Converting a stair dimension between inches and mm
Four fields decide every output on this page — pick the right dimension type first, since that's what drives the code checks.
Pick the dimension type
Riser, tread, rise, run, stringer, width or a plain generic length. This only changes which code limits get checked — the conversion math is identical either way.
Enter the value in whatever unit you have it in
Inches, feet+inches, centimeters or millimeters — switch the unit dropdown and the value carries over converted, it doesn't reset to zero.
Read every unit at once
The results strip shows decimal inches, the nearest usable fraction, millimeters, centimeters, meters and feet+inches simultaneously — no repeat conversions needed.
Check the compliance chips
A green or red chip means the figure is checked against a hard US IRC number. A grey chip means it's checked against a metric limit this page hasn't independently verified for your local code — confirm it before you build.
The four conversions behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Inches → Millimeters
25.4 is an exact, internationally defined constant — not a rounded approximation. Every mm figure on this page derives from it.
Millimeters → Inches
The reverse of the same exact constant. Round-tripping a number through both formulas should return the original value, within display rounding.
Decimal → Nearest Fraction
N is your chosen denominator (8, 16 or 32). This is the step a tape measure or a saw actually needs — a decimal like 7.6693" isn't cuttable on its own.
Millimeter Display Rounding
P is your chosen mm precision (1mm or 0.5mm). Metric tapes and rules are typically graduated in whole millimeters, so this is the equivalent rounding step on the metric side.
Converting a 7-5/8" riser to millimeters
Same four formulas, filled in step by step, for a riser height measured as seven and five-eighths inches — a common real-world tape reading, not a round number.
Convert the fraction to decimal
7-5/8" = 7 + 5÷8 = 7.625". This is the decimal value every downstream formula on this page actually uses internally.
Convert to millimeters
7.625 × 25.4 = 193.675mm, which rounds to 194mm at 1mm precision, or 193.5mm at 0.5mm precision — the two precision settings genuinely disagree here, which is why the setting matters.
Check the US code limit
IRC caps residential riser height at 7.75". Since 7.625" ≤ 7.75", this riser passes — shown as a green chip.
Check the metric code limit
Against a [VERIFY]-flagged metric riser reference of 220mm maximum, 194mm also passes — but this chip renders grey, not green, because that 220mm figure hasn't been independently confirmed against your specific local adopted code. Confirm it before treating the pass as final.
Notice the two code checks used two completely different limit numbers — 7.75" and 220mm are not the same threshold converted, they're two independently set limits from two different code bodies. See the Metric Code Nuance section below for why that matters.
Common stair dimensions, both units
The figures a stair project reaches for most often, pre-converted so you don't have to run the calculator for the obvious ones.
| Dimension | Inches | Millimeters | Notes |
|---|---|---|---|
| IRC max riser | 7.75" | 196.85 → 197mm | Residential maximum, not a target. |
| Common target riser | 7.5" | 190.5mm | This site's Rise & Run calculator default. |
| IRC min tread/going | 10" | 254mm | Residential minimum, not a target. |
| 7-11 rule tread | 11" | 279.4mm | Common comfort-rule pairing with a 7" riser. |
| IRC min headroom | 80" | 2032mm | Measured perpendicular from the nosing line. |
| IRC min clear width | 36" | 914.4mm | Typical single-family residential minimum. |
| Standard stringer board | 192" (16') | 4876.8mm | Common stocked dimensional lumber length. |
US IRC vs. metric-code reference limits
This calculator checks whichever dimension you select against the pair of limits below. Only the US column is a figure this page has verified with confidence.
| Element | US Residential (IRC) | Metric-code Reference |
|---|---|---|
| Max riser height | 7.75" / 197mm | 220mm [VERIFY] |
| Min tread depth / going | 10" / 254mm | 220mm [VERIFY] |
| Min headroom | 80" / 2032mm | 2000mm [VERIFY] |
| Min clear width | 36" / 914mm | 900mm [VERIFY] |
[VERIFY: the "Metric-code Reference" column is a placeholder drawn from commonly cited UK Building Regulations Approved Document K residential figures and has not been independently confirmed against a current, jurisdiction-specific source. Every jurisdiction using metric units — UK, EU member states, Australia, and others — sets its own numbers, and they are not identical to each other. Always confirm against your own local adopted code before building, and treat this column as a rough planning reference only.
Why you can't just convert a US code number
A converted number and a metric code limit look similar but answer two different questions — this section covers the three ways that distinction actually bites people.
Metric codes aren't translated, they're native
A country that writes its building code in millimeters set its own riser and going limits independently — they were never derived by converting the IRC's inch figures. Converting 7.75" gets you 196.85mm, a number that has no special status under a metric code at all.
Fraction rounding compounds across a flight
Rounding each riser to the nearest 1/16" individually, then stacking 14 of them, can drift from the total rise by more than rounding the total once. Convert the whole-flight total first, then divide, rather than rounding every intermediate step.
1/16" and 1mm aren't interchangeable precision
A measurement read to the nearest millimeter carries roughly 20% finer resolution than one read to the nearest 1/16 inch. Converting a metric spec to imperial at 1/16" precision silently discards some of that resolution.
Cross-checking against a sibling calculator
The 7.75" riser max and 10" tread min used here are the same numbers this site's Stair Rise and Run Calculator checks against — deliberately, so a figure never silently differs between two pages answering the same question.
If your project is being inspected under a metric-native code, treat the Metric Code Reference chip on this page as a starting point for research, not a substitute for reading your actual adopted code text.
Common mistakes when converting stair dimensions
Most conversion errors on a stair project aren't arithmetic mistakes — they're the wrong assumption applied to a correct number.
Assuming a converted IRC number is a metric code limit
196.85mm is what 7.75" converts to — it is not automatically what a metric-code jurisdiction allows. Look up the actual local figure.
Rounding every step, then adding them up
Round the total rise or total run once, at the end, rather than rounding each individual riser or tread and letting the errors stack across a full flight.
Mixing up centimeters and millimeters
A 10x factor error is easy to make when switching between mm and cm mid-project — double-check the unit label on every figure you copy into a cut list.
Trusting a fraction beyond your tape's markings
Most tape measures are marked to 1/16", occasionally 1/32". Calculating a result to 1/64" precision is meaningless if nothing in the shop can mark it that fine.
Cutting to the unrounded decimal instead of the fraction
7.6693" isn't a cut line. Always read the Nearest Fraction output, not the Decimal Inches output, when it's time to actually mark material.
When to use a stair unit converter
Unit conversion shows up any time a stair project crosses between an imperial and a metric source.
Reading a metric plan set on a US job
Imported drawings, prefab components, or an architect's metric-native CAD export landing on an imperial-built site.
Ordering imported stair parts
Glass panels, metal balusters, or LED strip specified in millimeters that need to fit an inch-measured opening.
Cross-border collaboration
Comparing a renovation plan with a contractor, architect, or supplier working in the other unit system.
Checking against a foreign code
An expat or international project where the local adopted building code is written in millimeters, not the builder's home unit.
Converting old blueprints
Older drawings sometimes mix systems, or a renovation needs to match an original spec recorded in the other unit.
Teaching and code study
Comparing US and international stair standards side by side while studying for a licensing or inspection exam.