How to Calculate Stairs: Rise, Run and Landing
To calculate a staircase, measure the finished floor-to-floor height, divide it by a target riser of about 7 1/2 in (190 mm) and round to a whole number of risers; the height divided by that number is your exact riser. Then pick a run so that two risers plus one run land near 24 13/16 in (630 mm), multiply it by the number of treads to get the total run, and check the headroom under the floor opening and your country's limits.
Summary table: the limits that drive the numbers
Values for stairs in a house. UK: private stair in England. Australia: stairs other than spiral.
| Limit | US (IRC 2021) | Canada (NBC 2020) | UK (Approved Document K) | Australia (NCC 2025) |
|---|---|---|---|---|
| Riser, maximum | 7 3/4 in (196 mm) | 7 7/8 in (200 mm) | 8 5/8 in (220 mm) | 7 7/16 in (190 mm) |
| Riser, minimum | none set | 4 15/16 in (125 mm) | 5 15/16 in (150 mm) | 4 9/16 in (115 mm) |
| Run (going), minimum | 10 in (254 mm) | 10 1/16 in (255 mm) | 8 11/16 in (220 mm) | 9 1/2 in (240 mm) |
| Run (going), maximum | none set | 13 15/16 in (355 mm) | 11 13/16 in (300 mm) | 13 15/16 in (355 mm) |
| 2R + G | not regulated | not regulated | 21 11/16–27 9/16 in (550–700 mm) | 21 11/16–27 9/16 in (550–700 mm) |
| Where | R311.7.5 | Tables 9.8.4.1, 9.8.4.2 | Table 1.1 | Table 11.2.2a |
Inch values for metric codes are conversions rounded to 1/16 in.
Step 1: measure the total rise
Measure from the finished floor at the bottom to the finished floor at the top. If the floors are not finished yet, add the finishes now: if the upper floor and the treads get 3/4 in (19 mm) of hardwood but the bottom floor stays bare, the first riser grows by 3/4 in. Also note the thickness of the upper floor, from the finished ceiling below to the finished floor above: you need it in step 7.
Example: 97 1/2 in (2476.5 mm) floor to floor, with an 11 in (279.4 mm) floor upstairs.
Step 2: find the number of risers
Divide the total rise by a comfortable target riser and round to a whole number: 97 1/2 ÷ 7 1/2 = 13 risers. Then check the bounds. Dividing by your code's maximum riser and rounding up gives the fewest risers allowed: 97 1/2 ÷ 7 3/4 = 12.6, so at least 13 under the IRC; 2476.5 ÷ 200 = 12.4, so at least 13 in Canada.
Step 3: work out the exact riser height
Divide the total rise by the number of risers: 97 1/2 ÷ 13 = 7 1/2 in, or 2476.5 ÷ 13 = 190.5 mm. Every riser gets this height, within your code's tolerance: 3/8 in (9.5 mm) between tallest and shortest under the IRC; 5 mm between neighbours and 10 mm per flight in Canada.
In Australia, 190.5 mm is over the 190 mm maximum, so the same floor height needs 14 risers of 176.9 mm (about 6 15/16 in).
Step 4: choose the run
The run, or going, is measured horizontally from nosing to nosing. The tread board is deeper by the nosing projection: 3/4 to 1 1/4 in (19 to 32 mm) under the IRC when the tread is under 11 in (279 mm) deep, and at most 25 mm in Canada.
Example: 10 1/16 in (255.6 mm), just above Canada's 255 mm minimum and the IRC's 10 in. With a 7 1/2 in riser the stair climbs at 36.7°, under the 42° pitch limit of England's Approved Document K.
Step 5: check comfort with 2R + G
Twice the riser plus the run approximates one natural stride; in French-speaking countries it is called Blondel's formula. The UK and Australia make it a rule, 550 to 700 mm (21 11/16 to 27 9/16 in). Elsewhere it is a design target: Stair3D aims for about 630 mm (24 13/16 in) and flags results outside 600 to 650 mm (23 5/8 to 25 5/8 in).
Example: 2 × 7 1/2 + 10 1/16 = 25 1/16 in, or 2 × 190.5 + 255.6 = 636.6 mm. Comfortable, and inside the UK and Australian range.
Step 6: total run, with or without a landing
A straight flight has one tread fewer than risers, because the upper floor is the last step. Total run = treads × run: 12 × 10 1/16 = 120 3/4 in, or 12 × 255.6 = 3067 mm. Add the nosing of the first tread for the footprint.
With a landing, place it at a whole number of risers and treat each flight separately. A landing at 45 in (1143 mm) takes 6 risers: the lower flight has 5 treads, 50 5/16 in (1278 mm) of run, and the upper flight 7 risers and 6 treads, 60 3/8 in (1534 mm). The landing adds its own depth: at least as long as the flight is wide in the UK (and in Canada when it turns 90° or more), 36 in (914 mm) on a straight run in the US, 750 mm in Australia. The half-turn layout is worked out in U-shaped stairs with a landing.
Step 7: size the stairwell opening for headroom
Headroom is measured vertically above the line of the nosings: at least 6 ft 8 in (2032 mm) under the IRC, 1950 mm (76 13/16 in) in Canada and 2.0 m (78 3/4 in) in England. For a straight stair, the opening in the upper floor must extend from the top nosing, measured horizontally, at least:
opening length = (headroom + floor thickness) × run ÷ riser
Example in Canada: (1950 + 279.4) × 255.6 ÷ 190.5 = 2991 mm, or about 117 13/16 in. The stair runs 120 3/4 in, so the floor above may cover only its first 3 in. Under the IRC, the same stair needs 122 1/8 in (3101 mm), slightly more than the stair itself: the floor edge has to stop in front of the bottom nosing. A landing or a turn changes this geometry; the U-shaped stair guide works one out.
Step 8: check your country's code
Run your numbers against the rules where you build, including width, handrails and guards: US (IRC), Canada and Quebec (NBC), UK (Approved Document K) and Australia (NCC). Your local building department has the final word on editions and amendments.
Worked example at a glance
| Step | Imperial | Metric |
|---|---|---|
| Total rise | 97 1/2 in | 2476.5 mm |
| Risers × height | 13 × 7 1/2 in | 13 × 190.5 mm |
| Run | 10 1/16 in | 255.6 mm |
| 2R + G | 25 1/16 in | 636.6 mm |
| Straight flight: 12 treads | 120 3/4 in | 3067 mm |
| With a landing at 45 in (1143 mm): flights | 50 5/16 in + 60 3/8 in | 1278 mm + 1534 mm |
| Stairwell opening, straight stair, Canada | 117 13/16 in | 2991 mm |
Common mistakes
- Counting as many treads as risers in a straight flight.
- Measuring from the subfloor, then finishing the floors: the first or last riser ends up off.
- Leaving the ceiling finish out of the floor thickness: the header then cuts into the headroom.
- Combining the maximum riser with the minimum run: legal, but the steepest stair your code allows.
FAQ
How many steps do I need for a 9-foot rise?
108 in (2743 mm) ÷ 7 3/4 in = 13.9, so at least 14 risers of about 7 11/16 in (195.9 mm) under the IRC. A straight flight then has 13 treads, at least 130 in (3302 mm) of run with 10 in treads.
What is a good riser and run combination?
Keep 2R + G near 24 13/16 in (630 mm). For example, 7 1/2 in risers with 10 in runs, or 7 in risers with 11 in runs, both give 25 in (635 mm).
What is the 2R + G rule?
Two risers plus one run, a measure of a natural stride. England's Approved Document K (Table 1.1) and Australia's NCC (Table 11.2.2a) require 550 to 700 mm.
How do I calculate stairs with a landing?
Set the landing at a whole number of risers, calculate each flight with treads = risers − 1, then add the landing depth required by your code.
How long should the stairwell opening be?
For a straight stair, (headroom + floor thickness) × run ÷ riser, measured from the top nosing. In the example above, 117 13/16 in (2991 mm) in Canada.
Stair3D gives you a design and a code check to guide your work. Check the measurements on site and with your local building authority before cutting.