How to use the stair calculator
Every stair design starts with one fixed number: the vertical distance between the two floors. Everything else, from the number of steps to how much floor space the stair takes, follows from that height and the limits set by your building code. This calculator works through those steps for you.
- Choose your units and a code preset. Presets fill in a maximum riser and tread depth for the IRC (US homes), the IBC (most other US buildings), UK private stairs, or a comfortable everyday stair. Pick Custom to enter your own limits.
- Enter the total rise, measured from finished floor to finished floor, including flooring on both levels.
- Adjust the tread depth if you have space to spare. Deeper treads make a gentler, safer stair but a longer one.
Results show the riser count, exact riser height, number of treads, total horizontal run, stair angle, stringer length and a 2R + T comfort check. Warnings appear if the stair falls outside the preset's limits.
Stair formulas explained
Risers = ⌈Total rise ÷ Max riser⌉
Riser height = Total rise ÷ Risers
Treads = Risers − 1
Total run = Treads × Tread depth
Angle = atan(Riser ÷ Tread)
Stringer ≈ √(Total rise² + Total run²)
The key step is rounding the riser count up. Rounding down would make every riser taller than the code allows. Because every riser in a flight must be the same height (within about 9.5 mm or 3/8"), the total rise is then divided evenly across that whole number of risers.
The comfort rule: 2R + T
A stair can meet the maximum and minimum limits and still feel wrong. The long-standing rule of thumb, first written down by the French architect François Blondel in the 17th century, is that two risers plus one tread should equal one natural stride: about 600–650 mm, or 24–25.5 inches. Steep stairs with shallow treads and shallow stairs with very deep treads both fail it.
Worked examples
A metric house stair
Floor to floor is 2,800 mm with a comfortable 175 mm target riser. 2,800 ÷ 175 = 16 risers exactly, each 175 mm. With 280 mm treads, 15 treads give a total run of 4,200 mm. 2R + T = 630 mm, right in the comfort band, and the angle is 32°.
A US home built to the IRC
The total rise is 108 inches (9 ft). 108 ÷ 7.75 = 13.9, so 14 risers of 7.71" each. With 10" treads, 13 treads give a 130" (10'-10") run at about 37.6°. That's legal, but a 10 1/2" or 11" tread would feel noticeably better if the plan allows the extra 6 to 13 inches.
A commercial stair to the IBC
With a 13'-0" (156") floor to floor, 156 ÷ 7 = 22.3, so 23 risers of 6.78". Because the IBC limits a single flight to 12 ft of rise, add a landing midway and split it into flights of 12 and 11 risers.
Who this calculator helps
- Architects fitting a stair into an early floor plan, checking whether the floor-to-floor height leaves enough run, or sizing the stairwell opening.
- Architecture students getting stairs right in studio projects, one of the most common things reviewers pick up on.
- Interior designers planning a feature stair, a loft access stair or a renovation where the stair is being moved.
- Builders, carpenters and stair fabricators laying out stringers and checking site dimensions against the drawings.
- Homeowners and DIYers building deck steps, garden stairs or a loft conversion, who want to understand what the inspector will check.
Practical tips
- Measure the total rise on site, not just from drawings. Floors are rarely exactly where they were drawn.
- Check headroom along the whole flight, typically 2.0 m in the UK or 6'-8" under the IRC.
- Remember handrails, guards and nosings. Many codes also require a nosing of 3/4" to 1 1/4" on solid risers.
- Plan the floor area the stair uses: total run times stair width, plus landings at the top and bottom. Run the result through the FAR calculator if floor area is tight, and use the scale converter to draw the stair at 1:50 or 1/4" = 1'-0".
Want to see how a new stair will look in the room? Upload a photo and try materials and finishes with Room Redesign.
