Stair Calculator: Risers, Treads and Stringer Length
Total rise divided into equal risers, treads one fewer, and the stringer as the hypotenuse. It also flags the IRC limits, because stairs are the most commonly failed inspection item in residential framing.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Total rise, finished floor to finished floor108 in
- Tread depth you want10.5 in
- Stair width36 in
Risers14 at 7.71 in each
Two risers plus one tread is 25.93 in. Comfortable stairs land between 24 and 25.
| Total rise | 108 in |
|---|---|
| Number of risers | 14 |
| Riser height | 7.71 in |
| Number of treads | 13 |
| Total run | 136.5 in (11.38 ft) |
| Stringer length, minimum | 14.5 ft |
| Headroom needed above nosing | 80 in minimum |
The material list this produces
| Stringers, 15 ft | 4 |
|---|---|
| Treads, 36 in | 13 |
| Risers, 36 in | 14 |
What this calculator assumes
- Riser count is the total rise divided by a 7.5 inch target, rounded to a whole number, and then increased by one if that would have put a riser over the code maximum. The rise is then divided evenly among them. Every riser in a flight must be within 3/8 inch of every other one.
- Treads are always one fewer than risers when the top riser lands on the finished floor above.
- The IRC limits are a 7.75 inch maximum riser and a 10 inch minimum tread. Your local code may be stricter, and commercial work certainly is.
Equal risers, and why an inspector cares
Every riser in a flight must be within 3/8 of an inch of every other riser. That is not fussiness, it is the single biggest cause of falls on stairs. People climb by rhythm and a riser that is different by half an inch is a trip.
Which means the rise cannot be divided into a convenient round number and the remainder dumped into the bottom step. Total rise divided by the riser count, exactly, every time. If that gives 7.3125 inches, cut every riser at 7.3125.
The IRC numbers
Residential, under the IRC: maximum riser 7.75 inches, minimum tread 10 inches, minimum headroom 6 foot 8 inches measured vertically from the nosing line, minimum width 36 inches above the handrail.
Commercial is tighter, usually a 7 inch riser and an 11 inch tread. Many jurisdictions amend either one. Check locally before cutting stringers, because a stringer cut wrong is firewood.
The comfort rules the code does not enforce
Two risers plus one tread should land between 24 and 25 inches. Riser plus tread should be about 17 to 18. Riser times tread should be about 75. These are old carpenter's rules and they all describe the same thing, which is a stair that matches the human stride.
A stair that passes code but violates all three feels wrong to walk on and the customer will say so without knowing why.
Questions people ask
How many stairs for 9 feet?
One hundred and eight inches of rise divided by a 7.5 inch target gives 14 risers at 7.71 inches each, with 13 treads. At a 10.5 inch tread the total run is 136.5 inches, or 11 feet 4.5 inches.
How do I calculate stringer length?
It is the hypotenuse of the total rise and total run. Square each, add them, take the square root.
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Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.