in is measured vertically from the nosing line, and the floor opening above must be framed to suit. Where a flight would exceed 12 ft of rise, the code calls for a landing. Check framing, opening and headroom before you cut.

Frequently Asked Questions

How many risers do I need?

Divide the total rise by a target riser height near 7-1/2 in and round to the nearest whole number. A 108 in rise divided by 7.5 is 14.4, which rounds to 14 risers of 7.71 in each. Fourteen risers sits inside the 7-3/4 in IRC maximum and close to the target, so it feels comfortable.

How do I calculate the stringer length?

Use the Pythagorean theorem on the rise and the total run. For a 108 in rise and a 130 in run, 108 squared is 11,664 and 130 squared is 16,900; the square root of their sum is 169 in, about 14 ft 1 in. Add for the bearing at each end before you buy the board.

What are the IRC stair limits?

The IRC requires a maximum riser height of 7-3/4 in, a minimum tread depth of 10 in, a minimum clear width of 36 in, and headroom of 6 ft 8 in measured vertically from the nosing line. The tallest riser in a flight may not exceed the shortest by more than 3/8 in. Confirm with your local building department.

How much headroom do I need?

The IRC calls for 6 ft 8 in of headroom measured vertically from the sloped plane of the nosings to the ceiling or the underside of the floor above. It is measured along the whole flight, not just at the top step, so a landing placed too close to the stair can fail even when the rest is fine.

Can I use a landing to break a long run?

Yes, and it often helps. Most U.S. codes limit a single flight to 12 ft of vertical rise between landings, so a taller floor-to-floor height needs a landing and a second stair section. A landing also gives you a natural place to turn, which is often the only way to fit a staircase into a narrow space.