What this estimate does and does not do
This calculator estimates straight-run stair geometry from total rise, target riser height, tread depth, available run, headroom, and stair width. It provides basic profile checks for common US IRC-style and UK domestic-style limits.
It does not replace local building code, a permit review, or structural design. Stair rules vary by jurisdiction and project type. Landings, handrails, guards, balusters, nosings, stringer strength, fire egress, winders, spiral stairs, ships ladders, and space-saver stairs have additional requirements not modelled here.
Formula used
minimum risers by code = ceil(total rise / max riser height)
target risers = round(total rise / target riser height)
selected risers = target risers clamped to code min/max
actual riser height = total rise / selected risers
treads = risers - 1
total run = treads × tread depth
stringer length = sqrt(total rise² + total run²)
stair angle = atan(total rise / total run)
comfort value = 2 × riser height + tread depthWorked example
For a 108 inch total rise, 7.25 inch target riser, 10.5 inch tread depth, and US IRC-style profile:
target risers = round(108 / 7.25) = 15
actual riser = 108 / 15 = 7.2 in
treads = 15 - 1 = 14
total run = 14 × 10.5 = 147 in
stringer length = sqrt(108² + 147²) ≈ 182.4 in
2R+T = 2 × 7.2 + 10.5 = 24.9 inThat geometry is within the common 7¾ inch max riser, 10 inch min tread, and 24–25.5 inch comfort band.
Basic profile limits used
US IRC-style profile:
- maximum riser: 7.75 in
- minimum riser: 4 in
- minimum tread: 10 in
- minimum headroom: 80 in
- minimum stair width: 36 in
- comfort check: 2R+T around 24–25.5 in
UK domestic-style profile:
- maximum rise: 220 mm
- minimum going: 220 mm
- maximum pitch: 42°
- minimum headroom: 2000 mm
- comfort check: 2R+G between 550–700 mm
Always verify the exact rules with your local building authority.
How to use the result
- Measure finished floor to finished floor accurately.
- Check that actual riser height is within your local maximum.
- Check that tread depth/going meets the minimum.
- Check total run against available floor space before cutting stringers.
- Check headroom along the whole stair path, especially under beams, ducts, and floor openings.
- Use the 2R+T value to catch stairs that technically fit but feel awkward.
Assumptions and limitations
- This is for straight-run stairs only.
- Winder, spiral, curved, ships ladder, alternating tread, and space-saver stairs need different rules.
- Riser and tread uniformity is critical; this calculator assumes every riser and tread is equal.
- The stringer length is the geometric diagonal and does not include extra lumber for cuts, bearing, waste, or connection details.
- Local code may be stricter than the built-in profiles.
Frequently asked questions
How do you calculate the number of stair risers?
Divide the total floor-to-floor rise by a target riser height, then round to a whole number and check that the actual riser height stays within the selected code profile limits.
Why is there one fewer tread than riser?
In a straight stair from one floor to the next, the upper finished floor acts as the final walking surface, so tread count is usually risers minus one.
What is the 2R+T stair comfort rule?
The rule says two risers plus one tread should fall in a comfortable walking range. In US-style inches, many builders aim around 24–25 inches. In UK guidance, 2R+G is often checked against 550–700 mm.
Does this prove my stair is code-compliant?
No. It checks basic geometry only. Local codes also cover landings, handrails, guards, nosings, width, headroom, structural stringers, fire rules, and more.
Should I use subfloor or finished-floor measurements?
Use finished floor to finished floor. If floor finishes differ, include that difference before cutting stringers.