Imperial and metric: the dimension systems that collide in a house

Two grids, one house#

A house is built on a dimensional grid, and the grid is inherited rather than chosen.

North American construction runs on an imperial grid. Studs and joists at 16 in (406.4 mm) centres, sometimes 19.2 in (487.7 mm) or 24 in (609.6 mm). Sheet goods at 4 x 8 ft (1219 x 2438 mm). Base cabinets 24 in (610 mm) deep and 34.5 in (876 mm) high, in 3 in width steps. Doors 80 in (2032 mm) high. Ceilings at 8 ft because that is the length of a board.

European construction runs on a metric grid derived from a 100 mm module. Studs at 400 or 600 mm. Sheets at 1200 x 2400 mm. Kitchen carcasses 600 mm wide and 720 mm high, standing on 150 mm legs under a 40 mm top to reach 900 mm, coordinated by BS EN 1116. Doorsets 2040 mm high.

Appliances are the third party, sold into both markets from the same factories, and their sizes are the reason the two grids meet inside a single room. A dishwasher is 600 mm or 24 in depending on which catalogue you are reading, and those are not the same number.

The UK is the worst case because it uses both at once. A British house is framed and boarded in metric, fitted with kitchen units on a 600 mm module, and hung with doors that are 1981 mm tall, which is 6 ft 6 in written in millimetres.

Hard metric and soft metric#

The distinction that explains most of the confusion is between two kinds of conversion.

A soft conversion restates an existing product in the other system without changing it. A 1981 mm door leaf is a soft conversion of 6 ft 6 in. A 2438 mm sheet is a soft conversion of 8 ft. Nothing changed except the label, so the product still fits the imperial grid and looks like an odd number in metric.

A hard conversion redesigns the product to a round metric size. A 2040 mm doorset, a 2400 mm sheet, a 600 mm cabinet: these are new sizes, not translations. They fit the metric grid and are genuinely incompatible with the imperial one.

Both types are on sale, often from the same merchant, and the label does not tell you which you have. A "2.4 m" board and an "8 ft" board differ by 38 mm (1.5 in). If your ceiling was set by one and your boards are the other, you have a 38 mm strip to make good along every wall.

The near misses that cost money#

An obvious mismatch is cheap: you see it, you reject the part, nothing is cut. The expensive mismatches are the ones a drawing cannot show you.

Dimension	Imperial	Exact metric	Metric standard	Gap	Where it bites
Base cabinet width	24 in	609.6 mm	600 mm	9.6 mm	96 mm (3.8 in) across ten units
Base cabinet depth	24 in	609.6 mm	560 mm carcass	49.6 mm	Worktop overhang and pipe space
Worktop depth	25 in	635 mm	600 mm (650 available)	35 mm	Sink and hob templates do not transfer
Worktop height	36 in	914.4 mm	900 mm	14.4 mm	Splashback and appliance panel heights
Range or cooker	30 in	762 mm	600 mm	162 mm	A US range needs a whole extra unit of wall
Cooker gap set out in metric	30 in	762 mm	760 mm gap	2 mm short	A 758.8 mm appliance in a 760 mm gap has no scribe
Fridge opening	36 in	914.4 mm	900 mm	14.4 mm	US 908 mm body plus air gaps will not go in
Dishwasher opening	24 in	609.6 mm	600 mm	9.6 mm	Metric unit fits imperial hole, not the reverse
Door leaf, width	32 in	812.8 mm	826 mm (EU), 838 mm (UK)	13 to 25 mm	Lining, ironmongery and clear width all move
Door leaf, height	80 in	2032 mm	2040 mm (EU), 1981 mm (UK)	8 to 51 mm	Architrave and flooring transitions
Sheet goods	48 x 96 in	1219 x 2438 mm	1200 x 2400 mm	19 and 38 mm	Board edges miss the studs
Panel thickness	3/4 in	19.05 mm	18 mm	1.05 mm	Grooves, shelf pins, drawer slides
Stud centres	16 in	406.4 mm	400 mm	6.4 mm	32 mm over five bays
Joist and stud centres	24 in	609.6 mm	600 mm	9.6 mm	48 mm over five bays
Mattress	US queen 60 x 80 in	152.4 x 203.2 cm	UK king 150 x 200 cm	24 and 32 mm	Frames, bases and fitted bedding
Kitchen waste outlet	1.5 in	38.1 mm	40 mm	1.9 mm	Needs an adaptor, not a push fit
Basin tap hole	1 3/8 in	34.9 mm	35 mm	0.1 mm	A genuine match, and the only one here

The last two rows are the point in miniature. The basin tap hole is the one place the two systems agree to a tenth of a millimetre, so nobody thinks about it. The kitchen waste is 1.9 mm out, which is small enough to look like the same fitting and large enough to leak.

Studs, joists and the space between them#

Framing centres are where the two grids diverge first, because every later trade measures from them.

Element	North America	Europe	Clear bay
Wall stud, standard	16 in (406.4 mm) centres	400 mm centres	14.5 in (368 mm) vs 353 to 362 mm
Wall stud, wide	24 in (609.6 mm) centres	600 mm centres	22.5 in (572 mm) vs 553 to 562 mm
Stud thickness	1.5 in (38 mm)	38 mm CLS or 47 mm sawn	Metric bays are the narrower
Floor joist	16, 19.2 or 24 in centres	400 or 600 mm centres	Same pattern, 1.6% apart

For fixing something heavy the difference is usually harmless, and the reason is worth knowing. A stud is 38 to 47 mm wide, so a screw has roughly 15 mm of latitude either side of the centreline before it misses timber. A 6.4 mm error per bay stays inside that until you are four or five bays from your reference point. This is why a VESA 400 mm TV bracket lands on a pair of 16 in studs (they are 406.4 mm apart, 6.4 mm wider than the bracket) and a VESA 600 bracket usually still catches 24 in studs at 609.6 mm. Measure from a stud you have found, never from a corner, and the arithmetic stops mattering. The furniture delivery path is a better use of your worry.

For anything set into the wall the difference is fatal. A US shower niche insert is made 14 in (356 mm) wide to drop into a 14.5 in (368 mm) bay between 16 in centres. A UK stud bay at 400 mm centres with 47 mm timber is 353 mm clear, which is 3 mm narrower than the niche. It cannot go in, and the fix is cutting and trimming a stud in a wet wall. Metric niches are made at 300 x 300 mm and 300 x 600 mm for exactly this reason. Check the bay before you buy the niche, alongside the rest of the numbers in shower sizes and clearances. The same applies to recessed cabinets, in-wall cisterns and insulation batts, which come 15 in (381 mm) wide in one system and 570 or 370 mm wide in the other.

Sheet goods, and lining out in the wrong system#

A 1200 mm board fits a 400 mm grid perfectly: three bays, edges landing on stud centres. A 48 in board fits a 16 in grid perfectly: three bays, same logic. Cross them and neither works.

Studs at 406.4 mm centres put timber at 0, 406.4, 812.8 and 1219.2 mm. A 1200 mm board runs out at 1200 mm, which is 19.2 mm short of the fourth stud centre and, since the stud is only 38 mm wide, a fraction short of the stud itself. The edge has nothing behind it. You either cut every board to 1219 mm, throwing away 19 mm and a straight factory edge, or you add a noggin at every joint. Over a 12 m (39 ft) wall, cutting costs you the equivalent of most of a sheet in offcuts plus the labour.

The vertical version of the same problem sets your ceiling. Two 4 ft boards stacked give 8 ft (2438 mm). Two 1200 mm boards give 2400 mm, 38 mm lower. Neither is wrong, but the choice is made at framing stage and it is not adjustable afterwards, which is why the standard heights on the room sizes reference come in imperial and metric pairs that never quite line up.

Appliances, worktops and the one-way valve#

Cabinets and appliances produce the most expensive collisions because they are bought late, from different suppliers, in different systems.

The asymmetry is worth stating plainly. Metric appliances generally fit imperial openings; imperial appliances generally do not fit metric openings. A 598 mm European dishwasher slides into a 609.6 mm US opening with 11 mm to spare. A 603 mm US dishwasher will not enter a 600 mm European housing at all. Same for the 908 mm US fridge against a 900 mm tall housing, and the 758.8 mm US range against a 600 mm cooker gap.

Height is the quieter problem. A US worktop finishes at 36 in (914 mm) and a European one at 900 mm. That 14 mm shows up wherever a fitted appliance has to align with the worktop line: an integrated dishwasher door designed for a 900 mm system sits 14 mm low under an imperial top, and an integrated fridge panel misses the reveal on every neighbouring door. The build-up behind those numbers is in counter and cabinet heights, and the gaps appliances need around them are in appliance clearances.

One thing did go global. The 32 mm system, the row of 5 mm holes at 32 mm centres that carries hinges, shelves and drawer runners, is metric everywhere, including in North American frameless cabinets. Every kitchen built in the last thirty years is already partly metric on the inside.

Beds, doors and the sizes people assume#

Two categories catch buyers rather than builders.

Beds. A US queen is 60 x 80 in (152 x 203 cm). A UK king is 150 x 200 cm (59 x 79 in), so the grander name is on the smaller mattress: 2.4 cm narrower and 3.2 cm shorter. A UK double at 135 x 190 cm sits 2 cm from a US full at 137 x 190.5 cm. In both pairs the mattress physically fits the other system's frame, which is the trap, and then the fitted sheet lets go of a corner nightly. Buy frame, mattress and bedding in one system: the full cross-system table is in bed sizes and the room each one needs.

Doors. Three families are in circulation. US doors are 2032 mm (80 in) high. UK stock doors are 1981 mm (6 ft 6 in). European doorsets are 2040 mm. A US 32 in leaf is 812.8 mm; the nearest European leaf is 826 mm and the nearest UK one 838 mm, and both change the structural opening, the lining width and the resulting clear width. Since accessibility rules are written on clear width rather than leaf width, substituting across systems can quietly move you above or below the 32 in (813 mm) figure in door sizes and swing clearances.

Conversion discipline#

Five habits remove almost all of this risk.

1. Convert once, at the survey. Decide which system the job will be built in and record every site dimension in that system. A drawing with both units on it will be misread.
2. Convert the module, then multiply. Ten 600 mm units is 6000 mm exactly. Ten units converted individually to "23 5/8 in" and added up is 6000.75 mm and a false precision. Never convert a running total.
3. Never round a cumulative dimension. Rounding 609.6 to 610 costs 0.4 mm per unit and is harmless. Rounding it to 600 costs 9.6 mm per unit and is a rebuild. If a number will be multiplied, keep the decimal.
4. Round outward, not to the neater figure. Clearances round up, object sizes round up. A 42 in aisle is 1066.8 mm, so specify 1070 mm, not 1050 mm. A 908 mm fridge in a 910 mm hole is not a fit.
5. Tag every dimension with its system, including on site. "760" and "30 in" written on the same wall by two trades is how a 2 mm gap becomes a returned appliance.

Conversion reference#

The dimensions used most across this site, with the exact conversion and the metric size that is commonly substituted for it. The final column is what one substitution costs.

Imperial	Exact	Usual metric equivalent	Error per unit
1/2 in	12.7 mm	12 mm	0.7 mm
3/4 in	19.05 mm	18 mm	1.05 mm
1 in	25.4 mm	25 mm	0.4 mm
4 in	101.6 mm	100 mm	1.6 mm
6 in	152.4 mm	150 mm	2.4 mm
12 in	304.8 mm	300 mm	4.8 mm
15 in	381 mm	380 mm	1 mm
16 in	406.4 mm	400 mm	6.4 mm
18 in	457.2 mm	450 mm	7.2 mm
21 in	533.4 mm	530 mm	3.4 mm
24 in	609.6 mm	600 mm	9.6 mm
25 in	635 mm	600 or 650 mm	35 or 15 mm
30 in	762 mm	750 mm	12 mm
32 in	812.8 mm	800 or 826 mm	12.8 or 13.2 mm
36 in	914.4 mm	900 mm	14.4 mm
42 in	1066.8 mm	1050 or 1100 mm	16.8 or 33.2 mm
48 in	1219.2 mm	1200 mm	19.2 mm
60 in	1524 mm	1500 mm	24 mm
80 in	2032 mm	2000 or 2040 mm	32 or 8 mm
7 ft	2133.6 mm	2100 mm	33.6 mm
8 ft	2438.4 mm	2400 mm	38.4 mm

To go the other way: multiply millimetres by 0.03937 for inches, or divide by 25.4. For mental work, 100 mm is very nearly 4 in (it is 1.6 mm short) and 1 m is 39 3/8 in.

What to check before you order#

• The opening, not the appliance. Measure the finished gap in millimetres, write down which system the cabinets came from, and compare it to the appliance body plus its stated air gaps.
• The stud bay, by drilling or scanning, before buying anything that recesses into a wall.
• The board length against the framing centres, before the first sheet is lifted.
• The frame and the mattress together, in one system, in one order.
• The door leaf, the lining and the resulting clear width, as three separate numbers rather than one.
• Any pair of dimensions within 2% of each other. If two numbers on your drawing look interchangeable, one of them is about to be substituted for the other by someone who is not reading the units. The terms in the glossary are given in both systems for that reason.