Stair dimensions at a glance
- US homes (IRC): riser max 7¾″ (197 mm), tread min 10″ (254 mm), headroom min 6′ 8″ (2032 mm), width min 36″ (914 mm), handrail 34–38″ when there are 4 or more risers.
- UK homes (Approved Document K, private stair): rise 150–220 mm, going 220–300 mm, pitch max 42°, 2R + G between 550 and 700 mm, headroom 2.0 m (1.9 m at the centre of a loft-conversion stair).
- Canada (NBC, houses): rise 125–200 mm, run 255–355 mm, headroom 1950 mm. Australia (NCC): riser 115–190 mm, going 240–355 mm, 2–18 risers per flight.
- Comfortable stairs (guidance, not code): 2R + G of 24–25″ (600–640 mm) and an angle of 30–37°.
- Number of treads is always one fewer than the number of risers on a standard straight stair, because the upper floor acts as the last “tread”.
How to use the stair calculator
- 1
Measure the total rise
Measure vertically from the finished lower floor to the finished upper floor, including any flooring still to be laid.
- 2
Pick your building code
Choose IRC or IBC (US), Approved Document K (UK), NBC (Canada) or NCC (Australia) so the limits and checks match where you are building.
- 3
Enter target riser and tread going
A riser of 7–7½″ (175–200 mm) with a going of 10–11″ (250–280 mm) suits most homes. Or switch to Fit to a run if the horizontal space is fixed.
- 4
Add nosing, width and the stairwell opening
Nosing, stair width, upper-floor depth and opening length let the calculator check headroom and draw the stairwell on the diagram.
- 5
Read the diagram and code checks
The scaled diagram updates as you type. Fix anything marked red, then compare riser counts in the options table.
- 6
Mark out the stringer
Use the mark-out schedule (height and run to each tread) and cut the bottom riser short by one tread thickness so every step ends up equal.
Stair calculation formulas
| Result | Formula |
|---|---|
| Number of risers | total rise ÷ target riser, rounded to a whole number |
| Riser height | total rise ÷ number of risers |
| Number of treads | risers − 1 |
| Total run | treads × going |
| Stair angle | arctan(riser ÷ going) |
| Stringer diagonal | √(total run² + total rise²) |
| Comfort check | 2 × riser + going ≈ 24–25″ (600–640 mm) |
| Stringer throat | board depth − (riser × going ÷ √(riser² + going²)) |
| Headroom | ceiling height − pitch-line height at the opening edge |
Worked example (US, IRC)
Total rise 9′ 0″ (108″), target riser 7¼″, going 10 1/2″. 108 ÷ 7.25 = 14.9, so use 15 risers of 108 ÷ 15 = 7 3/16″ (under the 7¾″ maximum). That gives 14 treads and a total run of 12′ 3″ (147″). The angle is 34.4° and 2R + G is 24 7/8″, right in the comfortable range. The stringer diagonal is 15′ 2 7/16″ (182 7/16″), so buy 18 ft boards.
Worked example (UK, Part K private stair)
Total rise 2600 mm, target 200 mm, going 225 mm. 2600 ÷ 200 = 13, so 13 risers of 200 mm and 12 goings for a total run of 2,700 mm (2.70 m). The pitch is 41.6° – inside the 42° limit – and 2R + G = 625 mm, within 550–700 mm.
How to read the stair diagram
The diagram is a true-to-scale side elevation of one straight flight, redrawn every time you change a number.
- Treads and risers – brown boards show each tread with its nosing overhang; the vertical lines are the risers. Step numbers count up from the bottom.
- Stringer – the shaded board under the steps, drawn as a cut (notched) or housed (closed) stringer.
- Pitch line – the dashed orange line through the nosings. The angle at its base is the stair pitch.
- Handrail – the blue line, drawn parallel to the pitch line at your code's handrail height.
- Headroom – the red dotted line is the minimum clearance above the pitch line; the green or red vertical shows the real headroom under the edge of the stairwell opening.
- Dimensions – total rise, total run, one riser (R), one going (G) and the opening length.
Stair building code requirements compared
The headline stair rules for homes in the US, UK, Canada and Australia, plus US commercial buildings. Local amendments can be stricter, so always confirm with your building control body or inspector.
| Rule | IRC (US residential) | IBC (US commercial) | UK private stair | NBC Canada (houses) | NCC Australia |
|---|---|---|---|---|---|
| Maximum riser | 7 3/4″ (197 mm) | 7″ (178 mm) | 8 11/16″ (220 mm) | 7 7/8″ (200 mm) | 7 1/2″ (190 mm) |
| Minimum riser | — | 4″ (102 mm) | 5 7/8″ (150 mm) | 4 15/16″ (125 mm) | 4 1/2″ (115 mm) |
| Minimum going | 10″ (254 mm) | 11″ (279 mm) | 8 11/16″ (220 mm) | 10 1/16″ (255 mm) | 9 7/16″ (240 mm) |
| Maximum going | — | — | 11 13/16″ (300 mm) | 14″ (355 mm) | 14″ (355 mm) |
| Maximum pitch | Set by rise/going | Set by rise/going | 42° | Set by rise/going | Set by rise/going |
| 2R + G | — | — | 550–700 mm | — | 550–700 mm |
| Minimum headroom | 80″ (2032 mm) | 80″ (2032 mm) | 78 3/4″ (2000 mm) | 76 3/4″ (1950 mm) | 78 3/4″ (2000 mm) |
| Minimum width | 36″ (914 mm) | 44″ (1118 mm) | Not set | 33 7/8″ (860 mm) | Not set |
| Flight limit | 151″ (3835 mm) rise | 144″ (3658 mm) rise | No set limit | 145 11/16″ (3700 mm) rise | 2–18 risers |
| Handrail height | 864–965 mm | 864–965 mm | 900–1000 mm | 865–965 mm | 865–1200 mm |
UK utility stairs: rise 150–190 mm, going 250–400 mm. UK general access stairs: rise 150–170 mm, going 280–425 mm; both limited to 12 rises per flight (16 in small premises). IRC nosing of ¾–1¼″ is required where the tread is under 11″.
What makes a comfortable stair?
Code limits are minimums for safety, not targets. Stairs feel easiest when the riser is around 7–7½″ (175–190 mm), the going is 10½–11″ (265–280 mm), the angle is 30–37°, and twice the riser plus the going comes to about 24–25″ (600–640 mm) – the length of an average stride. A steeper stair saves floor space but is harder for children and older people, and makes fitting a stair runner trickier.
Stair terms explained
- Total rise
- Vertical distance from the finished lower floor to the finished upper floor.
- Riser (rise)
- The vertical height of one step. All risers in a flight should be equal (IRC allows ⅜″ variation).
- Going (run)
- The horizontal depth of one step measured nosing to nosing, excluding the nosing overhang.
- Tread depth
- The full depth of a tread board: going plus nosing.
- Nosing
- The front edge of a tread that overhangs the riser below.
- Total run
- Horizontal length of the flight: number of treads × going.
- Pitch line
- An imaginary line joining the nosings. Pitch angle, handrail height and headroom are measured from it.
- Stringer (string)
- The inclined board that supports the treads – cut (notched) or housed/closed (treads let into grooves).
- Headroom
- Clear height measured vertically from the pitch line to the lowest overhead obstruction, such as the stairwell trimmer.
- Flight
- An unbroken run of steps between floors or landings.
Stair calculator FAQs
How do I calculate how many steps I need?
Divide the total rise by a target riser height of about 7–7½″ (175–200 mm) and round to the nearest whole number; that is the number of risers. Divide the total rise by the risers to get the exact riser height and make sure it is within your code limit. The number of treads is one less than the risers. For a 9′ 0″ (108″) rise that gives 15 risers of 7 3/16″ and 14 treads.
What is the standard rise and run for stairs?
In US homes (IRC) the maximum riser is 7¾″ and the minimum tread is 10″. In UK homes (Approved Document K, private stair) the rise is 150–220 mm, the going 220–300 mm, with a maximum pitch of 42°. A comfortable stair is around 7–7½″ rise with a 10½–11″ run.
How do I calculate the angle of a staircase?
Stair angle = arctan(riser height ÷ tread going). For example, a 7 3/16″ riser with a 10 1/2″ going gives 34.4°. Most comfortable stairs are between 30° and 37°; UK private stairs must not exceed 42°.
How long does a stair stringer need to be?
The stringer diagonal is the square root of (total run² + total rise²). Add about 12″ (300 mm) for the top and bottom cuts and buy the next standard length. A 9′ 0″ (108″) rise with a 12′ 3″ (147″) run has a 15′ 2 7/16″ (182 7/16″) diagonal, so you would buy 18 ft boards.
How much headroom do stairs need?
The IRC and IBC require 6′ 8″ (2032 mm). Approved Document K requires 2.0 m in the UK, reduced to 1.9 m at the centre of a loft-conversion stair (1.8 m at the side). Canada's NBC requires 1950 mm for houses and Australia's NCC 2.0 m. Headroom is measured vertically from the pitch line to the lowest overhead obstruction.
How many stairs can you have before a landing?
Under the IRC one flight may rise up to 151″ (12′ 7″) before a landing; under the IBC 12′. Australia's NCC allows 2 to 18 risers per flight, and Canada's NBC limits a flight to 3.7 m of rise. In the UK, utility and general-access stairs are limited to 12 rises per flight (16 in small premises); private stairs have no set limit, but more than 16 is uncomfortable.
What is the 2R + G rule?
It is a comfort formula: twice the riser plus the going should be about 24–25″ (600–640 mm), the length of a normal stride. In the UK and Australia it is a legal requirement for 2R + G to fall between 550 and 700 mm. A 200 mm riser with a 225 mm going gives 625 mm.
Does the stair calculator work for deck and outdoor steps?
Yes. Choose IRC for US decks and set the stringer type to cut. The calculator checks the remaining stringer throat (at least 5″ is common guidance) and works out how many stringers you need at your chosen spacing.
Can I download or print the stair diagram?
Yes. Download the scaled diagram as an SVG or a high-resolution PNG, print the page, export the mark-out schedule as CSV or copy a share link that reopens the calculator with your numbers.
Is this stair calculator free?
Yes. The stair calculator and diagram are free to use with no sign-up, in imperial or metric units.
Related stair calculators
Sources
- International Code Council – 2021 International Residential Code, Section R311.7 Stairways
- International Code Council – 2021 International Building Code, Sections 1011 and 1014
- HM Government – Approved Document K: Protection from falling, collision and impact (England)
- National Research Council Canada – National Building Code of Canada 2020, Section 9.8
- Australian Building Codes Board – NCC 2022 Housing Provisions, Part 11.2
- American Wood Council – DCA 6 Prescriptive Residential Wood Deck Construction Guide
Results are estimates to help you plan. Building regulations vary by country, state and local authority and are updated regularly – check your design with your building inspector or building control before you cut timber.