How the stair calculator works
The calculator divides the total rise by the 7-3/4 inch maximum riser height in 2021 IRC Section R311.7.5.1, rounds up to a whole number of risers, then splits the rise evenly. Every riser comes out the same height, which keeps you inside the 3/8 inch variation the code allows within a flight.
A straight flight has one fewer tread than risers, because the top riser lands on the upper floor. Total run is the tread count multiplied by the tread depth you enter. Stringer length is the diagonal of the total rise and total run, so treat it as a minimum board length before you add material for the top and bottom cuts.
IRC stair limits used by this calculator
Table: 2021 IRC residential stairway limits, Section R311.7.
| Rule | Limit | 2021 IRC section |
|---|---|---|
| Maximum riser height | 7-3/4 in. | R311.7.5.1 |
| Minimum tread depth | 10 in. | R311.7.5.2 |
| Maximum rise of one flight | 12 ft 7 in. | R311.7.3 |
| Minimum clear width | 3 ft | R311.7.1 |
| Minimum headroom | 6 ft 8 in. | R311.7.2 |
| Nosing projection (tread under 11 in.) | 3/4 in. to 1-1/4 in. | R311.7.5.3 |
The calculator checks riser height, tread depth and flight rise. It cannot measure width, headroom or nosing, so check those on site. The full rules, including winders and spiral stairs, are in our guide to stair riser height and tread depth code.
How do you measure total rise?
Measure total rise from the finished floor at the bottom to the finished floor at the top, not from subfloor to subfloor. If flooring is not installed yet, add its thickness at the top and subtract it at the bottom, or the first and last risers will not match the rest.
- Set a straightedge level off the upper floor so it projects over the stair opening.
- Measure plumb from the underside of the straightedge to the lower finished floor.
- Adjust for any finish flooring still to be installed at either level.
- Enter the result in inches. For 9 ft 2-1/2 in., enter 110.5.
Laying out the stringer
Mark the stringer with a framing square set to the riser height on the tongue and the tread depth on the blade, stepping it along the board once per tread. Drop the bottom of the stringer by the thickness of one tread so the first step matches the others once treads are installed. The stair opening above must give the 6 ft 8 in. headroom in R311.7.2; our guide to framing a stair opening covers the headers and trimmers around it.
Handrails and guards are separate rules. See handrail height code for the R311.7.8 requirements, and minimum ceiling height for the room heights a stair often has to fit under.
Draw the stair opening into your framing plan
Framing Plan is a free browser tool for preliminary floor framing layouts. Sketch the joists, headers and opening around your stair and export PNG, PDF or DXF.
Open the framing studioFrequently asked questions
What is the maximum stair riser height under the IRC?
The 2021 IRC Section R311.7.5.1 limits residential stair risers to 7-3/4 inches, measured vertically between the leading edges of adjacent treads. Within one flight, the tallest riser may not exceed the shortest by more than 3/8 inch, so every riser should be cut to the same height.
What is the minimum stair tread depth?
The 2021 IRC Section R311.7.5.2 requires a tread depth of at least 10 inches, measured horizontally between the foremost projections of adjacent treads. The greatest tread depth in a flight may not exceed the smallest by more than 3/8 inch.
How do you calculate the number of stair risers?
Divide the total rise, from finished floor to finished floor, by the maximum riser height of 7-3/4 inches and round up to the next whole number. Then divide the total rise by that count to get the actual riser height. A straight flight has one fewer tread than risers.
How tall can one flight of stairs be?
The 2021 IRC Section R311.7.3 limits a single flight to a vertical rise of 12 feet 7 inches between floor levels or landings. A taller rise needs an intermediate landing that splits it into flights that each stay within that limit.
How wide must a residential stairway be?
The 2021 IRC Section R311.7.1 requires a clear width of at least 36 inches at all points above the permitted handrail height, and Section R311.7.2 requires at least 6 feet 8 inches of headroom. This calculator does not check width or headroom, so measure those separately.
Results are preliminary layout figures for planning. The building code edition and local amendments adopted where you build govern, and the building official has the final say.