Metric Stair Calculator Guide: Rise and Run in Millimetres
A complete metric guide to stair rise and run: measure in millimetres, apply code limits, use inch-to-mm conversion tables, and work through a full example for a 2800 mm floor-to-floor height.
Most stair calculators ask for rise and run in inches, but if you build, design, or renovate outside the United States, millimetres are the language of the tape measure. Working in millimetres removes the fraction arithmetic behind most stair mistakes, leaving clean base-10 numbers from the first measurement to the final cut. This guide shows you how to plan a complete staircase in metric units: the code limits converted to millimetres, quick inch-to-mm conversion tables, a full worked example for a typical floor-to-floor height, and the metric comfort formula that keeps stairs feeling right underfoot. If you want to skip the hand math, run the same numbers through our stair calculator and check its output against the steps below.
Why millimetres make stair math easier
An inch divides into halves, quarters, eighths, and sixteenths, and every one of those fractions is a chance to misread a tape or misplace a mark. A millimetre is one-thousandth of a metre and needs no subdivision at all. When you divide a 2800 mm floor-to-floor height by 16 risers, you get exactly 175 mm per riser, a number you can mark with a single tick. Do the same in inches, 110.24 inches divided by 16, and you get 6.89 inches, which you must then convert back to a fraction your tape understands. Metric tapes are also marked more densely: most show every millimetre, so the mark you read is the mark you cut, and small rounding errors do not accumulate across the flight.
The one rule: measure everything in millimetres first
Before any calculation, decide your datum points and measure them in millimetres, finished surface to finished surface. Total rise is the vertical distance from the finished lower floor to the finished upper floor, including tile, hardwood, or carpet buildup on both levels. Total run is the horizontal distance available for the stair, measured from the face of the upper floor edge to where the bottom riser will land. Stair width is measured between finished wall faces or stringers. Write each number down in millimetres only, with no centimetres mixed in: 280 cm and 2800 mm look similar on a notepad and have ruined more than one staircase. If your tape shows both scales, use the millimetre edge for every stair measurement and ignore the centimetre numbers entirely during layout.
Code limits for rise and run, in millimetres
Building codes set the legal boundaries for riser height and tread depth. Here are the key limits converted to millimetres.
Under the International Residential Code, which governs most US homes, Section R311.7.5.1 caps riser height at 7.75 inches, which is 196.85 mm (use 197 mm as your working maximum). Minimum tread depth is 10 inches, or 254 mm, measured nosing to nosing. Riser heights within a single flight must not vary by more than 0.375 inches, which is 9.525 mm. Nosing projection must fall between 0.75 and 1.25 inches, or roughly 19 to 32 mm. Headroom must be at least 80 inches (2032 mm) measured vertically from the nosing line, and minimum stair width is 36 inches (914 mm).
Other countries publish their codes natively in metric. In England and Wales, Approved Document K allows 220 mm maximum risers with 220 mm minimum goings, and caps pitch at 42 degrees. Australia's National Construction Code allows a maximum 190 mm riser and requires a minimum 240 mm going. Canada's National Building Code permits risers from 125 to 200 mm with runs from 255 to 355 mm. Our guide to standard stair dimensions covers the full set of width, headroom, and landing rules these limits sit inside.
Riser and tread conversion tables
When you inherit imperial plans or an American-made calculator, these tables convert the common sizes to millimetres. The exact factor is 1 inch = 25.4 mm.
| Riser (inches) | Riser (mm) | Notes |
|---|---|---|
| 7 | 177.8 | Comfortable residential target |
| 7.25 | 184.2 | Common US residential riser |
| 7.5 | 190.5 | Upper end of comfortable |
| 7.75 | 196.9 | IRC maximum (R311.7.5.1) |
| 8 | 203.2 | Exceeds IRC, steep |
| Tread (inches) | Tread (mm) | Notes |
|---|---|---|
| 10 | 254.0 | IRC minimum tread depth |
| 10.5 | 266.7 | Comfortable residential |
| 11 | 279.4 | IBC minimum for commercial |
| 11.5 | 292.1 | Generous residential tread |
| 12 | 304.8 | Deep, easy tread |
How to calculate risers and goings in millimetres
Step 1: find the riser count. Divide total rise by your target riser height, aiming for 170 to 180 mm for a comfortable residential stair and never exceeding your code maximum. Take a typical floor-to-floor height of 2800 mm: 2800 divided by 175 gives exactly 16 risers. Real heights rarely divide so neatly. Suppose the finished measurement is 2870 mm: 2870 divided by 175 gives 16.4, so round to 16 risers and recalculate. 2870 divided by 16 equals 179.4 mm per riser.
Step 2: check the comfort formula. The metric version of the classic 2R + T rule, sometimes called Blondel's formula, says twice the riser plus the going should land between 600 and 640 mm. With 179.4 mm risers, try a 260 mm going: 2 times 179.4 plus 260 equals 618.8 mm, right in the comfort band. If the result falls below 600, the stair will feel cramped; above 640, it will feel like a march up shallow ledges.
Step 3: find the total run. Multiply the going by one fewer than the riser count, because the top tread is the upper floor itself. With 16 risers and a 260 mm going, 15 times 260 equals 3900 mm of horizontal run.
Total run, stair length, and stringer length in metric
Total run tells you the footprint; the stringer length tells you how long to cut the structural boards. Treat the stair as a right triangle: total rise is the vertical side, total run is the horizontal side, and the stringer is the hypotenuse. For the example above, the square root of (2800 squared plus 3900 squared) equals the square root of 23,050,000, which is about 4801 mm, or 4.80 metres of stringer per side. Add a little extra for the top and bottom cuts before you buy lumber, then lay the stringer out with a framing square set to your riser and going in millimetres. Our stringer length walkthrough works the same triangle with additional examples if you want more practice.
Metric mistakes that ruin staircases
The most expensive error is mixing units mid-project: cutting stringers from an inch-based plan while measuring the opening in millimetres, or vice versa. Pick millimetres at the first measurement and never switch. Second, do not round riser heights until the final division is done; rounding 179.4 to 179 mm on each of 16 risers silently steals 6.4 mm from the total rise and leaves the top or bottom step noticeably off. Third, measure from finished floors, not subfloors: a 20 mm tile bed plus 12 mm of adhesive on the landing changes every riser in the flight if you ignore it. Fourth, watch the centimetre trap. Writing 280 cm when you mean 2800 mm, or reading the cm scale on a dual-marked tape during layout, produces a stair ten times too short or too tall. Fifth, remember the going is measured nosing to nosing, not the full board depth: with a 25 mm nosing projection, a 285 mm board gives a 260 mm going.
Frequently Asked Questions
What is the standard riser height in millimetres?
For US residential work under the IRC, the maximum is 197 mm (7.75 inches), and most builders target 175 to 185 mm for comfort. UK private stairs allow up to 220 mm, and Australia caps risers at 190 mm.
What is the standard tread depth in millimetres?
The IRC minimum tread depth is 254 mm (10 inches), measured from nosing to nosing. A 260 to 280 mm going feels comfortable in most homes.
How do I convert stair measurements from inches to millimetres?
Multiply inches by exactly 25.4. A 7.5 inch riser becomes 190.5 mm, and a 10.5 inch tread becomes 266.7 mm. For quick field work, multiply by 25 and add a small margin, then verify with the exact factor before cutting.
How many steps do I need for a 2800 mm floor-to-floor height?
Divide 2800 by your target riser. At 175 mm per riser you need exactly 16 risers. At 180 mm you get 15.6, so use 16 risers at 175 mm or 15 risers at 186.7 mm.
What is the 2R + T comfort rule in metric?
Twice the riser height plus the going (tread depth) should total between 600 and 640 mm. For example, 175 mm risers with 260 mm goings give 610 mm, which most people find a natural, easy stride.
Every staircase is a chain of small decisions, and metric units keep each decision simple. Measure finished surface to finished surface, divide total rise by a 170 to 180 mm target riser, verify the result with 2R + T between 600 and 640 mm, and check every figure against your local code limits. Do that, and the staircase you cut will match the staircase you planned, to the millimetre.
