Calculators
Interior unit converter
One inch is 25.4 mm exactly. Almost every other conversion indoors is a rounding decision.
On this page (6 sections)
Short answer
One inch is 25.4 mm (2.54 cm), by definition rather than by measurement, so every length conversion here is exact before it is rounded. The converter reports inches to the nearest 1/16 in because that is the finest division on a tape, and millimetres as whole numbers because joinery is set out in whole millimetres. Round too early and a run of cabinets drifts; round too late and you are quoting 91.44 cm to a joiner.
The numbers, at a glance
- Inch to millimetre
- 25.4 mm exactlyA definition since 1959, not an approximation
- Inch to centimetre
- 2.54 cmMetric lengths are shown to one decimal place
- Foot to metre
- 0.3048 m3 ft is 914 mm, not 900 mm
- Square foot to square metre
- 0.0929 m2One square metre is 10.764 sq ft
- Lumens per sq ft to lux
- 1 to 10.76The lighting mode reports both
- Yard to metre
- 0.9144 mFabric is cut in both systems
- Inch answers rounded to
- 1/16 in (1.6 mm)The resolution of a steel tape
- Millimetre answers rounded to
- 1 mmNo decimals; joinery works in whole mm
- The near miss to watch
- 24 in = 609.6 mmA 600 mm carcass is 9.6 mm narrower
Interior unit converter#
The conversions that come up while planning a room, in one place: length, area, lighting and fabric, with the rounding conventions the trade actually uses.
Millimetres are given without decimals because joinery is set out in whole millimetres, and inches are given to the nearest 1/16 in because that is the finest division on a tape.
Nothing you type leaves your browser: the maths runs on this page, and no values are stored or sent anywhere.
Four modes, one box. The converter does lengths, areas, lighting and fabric, and it rounds each answer to the resolution of the trade that will use it. The maths runs in your browser as you type: nothing is stored and nothing is sent anywhere. These are planning figures, not a substitute for the dimension printed on a product drawing.
What each mode does, and what it reads#
The Convert selector chooses the mode, and the From selector chooses the unit you typed. What happens next differs by mode, and it is worth being blunt about it.
Length is the general case. Whatever you type is converted to inches internally, then shown five ways: inches to two decimal places, feet and inches to the nearest 1/16 in, centimetres to one decimal place, whole millimetres, and decimal feet and metres to three places.
Area reads the From selector only as a flag. Choose any metric unit and your number is treated as square metres; choose inches or feet and it is treated as square feet. It returns both, plus the side length of a square room of that area, which is a fast sanity check on whether a floor area is the shape you are picturing. The relationship behind it is set out in full in the square feet to square metres tables.
Lighting ignores the From selector entirely. It takes the value as lumens and the second field as room area in square feet, then reports lumens per square foot, the equivalent in lux, and a rough incandescent and LED wattage.
Fabric converts to yards and metres and adds the cloth area at a 54 in (137 cm) width, which is the working width most upholstery and drapery fabric is sold in.
The arithmetic, worked once by hand#
Take a European door leaf listed as 2,400 mm tall and put it into a house measured in feet.
Divide by 25.4 to get inches: 2400 / 25.4 = 94.488 in. The converter shows 94.49 in.
For the feet and inches line it takes the whole feet first, 94.488 / 12 = 7 with 10.488 in left over, then rounds the remainder to the nearest sixteenth: 10.488 x 16 = 167.8, which rounds to 168 sixteenths, or 10.5 in. So 7 ft 10 1/2 in.
Multiply the inches back by 2.54 for centimetres: 94.488 x 2.54 = 240.0 cm. Multiply by 25.4 for millimetres and round: 2,400 mm, unchanged, because the round trip is exact.
Two things fall out of that. The metric answer returns to the exact number you started with, because 25.4 is a definition. The imperial answer does not land anywhere convenient, and 7 ft 10 1/2 in is a door height that no US supplier stocks against the standard 6 ft 8 in and 8 ft leaves listed in interior door sizes and rough openings.
Rounding: where it is safe and where it is not#
| Quantity | Shown as | Round to | Why |
|---|---|---|---|
| Length, imperial | 94.49 in, 7 ft 10 1/2 in | 1/16 in (1.6 mm) | Finest division on a tape |
| Length, metric | 240.0 cm, 2,400 mm | 1 mm | Joinery is set out in whole mm |
| Room dimensions in prose | 36 in (91 cm) | 1 cm | Below a centimetre is false precision |
| Cabinet and appliance openings | 24 in (610 mm) | 1 mm, never to 600 | 9.6 mm of drift per unit |
| Area | 150.0 sq ft, 13.94 m2 | 0.1 sq ft, 0.01 m2 | Flooring is ordered in whole packs |
| Fabric | 3.00 yd, 2.74 m | 0.05 m | Cut lengths need a tolerance anyway |
| Lumens | Whole numbers | 50 lm | Lamp output varies more than that |
The single rule underneath the table: convert once, at the end, at full precision, and round to the resolution of whoever uses the number next. Converting a converted number is where the drift starts.
Where judgement beats the arithmetic#
The converter is exact. Deciding which number to convert is not.
A 900 mm worktop height converts to 35.4 in, but nobody builds a 35.4 in counter. The right move is to pick the standard in the system you are building in, which is 36 in in the US and 900 mm in Europe, and accept that they differ by 14 mm. The reasoning behind the two families of heights sits in kitchen counter and cabinet heights, and the wider pattern of systems that nearly agree is set out in imperial and metric.
The same applies to sheet goods, tile modules and appliance openings. Conversion tells you what a number is worth; it does not tell you whether the trade you are buying from will make it. When the converted figure lands between two stocked sizes, the stocked size wins, and the difference goes into a filler or a scribe.
What the converter deliberately does not do#
- It does not convert square inches, square millimetres or cubic anything. The area mode handles square feet and square metres only.
- It does not know what you are measuring, so it will not warn you that 2,400 mm is an unusual door height or that 200 lumens will not light a kitchen.
- It does not apply a light loss factor, a maintenance factor or a coefficient of utilisation, so the lux figure is the raw arithmetic rather than a lighting design. Targets by room are in how many lumens a room needs and the underlying relationships in lumens, lux and watts converted.
- It does not allow for fabric repeat, seam allowance or fullness. Those multipliers are in curtain width, fullness and stack-back and the yardage tables in fabric and drapery conversions.
- It does not store anything. Reload the page and the fields return to their defaults.
The rest of the interior dimension calculators pick up where this one stops, and the standing tables of the conversions that recur indoors are in inches to centimetres.
Frequently asked questions#
How many mm are in an inch exactly?
Exactly 25.4 mm. The figure is a definition agreed in the International Yard and Pound Agreement of 1959, not a measured average, so there is no error term to worry about. Everything downstream follows from it: a foot is 304.8 mm, a yard is 914.4 mm, and a metre is 39.3701 in. The converter carries the full precision internally and only rounds at the point of display.
What is 36 inches in cm and mm?
Thirty-six inches is 91.4 cm and 914 mm, or 3.000 ft and 0.914 m. That single conversion explains most of the friction between the two systems indoors: a 36 in (914 mm) door, counter or walkway does not land on a round metric number, and a 900 mm equivalent is 14 mm narrower. Neither figure is wrong, but they are not interchangeable.
Should I round 91.44 cm to 91 cm or 90 cm?
To 91 cm for a written dimension and to 914 mm for anything being made. Dropping to 90 cm loses 14 mm, which is more than the tolerance on a fitted run of cabinets and roughly the thickness of a cabinet side. The rule the converter follows is to round once, at the end, to the resolution of the tool that will use the number.
Why does the area mode ignore which metric unit I pick?
Because it reads the From selector only as a flag for which system you typed in. Pick any metric option and the number is treated as square metres; pick inches or feet and it is treated as square feet. It is a deliberate simplification, and it means you cannot convert square inches or square millimetres here. Enter the room area in whole units instead.
How do lumens convert to lux?
Lux is lumens per square metre, so it depends on the area being lit and is not a fixed conversion. The lighting mode divides the lumens you enter by the room area in square feet, then multiplies by 10.76 to give lux. Three thousand lumens over 150 sq ft is 20 lumens per sq ft, which is about 215 lux at the floor before any loss.
How many watts is 800 lumens?
About 57 W as an old incandescent bulb and about 9 W as an LED, which is the ratio the converter uses: roughly 14 lumens per watt for incandescent and 90 lumens per watt for a domestic LED. Real LED efficacy ranges from about 80 to 120 lumens per watt, so treat the wattage as an order of magnitude rather than a specification.
Is 1 yard the same as 1 metre for curtains?
No, and the difference matters over a long drop. A yard is 0.9144 m, so a metre is about 3.4 in (8.6 cm) longer. Order five yards where you meant five metres and you are 43 cm short, which is a hem and a header. The fabric mode converts in both directions and also reports the area at a 54 in (137 cm) cloth width.
Does this converter send my measurements anywhere?
No. The arithmetic runs in your browser as you type, nothing is stored, and no values are transmitted. That is true of every calculator on the site. It also means the tool cannot check its answers against your drawings, so treat the output as a planning figure and verify anything critical against the product or the code before ordering.
Sources and standards referenced#
- NIST Special Publication 811: Guide for the Use of the International System of Units National Institute of Standards and TechnologyThe exact inch definition and the rules on significant figures in conversion
- International Yard and Pound Agreement, 1959 National Bureau of Standards and Commonwealth signatoriesDefines the international inch as exactly 25.4 mm
- The Lighting Handbook Illuminating Engineering SocietyFoot-candle and lux relationship used by the lighting mode
- Architects' Data Ernst Neufert, Wiley-BlackwellMetric coordinating dimensions for joinery and openings
- Time-Saver Standards for Interior Design and Space Planning De Chiara, Panero and Zelnik, McGraw-HillConventional rounding for room and furniture dimensions
We cite published codes, industry planning guidelines and manufacturer specifications. Where a figure is our own recommendation rather than a published standard, the text says so. See how we measure.