Keycap Size Reference
Every standard keycap width in units and millimeters, which keys use which size, and how to measure a keycap you already own.
Keycap widths are quoted in units, written u. One unit is the width of a letter key. Every other key is a multiple of it, and a keycap set that does not include the sizes your board uses will leave gaps you cannot fill.
This page is the conversion table, plus how to measure a cap you already have.
The unit
One unit is 19.05 mm of key spacing, which is exactly three quarters of an inch. That figure is the spacing between the centres of two adjacent keys, not the size of the plastic.
The keycap itself is smaller than its spacing, so neighbouring caps do not rub. A 1u cap measures roughly 18 mm across the base and about 12 mm across the top surface, depending on profile.
So there are two ways to measure, and they give different numbers:
- Measure the board. Count from the left edge of one key to the left edge of the next and divide by 19.05. This is the reliable method.
- Measure the cap. Measure the base, add about 1 mm, and divide by 19.05. Less reliable, because base dimensions vary by profile.
Standard widths
| Size | Millimeters (spacing) | Keys that use it |
|---|---|---|
| 1u | 19.05 | Letters, numbers, function row, arrows, most numpad keys |
| 1.25u | 23.8 | Ctrl, Alt, Super and Fn on a standard bottom row |
| 1.5u | 28.6 | Tab, backslash, and Ctrl on some layouts |
| 1.75u | 33.3 | Caps Lock on most boards, right Shift on 65 and 75 percent boards |
| 2u | 38.1 | Backspace, numpad plus and enter (vertically), numpad zero on some layouts |
| 2.25u | 42.9 | Left Shift and ANSI enter on a standard layout |
| 2.75u | 52.4 | Right Shift on a full-size, tenkeyless or 60 percent board |
| 6u | 114.3 | Spacebar on some older and compact boards |
| 6.25u | 119.1 | Spacebar on a standard ANSI board |
| 7u | 133.4 | Spacebar on a great many custom boards |
Vertical sizes exist too, but only on the numpad: the plus and enter keys are 2u tall and 1u wide.
The spacebar problem
Read this part even if you skip the rest.
The spacebar is the one key where a mismatch cannot be worked around, and boards disagree about it. A standard prebuilt uses 6.25u. A large share of custom boards, particularly 60 and 65 percent kits, use 7u.
A keycap set’s base kit normally includes one spacebar, usually 6.25u. If your board takes 7u you need a set that offers it, and many sets sell it in a separate spacebar kit. Check your board’s spacebar width before you buy a set, not after.
Sizes that are not about width
Three keys are incompatible for reasons other than their measurement:
- ISO enter. ISO layouts use a tall enter shaped like an upside-down L, plus a 1.25u left shift, where ANSI uses a 2.25u enter and a 2.25u shift. An ANSI set cannot fill an ISO board without an international kit.
- Stepped Caps Lock. Some boards use a caps lock with a raised step at one end. It is still 1.75u wide but it is a different cap, and only some sets include one.
- Split keys. Some boards split backspace or left shift into two smaller keys. That needs 1u caps in positions a standard set does not anticipate, though a base kit usually has spares.
Rows, on a sculpted set
If your keycaps are a sculpted profile such as Cherry, OEM, KAT or MT3, each row of the keyboard has a different height and angle and the caps are not interchangeable between rows. Sets label them R1 to R4, counting from the function row down, though the numbering varies between manufacturers.
A uniform profile such as DSA, XDA or KAM has one shape for every key, so any cap fits any row. That is the only case where you can freely rearrange your letters.
See keycaps for what the profiles feel like and which plastic to choose, and the layout sheet for identifying your board’s layout in the first place.