Stair Calculator: Rise, Run, Stringer and Angle
Lay out a stair from the total rise. Get the number of risers and treads, riser and tread sizes, total run, stringer length and angle, with warnings against residential code limits.
Finished floor to finished floor.
Blank uses 10.5 in treads.
Enter the total rise from the lower floor to the upper floor.
About the stair calculator
The Stair Calculator plans a straight flight of stairs from the height it has to climb. Enter the total rise, the floor-to-floor height, and the calculator works out how many risers you need, how tall each riser is, how many treads there are, how deep each tread is, the total run, the length of the stringer and the angle of the stair. It also checks the result against the main residential limits in the International Residential Code and tells you which ones are exceeded.
Results are shown in inches, to two decimal places, with the metric equivalent in whole millimetres. You can type the rise and run in feet, inches, yards, metres, centimetres or millimetres, and each field has its own unit.
How to plan a stair
Steps
- Enter the total rise, measured from the finished lower floor to the finished upper floor.
- Optionally enter the total run, the horizontal space you have for the stair. Leave it empty to use the default tread depth of 10.5 in (about 26.7 cm).
- Set the target riser height. The default is 7.5 in (19 cm).
- Choose the mounting. A standard stair has one tread fewer than risers, because the upper floor acts as the last tread. A flush mounting has as many treads as risers.
- Optionally enter the headroom you have, and the calculator will tell you whether it reaches the code minimum.
- Read the risers, treads, depths, stringer length, angle and any warnings.
Formulas
Risers and treads
The number of risers is the total rise divided by the target riser height, rounded to the nearest whole number, with a minimum of one:
Because is a whole number, the actual riser height differs slightly from the target. Treads are for a standard stair and for a flush one. The tread depth is the total run divided by the number of treads, or 10.5 in if you did not enter a run.
Run, stringer and angle
With treads of depth :
is the straight-line stringer length, from the foot of the stair to the top.
Worked examples
Default stair. A total rise of 108 in divided by a 7.5 in target is 14.4, so there are 14 risers of 7.71 in (196 mm). A standard stair has 13 treads. At the default 10.5 in (267 mm) tread, the total run is 136.5 in, the stringer is 174.06 in and the angle is 36.3 degrees. This passes the riser, tread and angle checks.
Short run. The same rise with only 120 in of run gives 13 treads of 9.23 in (234 mm). The stringer is 161.44 in and the angle is 39.9 degrees. The calculator warns that the tread is under 10 in and that the angle is steeper than 37.5 degrees.
Flush mounting. The same 108 in rise with flush mounting has 14 treads, so the run at 10.5 in is 147 in and the stringer is 182.41 in. The angle stays at 36.3 degrees.
Metric. A rise of 2,700 mm is 106.30 in. With a 19 cm target there are 14 risers of 7.59 in (193 mm).
Code checks
The calculator flags results that go beyond the residential limits in IRC section R311.7: a riser taller than 7.75 in (197 mm), a tread shallower than 10 in (254 mm), and a stair angle steeper than 37.5 degrees. The minimum headroom is 6 ft 8 in, which is 80 in or 2,032 mm. If you enter the headroom you have, the calculator reports whether it meets that figure.
The tool does not check local code. Rules differ between countries and cities, including Canada and the UK Building Regulations, and your inspector has the final say. Treat the warnings as a first screen, not approval.
Measuring total rise and why uniform risers matter
Measure from the top of the finished lower floor to the top of the finished upper floor, including flooring that will be added later. Take the measurement with a level and a straight board or a laser, and check it in more than one place, because floors are rarely level.
Every riser in a flight should be the same height, and the code limits variation to 3/8 in between the tallest and shortest. People learn the rhythm of a stair quickly, and an uneven step is a common cause of falls. Tread depth is the horizontal distance from one nosing to the next. A nosing of 3/4 in to 1-1/4 in is normally required on solid-riser stairs with treads shallower than 11 in.
Rounding
Risers are the nearest whole number, so the riser height can differ from the target. If you need a riser closer to 7.5 in, change the rise or the target and compare. A standard stair needs at least two risers, because the top floor is the last tread. If the rise is so small that it rounds to a single riser, which happens below about 11.25 in with the default target, the calculator reports an error instead of a result.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
To find the length of the cut board and the layout numbers, see the Stair Stringer Calculator. The Length Converter converts rise and run between units, and the Angle Converter changes the stair angle into other angle units. For the concrete in a stair footing, try the Concrete Calculator.
Frequently asked questions
How many risers do I need?
Divide the total rise by your target riser height and round to the nearest whole number. A rise of 108 in with a 7.5 in target gives 14.4, so 14 risers. The riser height is then 108 divided by 14, which is 7.71 in.
Why are there fewer treads than risers?
On a standard stair the upper floor acts as the last tread, so treads equal risers minus one. A 14 riser stair has 13 treads. A flush mounting has the same number of treads as risers because the top tread sits at floor level.
What is the total rise and how do I measure it?
Total rise is the vertical distance from the finished lower floor to the finished upper floor. Measure with a level and a straight board or a laser at more than one point, and include any flooring that will be installed later.
Which code limits does the calculator check?
It warns when the riser is taller than 7.75 in, the tread is shallower than 10 in, or the stair angle is steeper than 37.5 degrees, following IRC R311.7. It also compares any headroom you enter with the 6 ft 8 in minimum. It does not check local code, which may differ.
Why is the riser height not exactly my target?
The number of risers must be a whole number, so the calculator rounds and then divides the rise equally. With a 108 in rise and a 7.5 in target you get 7.71 in, not 7.5 in. To get closer, adjust the target or the rise.
Does it work for metric or UK building regulations?
The maths works in any unit, and results are shown in inches and millimetres. The warnings are based on the US residential code, so for the UK, Canada or elsewhere, check your own regulations and use the numbers only as a starting point.
Is my data sent anywhere?
No. The stair sums run in your browser, so your rise, run and headroom values are never uploaded or stored on a server. They stay on this device and are not shared with anyone.