Stair Calculator

How this calculator works

Stairs start from one fixed number: the total rise, the finished floor-to-floor height. The calculator divides that by your preferred riser height, rounds to a whole number of steps, then shares the height out evenly so every riser is identical (which every building code requires):

risers   = round( total rise ÷ preferred riser )
rise each = total rise ÷ risers
treads   = risers − 1
total run = treads × going
stringer = sqrt( total run² + total rise² )
pitch    = atan( rise each ÷ going )

There is always one fewer tread than riser in a standard flight, because the top floor itself acts as the last “step.” It then checks the result against the stair rules for the country you pick.

Worked example

This example follows the unit system you pick in the calculator above.

An 8.5 ft floor-to-floor rise, aiming for a 7.5 in riser with a 10 in tread (checked against US IRC R311.7):

1. Total rise = 8.5 ft = 102 in; risers: 102 ÷ 7.5 = 13.6 → round to 14 risers.
2. Rise per step: 102 ÷ 14 = 7.29 in (≤ 7¾ in ✓).
3. Treads: 14 − 1 = 13 treads; total run = 13 × 10 = 130 in = 10.83 ft.
4. Tread depth 10 in meets the 10 in minimum ✓ (the IRC sets no 2R+G rule).
5. Pitch = atan(7.29 ÷ 10) = 36.1°; stringer = √(10.83² + 8.5²) = 13.77 ft.

Stair rules by country

Stair geometry is one of the most heavily regulated parts of a home, and the limits differ enough that the same flight can be legal in one country and not another. These are the limits for private stairs serving a single dwelling:

Country	Max riser	Min going (run)	2R + G	Max pitch	Notes
🇺🇸 US - IRC R311.7	7¾ in (197 mm)	10 in (254 mm)	not used	-	Landing required above 12 ft 7 in of rise
🇬🇧 UK - Approved Doc K	220 mm	220 mm	550–700 mm	42°	Rise also has a 150 mm minimum
🇦🇺 Australia - NCC 11.2	190 mm	240–355 mm	550–700 mm	≈38°	Max 18 risers per flight
🇨🇦 Canada - NBC 9.8	200 mm	255 mm	not used	-	NBC 2020 raised min run from 210 mm
🇳🇿 New Zealand - D1/AS1	190 mm	280 mm	not used	37°	Caps pitch; sets no minimum riser

Two differences catch people out. The US and Canada do not use the 2R + G formula - they cap rise and run independently - while the UK and Australia require 2R + G between 550 and 700 mm on top of those limits. New Zealand takes a third path: no 2R + G, no minimum riser, but a hard cap on the pitch angle.

The figures above are summarised from each country’s official building code. Always confirm against the current edition adopted in your area:

• US - IRC Section R311, Means of Egress (ICC Digital Codes)
• UK - Approved Document K: protection from falling, collision and impact (GOV.UK)
• Australia - NCC Part 11.2: stairway and ramp construction (ABCB)
• Canada - National Building Code of Canada 2020 (NRC)
• New Zealand - Building Code clause D1 Access Routes (MBIE)

Beyond the step geometry

The calculator covers rise, going, run, stringer and pitch. A full code-compliant stair also needs:

• Headroom - at least 2 m (UK, AU, NZ), 1.95 m in Canada, or 6 ft 8 in (2.03 m) in the US, measured vertically above the pitch line.
• Handrails - typically 865–1070 mm (Canada), 900–1000 mm (UK, NZ) or 34–38 in (US) above the nosing line.
• Landings - break the flight before the rise exceeds the limit (12 ft 7 in US, 3.7 m Canada, 4.0 m NZ) or 18 risers (Australia).
• Open risers and gaps - a 100 mm sphere (UK, NZ) or 125 mm sphere (Australia) must not pass through any gap, to protect small children.

Tips for setting out

• Buy your riser timber to the exact rise per step the calculator gives, not your rounded preference - evenness is a code requirement, and the human foot notices a few millimetres.
• Confirm the total run fits the floor space before committing: a shallower riser is more comfortable but eats more floor.
• Check the adopted code edition for your area - Canada’s minimum run, in particular, jumped from 210 mm to 255 mm in NBC 2020, and provinces adopt updates on their own timetable.