Stair Calculator
A 108 in floor-to-floor rise with a 10 in tread takes 15 risers of 7.2 in (14 treads), a 140 in run, a 35.8 degree angle and a 176.8 in stringer. It meets the US IRC limits (riser up to 7-3/4 in, tread at least 10 in).
How to use
- Measure the total rise: the vertical distance from the finished lower floor to the finished upper floor. Choose inches or millimetres.
- Enter the riser height you would like (7 in or 180 mm is a common target) and the tread depth. The calculator picks the whole number of risers closest to your target and divides the rise evenly.
- Read the riser height, run, angle and stringer length, then check the code lines. The side view shows the flight to scale.
How it works
Number of risers = total rise ÷ target riser height, rounded to a whole number (at least 2). Riser height = total rise ÷ number of risers, so every riser is identical.
Treads = risers − 1, because the upper floor is the last tread. Total run = treads × tread depth, measured nosing to nosing. Angle = arctan(riser ÷ tread). Stringer length = √(total rise² + total run²), measured along the nosing line.
Comfort rule: 2 × riser + tread should fall between 24 and 25 in (600 to 650 mm). US residential code (IRC R311.7.5) allows a riser of at most 7-3/4 in (196 mm) and a tread of at least 10 in (254 mm). Only these two limits are checked here.
Code and comfort limits
| Measure | Imperial | Metric |
|---|---|---|
| Maximum riser (IRC R311.7.5.1) | 7-3/4 in | 196 mm |
| Minimum tread (IRC R311.7.5.2) | 10 in | 254 mm |
| Comfort rule 2R + T | 24 to 25 in | 600 to 650 mm |
| Riser or tread variation in one flight | no more than 3/8 in | no more than 9.5 mm |
What this calculator does not check
- Nosing, headroom (6 ft 8 in), stair width, handrails and guards, winders, and landings.
- Local amendments. Many jurisdictions adopt a different code edition or a lower riser limit, so confirm with your building department.
- The stringer length is the nosing line. A real stringer board must be longer and has the notches cut from a wider plank.
Sources: IRC R311.7 (ICC)Last reviewed 2026-10-16
FAQ
How do I calculate the number of stairs?
Divide the total rise by your target riser height and round to the nearest whole number. A 108 in rise at 7 in gives 15.4, so use 15 risers of 7.2 in each.
What is the maximum riser height allowed?
The US International Residential Code limits residential risers to 7-3/4 in (196 mm) and requires treads of at least 10 in (254 mm). Other countries and local codes differ, so check yours.
Why is the number of treads one less than risers?
The top step is the upper floor itself, so a flight of 15 risers has 14 treads. Counting 15 treads would make the run too long by one tread.
What is the 2R + T rule?
Twice the riser plus the tread should be about 24 to 25 in (600 to 650 mm), which matches a natural walking stride. A 7.2 in riser with a 10 in tread gives 24.4 in.
Is the stringer length the board length?
No. It is the length along the line through the nosings, from floor to floor. Allow extra board for the cut at the top and bottom and for the plank depth left under the notches.
Does it work in millimetres?
Yes. Switch the unit to mm and enter the rise in millimetres. The code limits are shown converted (196.85 mm riser, 254 mm tread). Values you already typed are converted for you.