Stair Calculator | Number of Steps, Riser Height, Tread Depth and Angle from Floor-to-Floor Height, Checked Against the Japanese Building Standards Act, Housing Performance Grades and the T + 2R Comfort Rule
The Stair Calculator answers questions such as “how many steps do I need for a 2,900 mm floor-to-floor height?”, “what riser and tread sizes are comfortable?” and “do my current stairs meet the building code?”. From the floor-to-floor height and the tread depth (or the horizontal length available for the stairs), it picks, among the step counts that meet the Building Standards Act, the one where tread + 2 × riser (T + 2R) is closest to 630 mm, and shows the riser, tread, angle, total run and sloped length. You can also set the number of steps yourself and compare step counts in a table. For each stair type (houses, elementary schools, junior and senior high schools and theatres, ground floors of large buildings, and others) it checks the maximum riser, minimum tread and minimum width in Article 23 of the Enforcement Order and whether a landing is needed, and compares the result with the housing performance grades for elderly-friendly design (slope of 22/21 or 6/7 and T + 2R of 550-650 mm). Includes a tab for checking existing stairs from measured riser and tread, a riser chart by floor height and step count, and a side-view drawing of the stairs.
Calculate steps, riser and tread
From the floor height and tread (or the run available), it picks the most comfortable step count that meets the building code. You can also set the number of steps yourself.
Stair type (Building Standards Act Enforcement Order, Article 23)
Houses (detached houses and inside dwellings; not shared stairs in apartment buildings) — riser 230 mm or less, tread 150 mm or more, width 750 mm or more
How to set the tread
Steps
Steps
14 steps
13 treads (straight flight)
Riser
207.1 mm
2,900 ÷ 14 steps
Tread
225 mm
Angle (slope)
42.6°
Typical for houses
T + 2R
639.3 mm
Comfortable
Run (straight flight)
2,925 mm
tread × 13
Sloped length
4,119 mm
Riser for T + 2R = 630 mm
202.5 mm
- Riser 207.1 mm (max 230 mm)
- Tread 225 mm (min 150 mm)
- Width 780 mm (min 750 mm)
- Housing performance (elderly-friendly design): meets the grade 3 guide
- T + 2R = 639.3 mm: Comfortable (guide 600-650 mm)
Compare step counts
Tap a row to use that step count. The highlighted row is the current one and ★ marks the recommendation.
| Steps | Riser | Tread | Angle | T + 2R | Run | Building code | Grade |
|---|---|---|---|---|---|---|---|
| 11 | 263.6 | 225 | 49.5° | 752.3 | 2,250 | × | – |
| 12 | 241.7 | 225 | 47.0° | 708.3 | 2,475 | × | – |
| 13 | 223.1 | 225 | 44.8° | 671.2 | 2,700 | ○ | – |
| ★ 14 | 207.1 | 225 | 42.6° | 639.3 | 2,925 | ○ | 3 |
| 15 | 193.3 | 225 | 40.7° | 611.7 | 3,150 | ○ | 3 |
| 16 | 181.3 | 225 | 38.9° | 587.5 | 3,375 | ○ | 4·5 |
| 17 | 170.6 | 225 | 37.2° | 566.2 | 3,600 | ○ | 4·5 |
| 18 | 161.1 | 225 | 35.6° | 547.2 | 3,825 | ○ | – |
| 19 | 152.6 | 225 | 34.2° | 530.3 | 4,050 | ○ | – |
| 20 | 145 | 225 | 32.8° | 515 | 4,275 | ○ | – |
| 21 | 138.1 | 225 | 31.5° | 501.2 | 4,500 | ○ | – |
All lengths in mm. Red numbers fail the building code or are awkward to walk.
ABOUT
About Stair Calculator | Number of Steps, Riser Height, Tread Depth and Angle from Floor-to-Floor Height, Checked Against the Japanese Building Standards Act, Housing Performance Grades and the T + 2R Comfort Rule
The Stair Calculator is a free tool that works out the number of steps, riser height, tread depth, angle and total run from the floor-to-floor height and the tread depth (or the available horizontal space), and checks them against Article 23 of the Enforcement Order of Japan’s Building Standards Act, the housing performance grades for elderly-friendly design and the T + 2R comfort rule.
How to use Stair Calculator | Number of Steps, Riser Height, Tread Depth and Angle from Floor-to-Floor Height, Checked Against the Japanese Building Standards Act, Housing Performance Grades and the T + 2R Comfort Rule
- 1Choose the stair type (house, elementary school, junior/senior high school or theatre, ground floor of a large building, or other)
- 2Enter the floor-to-floor height (from the lower floor surface to the upper floor surface) in mm
- 3Enter the tread depth, or choose “From the available run” and enter the horizontal length you have for the stairs
- 4Check the recommended number of steps, the riser, tread, angle, T + 2R and total run, and the building code and grade checks
- 5Tap rows in the comparison table to try other step counts. For existing stairs, enter measured dimensions in the “Check existing stairs” tab
Great for
- Finding out how many steps a staircase will need for a given floor height before building or renovating a house
- Deciding the tread depth and number of steps from the length available on a floor plan (such as 1.5 or 2 ken)
- Setting the riser height for stairs you build yourself, such as deck or garden steps
- Comparing stairs at home that feel steep with the building code and housing performance standards
- Checking riser, tread and width requirements for shops, offices or shared stairs in apartment buildings
- Seeing whether a parent’s stairs are easy for older people to use, judged by T + 2R and slope
Good to know
- The number of steps is the number of risers, and riser = floor-to-floor height ÷ number of steps. A straight flight has one tread fewer than steps (the top step is the upper floor), so the total run is tread × (steps − 1). For example, with a 2,900 mm floor height and 225 mm treads, the recommendation is 14 steps: a 207.1 mm riser, 42.6°, T + 2R of 639.3 mm and a run of 2,925 mm.
- The recommended step count is the one, among those meeting the code’s maximum riser and minimum tread, where tread + 2 × riser (T + 2R) is closest to 630 mm. Stairs are said to be comfortable when T + 2R is close to a person’s stride, so 600-650 mm is used as the guide. Stairs steeper than a 22/21 slope (about 46.3°) are rated “average” even if T + 2R is in range.
- Article 23 of the Enforcement Order sets these minimums: houses (excluding shared stairs in apartment buildings) riser 230 mm or less, tread 150 mm or more, width 750 mm or more; other stairs 220 mm, 210 mm and 750 mm; ground floors where the rooms on the floor above exceed 200 m² and similar, 200 mm, 240 mm and 1,200 mm; junior and senior high schools and theatre audiences, 180 mm, 260 mm and 1,400 mm; and elementary school pupils, 160 mm, 260 mm and 1,400 mm. The house minimum (230 mm riser, 150 mm tread) means an angle of 56.9°, so stairs can be legal and still steep.
- Stairs taller than 4 m (3 m for schools and similar) need a landing within every 4 m (or 3 m) of height, at least 1,200 mm deep on a straight flight. On winder stairs the tread is measured 300 mm from the narrow end, and up to 100 mm of a handrail’s projection is ignored when measuring width.
- For stairs inside a dwelling, the housing performance grades for elderly-friendly design use a slope of 22/21 or less, T + 2R of 550-650 mm and a tread of 195 mm or more for grade 3, and a slope of 6/7 or less with T + 2R of 550-650 mm for grades 4 and 5. There are other requirements such as a toe recess of 30 mm or less and handrails; this tool only checks slope, T + 2R and tread.
- Results are a design guide. Dimensions can be relaxed under ministry notices (handrails on both sides, non-slip treads and so on), and local ordinances may set other rules, so confirm actual plans with an architect or building inspector.
Questions this tool answers
Use Stair Calculator | Number of Steps, Riser Height, Tread Depth and Angle from Floor-to-Floor Height, Checked Against the Japanese Building Standards Act, Housing Performance Grades and the T + 2R Comfort Rule for searches like these:
- Calculate the number of steps and riser height from floor height
- Learn the building code rules for riser and tread
- Find comfortable stair dimensions (T + 2R)
- Decide tread depth and steps from the space available
- Calculate stair slope and angle
- Check whether existing stairs are too steep
FAQ
- How many steps do I need for a 2,900 mm floor-to-floor height?
- With 225 mm treads, 14 steps is recommended: a 207.1 mm riser, 42.6°, T + 2R of 639.3 mm and a straight run of 2,925 mm. With 13 steps the riser is 223.1 mm (T + 2R of 671.2 mm), which meets the code but is steep and does not match a normal stride.
- What are the riser and tread rules for houses?
- Article 23 of the Enforcement Order of the Building Standards Act sets a riser of 230 mm or less, a tread of 150 mm or more and a width of 750 mm or more for stairs in houses. That minimum works out to 56.9°, so real houses usually use risers of around 200 mm and treads of about 220-250 mm.
- What stair dimensions are comfortable?
- Stairs are considered comfortable when tread (T) plus twice the riser (R), T + 2R, is 600-650 mm (around 630 mm). A 200 mm riser with a 230 mm tread gives T + 2R of 630 mm and 41.0°. The housing performance standards use T + 2R of 550-650 mm.
- Can I work out the steps from the space available?
- Yes. Choose “From the available run” and the tread is calculated as run ÷ (steps − 1). For a 2,900 mm floor height, 2,730 mm (1.5 ken) gives 14 steps with 210 mm treads, 207.1 mm risers and 44.6°, and 3,640 mm (2 ken) gives 15 steps with 260 mm treads, 193.3 mm risers and 36.6°.
- When is a landing required?
- When the stairs rise more than 4 m (3 m for elementary, junior and senior high schools and theatre audiences), a landing is needed within every 4 m (or 3 m). A landing on a straight flight must be at least 1,200 mm deep. Typical house floor heights (2,700-3,000 mm) do not need a landing under the code.
- How can I tell whether my stairs are steep?
- In the “Check existing stairs” tab, enter the riser (from one step surface to the next) and tread (from one nosing to the next) you measured with a tape. It shows the angle, T + 2R and whether the stairs meet the building code and housing performance standards. Over 45° is steep, and about 30-37° is easy to climb.
Special Thanks