Conversions

Interior dimension conversion tables

Converting is the easy half. Rounding is the other half, and it compounds along a wall.

By the Refined Space editorial desk · Updated September 6, 2026 · How we work

On this page (5 sections)

25.4 mm exactly, and everything downstream of it
600 mm against 24 in, and the other near-misses
Round to the tolerance of the trade
10.764, not 3.281: the area conversion people invert
Quantities: fabric, tile and paper

Short answer

One inch is exactly 25.4 mm, and every other interior conversion derives from it: 1 ft is 304.8 mm, 1 m is 3.281 ft, 1 sq ft is 0.0929 m2, and 1 foot-candle is 10.764 lux. The trap is the rounding, not the arithmetic. Substituting 600 mm for 24 in (609.6 mm) shortens every unit by 1.6 per cent, which is invisible on one cabinet and 48 mm (1 7/8 in) across a run of five.

The numbers, at a glance

Inch to millimetre: 1 in = 25.4 mm exactlyDefined, not measured; everything else derives
Foot to millimetre: 1 ft = 304.8 mmSo 3 ft is 914.4 mm, not 900 mm
Metre to feet: 1 m = 3.281 ft (39.37 in)3.28 is fine for a room, not for a run
Square metre to square feet: 1 m2 = 10.764 sq ftArea converts as the square of the length ratio
The cabinet near-miss: 24 in = 609.6 mm against 600 mm9.6 mm (3/8 in) per unit, 1.6 per cent
Drift over five units: 48 mm (1 7/8 in)Five 24 in units against five 600 mm units
Foot-candle to lux: 1 fc = 10.764 luxThe same ratio as sq ft to m2, for the same reason
Working rounding: 5 mm or 1/8 in (3 mm)1 mm only for joinery and appliance cutouts

Converting between systems is arithmetic, and arithmetic is not where projects go wrong. They go wrong at the rounding, in three ways: rounding to a neater number than the trade works to, rounding a minimum in the wrong direction, and rounding the same dimension twice so a figure reaches the drawing that was never anybody's measurement. The order below avoids all three. Establish the exact factor, decide which system the object is manufactured in, convert once, round to the tolerance of the trade, then leave it alone.

25.4 mm exactly, and everything downstream of it#

An inch is exactly 25.4 mm. That is a definition agreed in 1959, not a measured value, so it never rounds and never drifts. Every other imperial to metric figure in building follows: a foot is 304.8 mm, a yard is 914.4 mm, a mile is irrelevant indoors. Going the other way, a metre is 39.37 in or 3.281 ft, and a centimetre is 0.3937 in.

Two of those deserve to be memorised, because they answer most questions in the head. Three feet is 914 mm, not 900 mm. Eight feet is 2438 mm, not 2400 mm. The full set for interiors is in the interior dimension chart and, for whole rooms, in feet and metres room size conversions. The interior unit converter does the arithmetic without the rounding decisions.

600 mm against 24 in, and the other near-misses#

The dangerous conversions are not the ones that are obviously different. They are the ones that are almost the same, because a difference of 1.6 per cent passes every eye test and fails every fitted run.

Imperial size	Exact metric	Metric size actually used	Gap	Where the gap appears
16 in	406.4 mm	400 mm	6.4 mm (1/4 in)	Stud and joist centres
24 in	609.6 mm	600 mm	9.6 mm (3/8 in)	Cabinet width and depth
30 in	762 mm	750 mm	12 mm (1/2 in)	Door leaves, table height
36 in	914.4 mm	900 mm	14.4 mm (9/16 in)	Worktop height, route width
48 in	1219.2 mm	1200 mm	19.2 mm (3/4 in)	Sheet width, panel modules
80 in	2032 mm	2000 mm	32 mm (1 1/4 in)	Door leaf height
96 in	2438.4 mm	2400 mm	38.4 mm (1 1/2 in)	Sheet length, ceiling height

Read the last row as a practical warning: a 2400 mm plasterboard or panelling sheet is 38 mm short of an 8 ft ceiling, which is why imported panelling leaves a gap at the top of a US room and why the fix is a cornice rather than a filler. The same list, treated as a collision between two manufacturing systems rather than a table, is in imperial and metric in a house, and the cabinet consequences are worked through in millimetre cabinet and joinery sizes and kitchen cabinet sizes, US against European.

Convert the run, never the unit

The 24 in against 600 mm substitution is a fixed proportional error: 609.6 divided by 600 is 1.016, so every swap shrinks the element by exactly 1.6 per cent, and that one figure predicts the rest. One unit loses 9.6 mm, which disappears into a scribe. Three lose 29 mm, a survivable filler. Six units along a 12 ft (3658 mm) wall lose 58 mm (2 1/4 in), a whole filler panel, with nowhere to hide it if the run is captured between two walls.

The failure is not gradual, though, which is the part the tables do not say. A run of cabinets fails at the appliance, because appliance apertures are fixed in each system and do not scale: a 600 mm dishwasher needs a 600 mm hole wherever the drift has moved it to. A converted layout looks right for two units, arrives at the appliance 20 to 30 mm out, and every unit after it inherits the error.

So the rule is not the one people apply. Set out the total run length in the system the units are manufactured in, place appliances by counting units rather than by converting dimensions, and convert only the finished overall figure back for the reader. Converting each dimension in turn guarantees the drift. Converting once, at the end, cannot produce it.

Round to the tolerance of the trade#

A converted number carries false precision. The job of rounding is to strip it without losing anything that matters, and how much you may strip depends entirely on what the number is for.

What is being converted	Round to	Reason
Overall room dimensions	10 mm or 1/2 in	The room is not that accurate anyway
Clearances and furniture sizes	5 mm or 1/8 in (3 mm)	Below the thickness of a cushion cover
Carcass and joinery sizes	1 mm	Panels are cut to the millimetre
Appliance cutouts	1 mm, and never optimistically	The hole either takes the machine or does not
Areas for ordering	Whole m2 or sq ft, then add waste	Waste percentage dwarfs the rounding
Lighting levels	10 lux or 1 fc	Perception is logarithmic, not linear

10.764, not 3.281: the area conversion people invert#

Area does not convert with the linear factor. One square metre is 10.764 sq ft, because the length ratio of 3.281 is squared. A 12 x 14 ft room is 168 sq ft and 15.6 m2, and anyone who divides 168 by 3.281 gets 51 and a room that does not exist. The worked examples, including the per-person floor areas that housing standards use, are in square feet to square metres for rooms.

The same square relationship explains the lighting conversion. A foot-candle is one lumen per square foot and a lux is one lumen per square metre, so a foot-candle is 10.764 lux, exactly the area ratio. A 50 fc worktop target is about 540 lux, normally specified as 500.

Watts convert to neither, because a watt measures the power a lamp draws rather than the light it emits. Efficacy runs from roughly 15 lumens per watt for halogen to over 100 for a good LED, so old wattage habits translate only through a lumen figure: a 60 W incandescent was about 800 lumens. The equivalent wattage tables are in lumens, lux and watts converted, with room targets in how many lumens a room needs.

Quantities: fabric, tile and paper#

Three families of conversion behave differently from dimensions, because they end in an order rather than a drawing.

Fabric is sold by the linear metre or yard off a fixed roll width, so the conversion that matters is width. A yard is 0.9144 m, curtain and upholstery cloth arrives at 137 cm (54 in) or 140 cm, and both the fullness multiplier and the pattern repeat act on the quantity before any unit conversion. The order of operations is in fabric and drapery conversion tables.

Tile converts through area. Tiles per square metre is 10,000 divided by the tile area in centimetres, giving 2.78 for a 600 x 600 mm tile and 5.56 for a 300 x 600 mm. Add 10 per cent waste for a straight lay and 15 per cent for diagonal or herringbone, then round up to whole boxes, which is a larger adjustment than any rounding in the conversion itself. The full arithmetic is in tile and flooring quantity conversions and the tile and flooring quantity calculator.

Paper converts through ratio. A drawing at 1:50 on A4 covers 10.5 x 14.85 m of building, and the 4 per cent gap between 1:48 and 1:50 is small enough to read one with the other's scale rule and too large to scale a dimension from. Sheets against scales are in plan scales and paper sizes converted.

Where this goes wrong

Never round a limit as though it were a measurement. A minimum rounds up and a maximum rounds down, always, and the direction is not negotiable because the figure is a threshold rather than a target. The 36 in minimum clear route in the ADA is published in metric as 915 mm, not 900 mm, and the 34 in maximum worktop height is 865 mm, not 870 mm. Convert a minimum downwards to a tidy metric number and you have quietly written a non-compliant dimension into your own drawing.

The order that prevents it: identify what kind of number you are holding, a minimum, a maximum or a target, before you convert it. Then convert exactly. Then round in the safe direction. Then write down which it was, because the next person to read the drawing will not know. Where the figure came from and whether it is law, guidance or trade practice is exactly what the standards and codes reference exists to record.

Frequently asked questions#

How many mm is an inch?

Exactly 25.4 mm. The figure is a definition rather than a measurement, agreed internationally in 1959, so it never rounds and never varies. Every other imperial to metric conversion in building follows from it: a foot is 304.8 mm, a yard is 914.4 mm, and a 4 x 8 ft sheet is 1219.2 x 2438.4 mm. Any conversion table that gives 25 mm is rounding, not converting.

Is 600 mm the same as 24 inches?

No, and the difference matters. Twenty-four inches is 609.6 mm, so a 600 mm unit is 9.6 mm (3/8 in) narrower, about 1.6 per cent. One unit absorbs that in a scribe. Five units in a row lose 48 mm (1 7/8 in), which is roughly one filler panel, and that is where a converted kitchen layout stops fitting between two walls.

How do I convert square feet to square metres?

Divide by 10.764, or multiply by 0.0929. A 12 x 14 ft room is 168 sq ft, which is 15.6 m2. The common error is converting the area with the linear factor of 3.281, which is wrong by a factor of three and a bit. Area scales as the square of the length ratio, so the two conversion numbers are unrelated in size.

What is the metric equivalent of a 4 by 8 foot sheet?

1219 x 2438 mm, against the 1200 x 2400 mm sheet sold in metric countries. The 38 mm (1 1/2 in) shortfall on the long side is why a 2400 mm board does not reach an 8 ft ceiling, and the 19 mm on the width is why metric and imperial stud modules, 400 mm against 406.4 mm, never line up over a long wall.

How many lux is a foot-candle?

One foot-candle is 10.764 lux, because a foot-candle is one lumen per square foot and a lux is one lumen per square metre. A kitchen worktop target of 50 fc is therefore about 540 lux, usually quoted as 500. Watts convert to neither: a watt measures power drawn, and lumens per watt vary from about 15 for halogen to over 100 for a good LED.

How precisely should I round a converted dimension?

To the tolerance of the trade. Room dimensions round to 10 mm or 1/2 in, furniture and clearances to 5 mm or 1/8 in, and joinery and appliance cutouts to 1 mm, where the true figure has to survive. Never round the same dimension twice, and never round a code minimum downwards: 36 in becomes 915 mm, not 900 mm.

How many tiles are in a square metre?

Divide 10,000 by the tile area in centimetres. A 600 x 600 mm tile gives 2.78 per square metre, a 300 x 600 mm gives 5.56, and a 100 x 100 mm gives 100. Order by area rather than by count, then add 10 per cent for a straight lay and 15 per cent for a diagonal or herringbone pattern, plus a spare box for future repairs.

Sources and standards referenced#

Guide for the Use of the International System of Units, Special Publication 811 US National Institute of Standards and TechnologyThe exact inch, foot and yard definitions and conversion factors
International Yard and Pound Agreement, 1959 National Bureau of Standards and equivalent national bodiesFixes the inch at exactly 25.4 mm
ISO 216: Writing paper and certain classes of printed matter International Organization for StandardizationA-series sheet sizes used with plan scales
The Lighting Handbook Illuminating Engineering SocietyLumen, lux and foot-candle relationships and recommended levels
Architects' Data Ernst Neufert, Wiley-BlackwellThe 100 mm building module, sheet sizes and metric joinery dimensions
BS EN ISO 80000-1: Quantities and units British Standards InstitutionHow derived units and rounding are handled in specification

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.