How to Size Stair Risers, Treads and the Number of Steps | Japan's Building Code Limits and the Comfortable-Stair Formula (T + 2R)
When you build or remodel a house, the same questions keep coming up: how many steps should the stair have, what riser and tread make it easy to climb, and is the stair you already have too steep? Stair dimensions come down to three things: dividing the floor-to-floor height by the number of steps, the maximum and minimum sizes in Article 23 of Japan's Building Standards Act Enforcement Order, and the comfort rule “T + 2R”. This guide covers how risers, treads and steps are counted, the legal limits for each type of building, the Japanese housing performance grades, a riser table for floor-to-floor heights of 2,600 to 3,000 mm, how to choose the number of steps from the length available in the floor plan, the pitfalls of winder stairs, and how to measure and check an existing stair — all with worked numbers.
The short answer: riser = floor height ÷ steps, and comfort = T + 2R of 600–650 mm
Stair dimensions are decided in three steps. First, divide the floor-to-floor height by the number of steps to get the riser (the height of one step), and pick a number of steps that keeps the riser within the legal maximum. Next, check that the combination of tread (the depth you put your foot on) and riser fits the human stride, using “T + 2R” (tread + riser × 2) of 600–650 mm. Finally, check that the horizontal length of the stair at that number of steps fits the floor plan.
| What to decide | Calculation / rule | House example (floor height 2,900 mm, tread 225 mm) |
|---|---|---|
| Riser (R) | Floor-to-floor height ÷ number of steps | 2,900 ÷ 14 = 207.1 mm |
| Legal limits | In a house: riser 230 mm or less, tread 150 mm or more, width 750 mm or more (Enforcement Order Art. 23) | 207.1 mm and 225 mm, so it complies |
| Comfort (T + 2R) | Tread + riser × 2 between 600 and 650 mm (about 630 mm) | 225 + 207.1 × 2 = 639.3 mm |
| Angle | arctan(riser ÷ tread) | 42.6° |
| Horizontal length (straight stair) | Tread × (steps − 1) | 225 × 13 = 2,925 mm |
The figures in the Building Standards Act are minimums: “do not make it steeper than this”. A stair built to the minimum for houses — a 230 mm riser and a 150 mm tread — has an angle of 56.9°, which is closer to a ladder than a stair. Real Japanese houses usually use risers of 180–210 mm and treads of 220–260 mm, giving angles of roughly 35–43°.
Riser, tread, nosing and floor-to-floor height — and how steps are counted
Here are the terms that come up in any discussion of stairs. The way “steps” are counted in particular varies from person to person, so when reading drawings or quotes, check what is being counted.
| Term | Meaning | How to measure / notes |
|---|---|---|
| Riser | The height of one step, written R (rise) | From the top surface of one step to the top surface of the next |
| Tread (going) | The depth you put your foot on, written T | Horizontal distance from one nosing to the next. The overhang of the nosing is not included |
| Nosing | The front edge of the step | Where anti-slip strips go. If it is hard to see, people miss their footing |
| Kick-back (toe space) | The part set back behind the nosing, or the board under it | A deep set-back makes it easy to catch your toes when going up |
| Floor-to-floor height | From the floor of the lower storey to the floor of the upper storey | Not the ceiling height but the difference between finished floor levels. With a 2,400 mm ceiling it is usually 2,800–3,000 mm |
| Number of steps | The number of risers | The top step is the upper floor itself, so a straight stair has one fewer tread than steps |
A “14-step stair”, for example, has 14 risers and 13 treads (tread boards). Some people count the boards and call it “13 steps”, and if the count is off by one the riser changes a lot. For a 2,900 mm floor height, dividing by 14 gives 207.1 mm while dividing by 13 gives 223.1 mm — a 16 mm difference.
Legal limits on riser, tread and width in Japan (Enforcement Order Article 23)
The minimum stair dimensions in Japan are set by Article 23 of the Building Standards Act Enforcement Order, according to the use and size of the building. Stairs inside a house (excluding common stairs in apartment buildings) have the loosest rules, and buildings used by many people, such as schools and theatres, have the strictest.
| Type of stair | Riser | Tread | Width of stair and landing | Landing required when the stair rises |
|---|---|---|---|---|
| Elementary school, for pupils | 160 mm or less | 260 mm or more | 1,400 mm or more | More than 3 m |
| Junior and senior high schools for students; theatres, cinemas and assembly halls for audiences; shops over 1,500 m² for customers | 180 mm or less | 260 mm or more | 1,400 mm or more | More than 3 m |
| Stairs on above-ground floors where the habitable rooms on the floor above exceed 200 m², or basements with habitable rooms over 100 m², etc. | 200 mm or less | 240 mm or more | 1,200 mm or more | More than 4 m |
| Houses (excluding common stairs in apartment buildings) | 230 mm or less | 150 mm or more | 750 mm or more | More than 4 m |
| Any other stair | 220 mm or less | 210 mm or more | 750 mm or more | More than 4 m |
- Landings: when a stair rises more than 4 m (3 m for schools, theatres and similar), a landing is required at least every 4 m (or 3 m). A landing partway up a straight stair must be at least 1,200 mm deep (Enforcement Order Art. 24). A typical house with a floor height of 2,700–3,000 mm does not legally need one.
- Handrails: stairs must have a handrail, and any side without one needs a wall or similar side barrier (Art. 25). Parts of the stair up to 1 m high are exempt.
- Measuring width: handrails and stair-lift rails that project no more than 100 mm from the wall are ignored when measuring width.
- Winder treads: the tread of a winder is measured 300 mm from the narrow end (Art. 23, para. 2).
- Relaxations: ministerial notices can relax the figures under certain conditions — for example, an elementary school stair can have a riser of up to 180 mm with treads of 260 mm or more if it has handrails on both sides and slip-resistant treads.
Stairs in shops, offices and the common areas of apartment buildings usually fall under “any other stair” or the “above-ground floor” category, so the targets are a riser of 220 mm or less with a tread of 210 mm or more, or 200 mm or less with 240 mm or more. Escape stairs may also be subject to the Fire Service Act and local ordinances, so check early once the building use is decided.
Comfortable stair dimensions come from “T + 2R”
Meeting the legal limits does not make a stair easy to use. The usual measure of comfort is “T + 2R”: the tread (T) plus twice the riser (R). The idea is that lifting your foot takes roughly twice the effort of moving forward on the flat, and when the total is close to the human stride (about 600–650 mm) you can go up and down in an easy rhythm.
| Riser (R) | Tread (T) | T + 2R | Angle | How it feels |
|---|---|---|---|---|
| 230 mm | 150 mm | 610 mm | 56.9° | The legal minimum for houses. Ladder-like and scary going down |
| 220 mm | 210 mm | 650 mm | 46.3° | The steep stair common in older houses |
| 200 mm | 230 mm | 630 mm | 41.0° | A standard house stair |
| 190 mm | 240 mm | 620 mm | 38.4° | A generous house stair |
| 180 mm | 260 mm | 620 mm | 34.7° | Easy to use, also suits older family members |
| 160 mm | 300 mm | 620 mm | 28.1° | Close to the gentle stairs in public buildings |
As the table shows, the angle changes a lot even when T + 2R is almost the same. A stair steeper than 45° feels frightening because you cannot easily see your feet on the way down, even if T + 2R matches your stride. On the other hand, steps lower than about 150 mm feel tiring because your stride no longer fits. As a rule of thumb, a stair that satisfies both an angle of around 30–40° and a T + 2R of 600–650 mm is comfortable.
Stair criteria in Japan's housing performance indication (consideration for older people)
Japan's Housing Performance Indication System includes a category for “consideration for older people”, which sets stricter targets than the Building Standards Act for stairs inside the dwelling. If you plan to live in the house for a long time, or are remodelling with accessibility in mind, these are good targets.
| Grade | Slope (riser ÷ tread) | T + 2R | Tread | As an angle |
|---|---|---|---|---|
| Grades 4 and 5 | 6/7 or less | 550–650 mm | — | About 40.6° or less |
| Grade 3 | 22/21 or less | 550–650 mm | 195 mm or more | About 46.3° or less |
| Grades 1 and 2 | Roughly the same as the Building Standards Act | — | — | — |
A stair with a 207.1 mm riser and a 225 mm tread, for example, has a slope of 207.1 ÷ 225 = 0.92. That is steeper than 6/7 (about 0.857) but within 22/21 (about 1.048), and with a T + 2R of 639.3 mm it meets the grade 3 target. With the same 2,900 mm floor height, using 16 steps lowers the riser to 181.3 mm and the slope to about 0.81, which meets the grade 4 and 5 target. The grades also have conditions on kick-back depth, handrails and the shape of winder stairs.
Step by step: from floor height to steps, riser and tread
- 1
Confirm the floor-to-floor height
Take the floor-to-floor height from the drawings, or the height in millimetres from the finished ground floor to the finished first floor. For a two-storey timber house in Japan, 2,700–3,000 mm is typical.
- 2
Find the minimum legal number of steps
Divide the floor height by the maximum riser and round up. For a house with a 2,900 mm floor height, 2,900 ÷ 230 = 12.6, so the minimum is 13 steps. With 12 steps the riser would be 241.7 mm, over the limit.
- 3
Compare the riser and T + 2R for each step count
Assuming a 225 mm tread: 13 steps give a 223.1 mm riser and T + 2R of 671.2 mm, 14 steps give 207.1 mm and 639.3 mm, and 15 steps give 193.3 mm and 611.7 mm. The candidates are 14 or 15 steps, which fall within 600–650 mm, and 14 steps, closest to 630 mm, is the recommendation.
- 4
Check that the horizontal length fits the plan
The horizontal length of a straight stair is tread × (steps − 1): 2,925 mm for 14 steps with 225 mm treads, and 3,150 mm for 15 steps. If it does not fit, narrow the tread slightly, or turn the stair with an L-shaped or winder layout.
- 5
Check the angle and grade
The angle is arctan(riser ÷ tread): 42.6° for 14 steps and 40.7° for 15. If older family members live with you, consider a step count that meets the grade 4 and 5 target (slope of 6/7 or less).
Enter the floor height and tread into the stair calculator and all five of these results appear at once. The comparison table lists 13, 14 and 15 steps side by side, which is handy for a first estimate before drawing anything.
Riser table by floor height and number of steps (2,600–3,000 mm)
This table lists the riser for each number of steps at common floor heights for houses. Values over the 230 mm limit for houses are marked “(not allowed)”. The step count whose T + 2R is closest to 630 mm with a 225 mm tread is marked “◎”.
| Floor height | 12 steps | 13 steps | 14 steps | 15 steps | 16 steps |
|---|---|---|---|---|---|
| 2,600 mm | 216.7 mm | ◎ 200.0 mm | 185.7 mm | 173.3 mm | 162.5 mm |
| 2,700 mm | 225.0 mm | ◎ 207.7 mm | 192.9 mm | 180.0 mm | 168.8 mm |
| 2,800 mm | (not allowed) 233.3 mm | 215.4 mm | ◎ 200.0 mm | 186.7 mm | 175.0 mm |
| 2,900 mm | (not allowed) 241.7 mm | 223.1 mm | ◎ 207.1 mm | 193.3 mm | 181.3 mm |
| 3,000 mm | (not allowed) 250.0 mm | (not allowed) 230.8 mm | 214.3 mm | ◎ 200.0 mm | 187.5 mm |
The table shows that you should aim for a step count that gives a riser of about 200 mm. With a 2,700 mm floor height, 13 steps (207.7 mm) give a T + 2R of 640.4 mm and 14 steps (192.9 mm) give 610.7 mm, both within the comfortable range. When in doubt, adding one step lowers the riser and flattens the angle, making the descent easier. The cost is that the horizontal length grows by one tread (225 mm here).
Choosing steps and tread from the length in the floor plan
Japanese houses are usually planned on a 910 mm grid (one ken = 1,820 mm), so the length available for the stair is often a multiple of 910 mm. In that case, work out the tread for each step count as tread = available horizontal length ÷ (steps − 1).
| Floor height 2,900 mm, available length | Recommended steps | Tread | Riser | Angle | T + 2R |
|---|---|---|---|---|---|
| 2,730 mm (1.5 ken) | 14 steps | 210.0 mm | 207.1 mm | 44.6° | 624.3 mm |
| 3,185 mm (1.75 ken) | 15 steps | 227.5 mm | 193.3 mm | 40.4° | 614.2 mm |
| 3,640 mm (2 ken) | 15 steps | 260.0 mm | 193.3 mm | 36.6° | 646.7 mm |
A straight stair in 2,730 mm matches the stride in terms of T + 2R, but at 44.6° it is quite steep. With 3,640 mm the angle drops to 36.6° and meets the grade 4 and 5 target (slope of 6/7 or less). When a straight run does not fit, an L-shaped stair that turns 90° partway, or a switchback stair, lets you add steps within the same floor area.
Treads on winder and L-shaped stairs
Winder stairs, which have triangular steps (winders) at the turn, save space but leave the inside of each tread very narrow. Under Japan's Building Standards Act, the tread of a winder is measured 300 mm from its narrow end.
| Turn | Angle per step | Tread 300 mm from the end | House minimum of 150 mm |
|---|---|---|---|
| 90° in 3 steps | 30° | About 157 mm | Complies |
| 90° in 4 steps | 22.5° | About 118 mm | Does not comply |
| 180° in 6 steps | 30° | About 157 mm | Complies |
The table assumes the winders are divided into equal angles around the inside corner (300 mm × π × angle ÷ 180°). Even when it complies, the inside of a winder is too narrow for a foot, so prevent missteps by adding a handrail on the inside too, placing the winders near the bottom rather than the top, or turning on a flat landing instead. The higher housing performance grades also restrict the shape of winder stairs.
How to measure an existing stair and check whether it is too steep
- 1
Measure the riser
Stand a tape measure on one step and measure up to the top of the next. Averaging several steps reduces the error. Measure the bottom and top steps separately, as their height often differs because of the floor finish.
- 2
Measure the tread
Measure the horizontal distance from one nosing to the next. Measuring the depth of the tread board itself adds the nosing overhang and the kick-back, so be careful.
- 3
Work out the angle and T + 2R
A 220 mm riser and a 210 mm tread give an angle of 46.3° and a T + 2R of 650 mm. That is legal for a house, but at more than 45° it is a steep stair.
Rebuilding a steep stair is a big job, but you can improve safety a great deal by fitting handrails on both sides, adding contrasting anti-slip strips to the nosings so the edges are easy to see, and lighting the lower steps with foot lights. Handrails are commonly set 700–900 mm above the nosing.
Work out steps, riser and tread at once with the stair calculator
In the stair calculator, choose the type of stair (house, elementary school, junior high school or theatre, above-ground floor of a large building, or other), then enter the floor height and either the tread or the length available for the stair. It shows the recommended number of steps along with the riser, tread, angle, T + 2R, horizontal length and sloping length. It also checks the Article 23 maximums and minimums, whether a landing is required, and the housing performance grade targets.
- Comparison table by step count: riser, angle, T + 2R and horizontal length as the number of steps changes one at a time
- Side view: a drawing of the stair with dimension lines for the riser and tread
- Check an existing stair: enter the riser and tread you measured to see the angle and whether it meets the criteria
- Riser table: all combinations of floor height and step count in one list
To compare slopes as a percentage or angle, see also converting slope between percent and degrees.
FAQ
- How many steps should a stair have for a 2,900 mm floor-to-floor height?
- With a 225 mm tread, 14 steps is the recommendation: a 207.1 mm riser, a 42.6° angle, a T + 2R of 639.3 mm and a horizontal length of 2,925 mm for a straight stair. With 13 steps the riser becomes 223.1 mm (T + 2R of 671.2 mm) and the stair is steep; with 15 steps the riser is 193.3 mm at 40.7°, and the horizontal length grows to 3,150 mm.
- What are the legal riser and tread limits for a house in Japan?
- Under Article 23 of the Building Standards Act Enforcement Order, stairs in a house (excluding common stairs in apartment buildings) must have a riser of 230 mm or less, a tread of 150 mm or more and a width of 750 mm or more. Built to that minimum the angle is 56.9°, so in practice risers of 180–210 mm and treads of 220–260 mm are common.
- What does T + 2R mean?
- It is the tread plus twice the riser, a measure of how comfortable a stair is. A value of 600–650 mm (about 630 mm) matches the human stride and is considered easy to walk, and Japan's housing performance indication uses 550–650 mm. A 200 mm riser with a 230 mm tread gives 630 mm.
- Is the number of steps the number of tread boards?
- The number of steps usually means the number of risers. On a straight stair the top step is the upper floor itself, so there is one fewer tread board than steps. A 14-step stair has 13 treads, and its horizontal length is tread × 13.
- Does a house stair need a landing?
- Japan's Building Standards Act requires a landing at least every 4 m when the stair rises more than 4 m. A typical house with a 2,700–3,000 mm floor height does not legally need one, but a landing partway up a switchback stair improves safety because a fall does not continue all the way to the bottom.
- Where is the tread of a winder stair measured?
- It is measured 300 mm from the narrow end. Turning 90° in 3 steps around the inside corner gives a tread of about 157 mm at that point, which meets the 150 mm minimum for houses, but turning in 4 steps gives about 118 mm, which does not.
- What stair angle is safe?
- Stairs of about 30–40° are easy to use. Japan's housing performance grades 4 and 5 target a slope of 6/7 or less (about 40.6° or less), and grade 3 targets 22/21 or less (about 46.3° or less). Above 45°, it becomes hard to see your feet on the way down and the stair feels steep.
Tools in this article
Related articles
Ski DIN Setting Chart by Weight and Height | How Skier Type, Age and Boot Sole Length Change Your Release Value
Every ski binding has a release setting — the DIN value — that decides how hard a fall has to be before the ski comes off. Shops and rental forms ask for your weight, height and skier type to set it, but few people ever explain what a normal number looks like or what changes between Type 2 and Type 3. Using the table in the international standard ISO 11088, this guide gives a DIN chart by weight and height, explains how the skier code is chosen, why skier type, age and boot sole length move the value, typical settings for children and skiers aged 50 and over, what happens when the setting is too high or too low, how to read a binding's range, and what to tell the shop before the season — all with worked examples.
Read article →Why Vertical Text Breaks on X, LINE and Instagram — and How to Fix Crooked Columns, Sideways Dashes and Wrapped Lines
You saw a post or profile written vertically in Chinese, Japanese or Korean, tried it yourself, and the columns came out crooked, the long dash stayed horizontal, or the lines wrapped halfway and fell apart. Social apps have no vertical writing mode: the vertical text you see is ordinary horizontal text rearranged one character per line, so it breaks as soon as one of three things is off — character width, screen width or the direction of punctuation. This guide separates those three causes and shows how to fix each one, how line-breaking rules keep punctuation off the top of a column, how much longer a post gets on X once it is vertical (full-width characters count as 2, up to 280), what to watch for on X, LINE and Instagram, and how to post vertical text without losing screen readers and search — with numbers from real conversions.
Read article →How to Calculate Your One Rep Max (1RM) | 100 kg for 10 Reps Is What Max? 7 Formulas Compared, Charts, Rep Max Table, %1RM and Training Loads by Goal
“I benched 80 kg for 8 reps, so what is my max?” “My 1RM is 100 kg, so what should I load for sets of 10?” You don’t have to attempt a true max to know your one rep max (1RM): you can estimate it from a weight you lifted and how many reps you got. This guide covers the most common formula, Epley (1RM = weight × (1 + reps ÷ 30)), how the seven well-known formulas (Brzycki, O’Conner and others) and the NSCA chart differ, why they drift apart as reps go up, a weight × reps chart, a rep max table that works backward from your 1RM, how to adjust with RIR and RPE when you didn’t go to failure, loads for strength, hypertrophy and endurance, and how to warm up and load the bar when you test a real max. Every number was calculated, not copied.
Read article →