Keycap sizes are given in units. One unit (1u) is the 19.05 mm (0.75 inch) spacing between the centres of two letter keys; the cap itself is smaller, about 18 mm square at the base, or roughly 1.8 cm. Modifiers run from 1.25u to 2.75u, and the standard space bar is 6.25u, about 118 mm wide.
Almost every failed keycap purchase is a sizing failure, not a taste failure. The colours arrive exactly as pictured, the profile is the one that was ordered, and then four or five keys on the bottom row have nowhere to go. Sizing is the part of a keycap set that is fully knowable in advance, and it is the part buyers most often skip.
Width and wall thickness are separate measurements. For product-specific plastic dimensions, see keycap wall thickness by material.
The unit is the only real dimension
Keycap listings are written in units. A unit, written 1u, is the distance from the centre of one letter key to the centre of the next. Deskthority describes MX-mount keycaps as designed for a 0.75-inch unit size, and 0.75 inches is 19.05 mm (1.905 cm) exactly, which is where the figure quoted all over keycap listings comes from. Note what it is: a conversion of the inch figure, not a published metric specification.
It is worth knowing how weakly that number is standardised. ISO 1091 used to prescribe 19 mm ± 1 mm (or 0.75” ± 0.04”) both horizontally and vertically, but the standard was withdrawn in 1994 in favour of the ISO 9995 series, and ISO 9995 never restored a unit definition. National bodies now fill the gap themselves: German DIN keeps the figure alive in the preliminary standard DIN 2137-2 while waiting for an international ruling.
Two consequences follow, and both matter when you are checking a set against a board.
The tolerance does not accumulate. If the unit is 19 mm ± 1 mm, then the distance from the centre of Q to the centre of P, nine units apart, has to be 171 mm ± 1 mm. It is not 171 mm ± 9 mm. Keyboard grids are far more consistent across the width of a board than a naive reading of the tolerance suggests.
Vertical unit and horizontal unit are not required to be equal. On almost every full-size board they are, but a set sold for one board is not automatically correct for another simply because both call a letter key 1u.
The keycaps themselves are deliberately slightly smaller than their unit size, which is what creates the visible gap between keys. That is why measured widths in the tables below come in just under the arithmetic.
How big is a 1u keycap?
A 1u keycap is roughly 18 mm square at its base, about 1 mm narrower than the 19.05 mm grid it sits on, which leaves the visible gap between keys. Its top is smaller again, and its height depends on profile: published figures run from about 7.4 mm for DSA to 16.5 mm for SA.
| Dimension | Figure | Where it comes from |
|---|---|---|
| Grid pitch, centre to centre | 19.05 mm, 1.905 cm, 0.75 in | Conversion of the 0.75 in unit |
| Base footprint | 17.9 to 18.2 mm square | Three caps measured by one Deskthority member |
| Base, named products | 18.10 × 18.10 mm; 18.5 × 18 mm | Keycapsss THT low-profile MX blank; Jelly Key Cherry-profile R1 |
| Top surface | 10.7 × 14 mm | Jelly Key Cherry-profile R1 only |
| Height | About 7.4 to 16.5 mm | Varies by profile and row (see below) |
| MX socket cross | 4.1 mm wide (+0.05), arms 1.17 mm (±0.02) | Cherry’s specification, as reported by Deskthority |
The generic base range comes from a Deskthority thread on 1u keycap bases, where one member listed caps at 18.2, 18.0 and 17.9 mm square. The named figures are manufacturer listings: Keycapsss gives its THT low-profile MX blanks as 18.10 × 18.10 mm, and Jelly Key gives its R1 Cherry-profile cap an 18.5 × 18 mm bottom, a 10.7 × 14 mm top and a height of about 10.2 mm at most. Jelly Key reshaped the cap’s interior for its resin designs, so read those as one maker’s part rather than a Cherry drawing.
A row of caps makes a quick check in centimetres. In a Deskthority thread on switch spacing, one member found ten original Cherry keycaps laid side by side came to about 18.25 cm, roughly 18.25 mm a cap, against the 19.05 mm centre-to-centre spacing others in the thread cite. A listing that gives a 1u cap as 19.05 × 19.05 mm is describing that spacing, not the plastic.
Common key widths in mm, cm and inches
| Key | Width | Grid mm | Grid cm | Grid inches |
|---|---|---|---|---|
| Letters, numbers, function row | 1u | 19.05 | 1.9 | 0.75 |
| Standard 60% bottom-row modifiers | 1.25u | 23.81 | 2.4 | 0.94 |
| Tab, backslash | 1.5u | 28.58 | 2.9 | 1.13 |
| Caps lock | 1.75u | 33.34 | 3.3 | 1.31 |
| Backspace | 2u | 38.10 | 3.8 | 1.50 |
| ANSI enter, ANSI left shift | 2.25u | 42.86 | 4.3 | 1.69 |
| ANSI right shift | 2.75u | 52.39 | 5.2 | 2.06 |
| Standard space bar | 6.25u | 119.06 | 11.9 | 4.69 |
| Wide space bar | 7u | 133.35 | 13.3 | 5.25 |
Deskthority’s catalogue of measured Cherry MX space bars lists the 6.25u part as 118 mm wide and the 7u part as 133 mm, which is the expected undercut against the grid figures above.
To estimate the plastic rather than the grid, take about 1 mm off the grid figure. The measured 1u bases and the 118 mm 6.25u bar both fit that rule; the 133 mm 7u bar sits closer to its grid width, so use the kit drawing when a few tenths of a millimetre matter.
MX, Cherry and OEM keycap dimensions
MX, Cherry and OEM keycaps share one footprint: all of them sit on the 19.05 mm grid over the same cross socket. “MX” names the mount, while Cherry and OEM name profiles, which change height and sculpt rather than width. Das Keyboard puts OEM at 11.9 mm tall and Cherry at 9.4 mm.
The socket is what makes a cap MX-compatible. Deskthority’s keycap mount reference reports Cherry’s specification as a cross 4.1 mm wide (+0.05 mm) with arms 1.17 mm wide (±0.02 mm), and Signature Plastics’ as 0.159 inches (4.04 mm) with 0.047-inch (1.19 mm) arms. Neither figure tells you how tall or how wide the cap is.
Height is where profiles part company:
| Profile | Height quoted by Das Keyboard | Other published figure |
|---|---|---|
| DSA | 7.6 mm | 0.291 in (7.39 mm), Deskthority |
| XDA | 9.1 mm | None cited here |
| Cherry | 9.4 mm | About 10.2 mm max, Jelly Key R1 cap |
| OEM | 11.9 mm | None cited here |
| KAT | 13.5 mm | None cited here |
| SA | 16.5 mm | None cited here |
Das Keyboard gives one number per profile without naming the row or the measuring point. That matters because Deskthority describes Cherry’s G80 and G81 caps as contoured and Signature Plastics lists SA as sculptured, so their height changes from row to row. Even DSA, which is uniform, carries two figures about 0.2 mm apart, which shows how far the measuring point alone can move a number. For row labels and what a millimetre height actually describes, see keycap row codes and profile heights.
Where sets actually fail: the bottom row
Above the space bar row, mainstream boards agree with each other. Below it, they do not. Deskthority’s keycap documentation puts it plainly: even within a single keycap mount, keys of a given function differ in size and shape from one manufacturer to another and from one country to another, and the size of the various modifier keys varies widely by brand.
The mainstream ANSI arrangement is 1.25u for each of the modifiers flanking the space bar, with a 6.25u space bar between them. A great many enthusiast boards deviate: 1.5u modifiers with a 7u space bar, split space bars, 1u modifiers on 40% boards, or a stepped or shifted arrangement that no stock kit anticipates.
None of this is discoverable from a set’s photographs, because the photograph is always of a standard board. It is discoverable from the kit contents list, which is the page most buyers never open.
What is the standard space bar size?
The standard space bar is 6.25u: 119.06 mm on the grid and 118 mm as a measured Cherry MX part, about 11.8 cm or 4.6 inches. The layout Signature Plastics’ Universal Space Bar covers has a centre mount and two more 50 mm either side. Modern Western boards typically use 6 to 7 units.
| Size | Grid width | Measured Cherry MX bar | In cm | Documented on |
|---|---|---|---|---|
| 6u | 114.30 mm | 114 mm | 11.4 | Cherry G80-1800 and G80-11800 with Windows keys |
| 6.25u | 119.06 mm | 118 mm | 11.8 | Standard ANSI bottom row with 1.25u modifiers |
| 6.5u | 123.83 mm | 123 mm | 12.3 | CM Storm QuickFire TK, Corsair Vengeance K60, K65, K70, K90 and K95, Logitech G710+ |
| 7u | 133.35 mm | 133 mm | 13.3 | Cherry G80-1000 and G80-3000 without Windows keys |
The measured widths and board lists come from Deskthority’s space bar dimensions catalogue, which also records 8u, 9u and 10u bars on older boards, up to 190 mm. To identify a bar that is not listed, divide its measured width by 19.05 and round to the nearest quarter unit: 118 mm gives 6.19, which rounds to 6.25u, and 123 mm gives 6.46, which rounds to 6.5u.
The space bar is the single worst offender
Space bar width is the easy half. Modern Western boards sit between 6 and 7 units, and the measured widths converge tightly: every documented Cherry MX 6.25u space bar in Deskthority’s list is 118 mm.
The stabiliser mounts are the hard half, and they do not converge at all. Among documented 6.25u Cherry MX space bars alone, the catalogue records variants with three mounts 50 mm apart, three mounts 42.5 mm apart, three mounts 40 mm apart, one arrangement with three mounts and no centre mount, and one with six mounts.
That is the whole problem in one line. A kit page that says “6.25u space bar” has told you the width and nothing whatsoever about whether the stabiliser stems underneath will line up with your board. Deskthority flags this explicitly as the reason aftermarket space bars fail more often than any other key: the width of the space keycap and the positions of its mounts vary between keyboards, even between keyboards using the same switches.
Before ordering, pull your existing space bar and count the stems on its underside, then note whether there is a centre mount. That single observation resolves most space bar disappointments.
ANSI, ISO, and the enter key
The enter key shape and size differ between ANSI and ISO layouts, and left shift differs with them. An ISO board needs an ISO kit; the tall enter and the shorter left shift are separate mouldings, not a rearrangement of ANSI parts. Some sets list an ISO kit as an add-on and some do not offer one at all.
Stepped keycaps, where part of the top surface sits lower than the rest, are a further wrinkle. A stepped caps lock is a different part from a flat 1.75u caps lock and the two are not interchangeable if you care how the board looks.
Stabilised keys start at 1.75u
Stabilisation is frequently applied to keys 1.75 units wide and greater. On a typical full-size US board that means both shifts, enter, backspace, and the plus, zero and enter keys on the numeric keypad. Those caps carry moulded stabiliser sockets on their undersides in addition to the switch mount.
The practical implication is that a large key is never satisfied by a cap of merely the right width. It needs the right width and the right stabiliser socket positions. This is the same failure mode as the space bar, applied to seven or eight more keys.
Uniform profiles dodge part of the problem
A uniform profile such as DSA gives every row the same shape and height, which means caps can be rearranged between rows freely. Signature Plastics’ DSA family is 0.291 inches, or 7.39 mm, on every row.
That solves layout-agnostic rearrangement, which is genuinely useful for alternative layouts. It does not solve width. A uniform-profile set with only 1.25u modifiers is exactly as wrong on a 1.5u bottom row as a sculpted one.
A pre-purchase checklist
- Photograph your board straight down, in good light.
- Write down every key that is not 1u, with its width in units.
- Note whether the enter key is ANSI or ISO.
- Pull the space bar. Measure its width against the space bar table above, count the stems, note whether one is central.
- Open the kit contents page for the set and tick off your list against it, key by key.
- Confirm the switch mount. Most sets are cut for the MX cross stem; anything else needs its own caps.
- If the set is a tall profile, check clearance against the case lip before ordering.
Steps four and five are where the money is saved. Everything else is usually fine.
If you are buying a board partly so that keycap sets will fit it, the widest-supported target is the standard 60% bottom row: 1.25u modifiers around a 6.25u space bar. A board built to that arrangement, such as the Anne Pro 2, is covered by essentially every 60%-compatible kit on sale. It ships in several switch variants, so decide the switch first and the layout question takes care of itself.
Next steps
The keycap profile and wall thickness sizer provides illustrative height and thickness presets plus a vendor row-code lookup. Confirm the actual set’s dimensions and clearance against its documentation.
For height evidence and replacement-key labels, keycap row codes and profile heights separates vendor row names from millimetre dimensions. If a set already arrived and something feels wrong under your fingers rather than simply not fitting, keycap wobble: causes, diagnosis and fixes works through the cap, switch and stabiliser checks.
FAQ
what size is a keycap in cm
A standard 1u keycap sits on a 1.905 cm grid and measures about 1.8 cm across its base; the three caps measured in a Deskthority thread ran from 1.79 to 1.82 cm. A 2u backspace takes 3.81 cm of grid, and a standard 6.25u space bar measures about 11.8 cm. Height depends on profile.
are oem and cherry keycaps the same size
Yes in width, no in height. Both sit on the same 19.05 mm grid with the same MX cross socket, so a 1u cap of either profile fits the same position. Das Keyboard lists OEM at 11.9 mm tall and Cherry at 9.4 mm, so mixing rows from the two profiles gives an uneven surface.
how tall is a cherry profile keycap
Das Keyboard gives Cherry profile as 9.4 mm tall, against 11.9 mm for OEM. That is one figure for a profile whose height changes by row: Jelly Key’s R1 Cherry-profile cap is about 10.2 mm at its highest point. Check which row and measuring point a quoted height uses before comparing two numbers.
is a 6.25u space bar universal
No. The width is consistent, since every Cherry MX 6.25u bar Deskthority lists measures 118 mm, but the stabiliser mounts are not. Its catalogue records 6.25u variants with mounts 50, 42.5 or 40 mm apart, one with no centre mount and one with six mounts. Count the stems under your current bar before ordering.
