Keycap Kitting and Coverage: Base Kits, Extra Kits, Units

Our compatibility guide answers “will these fit my keyboard?” This guide answers the deeper, money-saving question behind it: how kitting itself works — why keycap sets are split into a base kit and extension kits, what the “u” unit system means, and how to plan coverage so you buy the right kits once instead of discovering blanks after a long group-buy wait. Kitting is the most underrated source of keycap regret, and it is entirely avoidable with five minutes of arithmetic.

The unit system: what “u” means

Keycap and keyboard widths are measured in units, written “u”. One unit (1u) is the width of a standard alphanumeric key — a single letter key. Wider keys are multiples:

• 1u — letters, numbers, F-keys, most arrows.
• 1.25u — standard bottom-row modifiers (Ctrl, Win/GUI, Alt) on many layouts.
• 1.5u — Tab on standard layouts; modifiers on some compact/“tofu-style” boards.
• 1.75u — common Caps Lock width on many layouts; right Shift on some.
• 2u — Backspace on many layouts; numpad keys.
• 2.25u — common left Shift / ANSI Enter-area width on standard layouts.
• 2.75u — common right Shift width on standard ANSI.
• 6.25u — the most common spacebar.
• 7u — an alternative spacebar width on some boards.

Heights are uniform within a profile; width in units is what kitting is organized around. The mismatch that bites people is almost always a width mismatch — a key on your board needs a width the kit you bought did not include.

Base kit vs extension kits

A keycap set is sold as a base kit plus optional extension kits:

• Base kit — designed to cover a standard keyboard. Most base kits target a standard full-size or tenkeyless ANSI layout: standard alphas, a standard bottom row, common modifier widths, F-row, and typically a 6.25u spacebar. The base kit assumes a conventional layout.
• Extension / extra kits — add the keys non-standard boards need. Common ones:
  • Spacebars kit — alternate spacebar widths (e.g., 7u, split spacebars) and sometimes extra accent spacebars.
  • 40s / ortho kit — extra 1u and 1.25u modifier legends in the quantities small and ortholinear boards need (a standard kit does not include enough 1u modifiers for a 40%).
  • ISO kit — ISO Enter, ISO-specific keys for ISO physical layouts.
  • Novelty / accent kit — themed or contrast keys, not coverage-critical but popular.
  • Numpad kit — for boards with a numpad if the base is TKL-oriented.

The base kit alone fully covers only a standard board. Base kit + the right extension kits, not the base kit alone, is how a non-standard board gets full coverage. This single sentence prevents most kitting regret.

Why renders mislead

A group-buy listing shows a gorgeous full-board render. That render almost always assumes every kit was purchased. It is a marketing image of maximum coverage, not of the base kit. If you buy only the base kit and your board is non-standard, your real board will not look like the render — it will have gaps where the render’s keys came from extension kits. As covered in the compatibility guide: read the kit breakdown images, not the hero render.

How to plan coverage (the arithmetic)

A reliable method, done before you pay:

1. List every key on your board with its width in units. Walk the keyboard left to right, row by row. Note each key and its u-width (your keyboard’s product page or a community layout reference usually states widths). Pay special attention to: bottom-row modifiers (1.25u vs 1.5u), spacebar width, Caps Lock width, Shift widths, Backspace, Enter, and any split keys.
2. Open every kit breakdown image for the set. Each kit usually has its own image showing exactly which keys and widths it contains.
3. Tick off each key on your list against the kits. For each key on your board, find it in the base kit or an extension kit. If a key is not in any kit you plan to buy, it is a gap.
4. Resolve gaps. Either add the extension kit that contains the missing keys, or knowingly accept the gap. The “spacebars” and ”40s/ortho” kits are the two most commonly forgotten — they are the usual culprits behind post-arrival blanks.
5. Count multiples. Coverage is not just “is this width present” but “are there enough.” Ortholinear and split boards often need more 1u/1.25u modifier keycaps than a standard kit ships; the 40s/ortho kit exists precisely for this.

Five minutes of this arithmetic, before payment, eliminates the most frustrating keycap outcome: waiting months for a group buy, then discovering blank positions you now have to chase on the aftermarket at a markup.

The uniform-profile shortcut

There is a structural escape from kitting headaches: uniform profiles (DSA, XDA, KAM — see profiles). Because every keycap in a uniform set is the same shape, any keycap can go in any position. There are no “wrong rows,” and coverage becomes purely a question of having enough keys of each width, not of matching sculpted rows. For people with multiple non-standard, ortholinear, or ergonomic boards, a uniform profile sidesteps an entire category of kitting and row-coverage problems. This is a major, under-appreciated reason uniform profiles are so popular for unusual layouts.

Sculpted profiles (Cherry, OEM, SA) add a second constraint on top of width: each row has its own sculpt, so you also need the correct row for each position, not just the correct width. Kitting for sculpted sets is therefore strictly more demanding than for uniform sets.

A quick kitting checklist

• Identify your layout’s every key width in units.
• Know your spacebar width (6.25u? 7u? split?).
• Know your bottom-row modifier widths (1.25u vs 1.5u).
• ANSI vs ISO — get the matching kit.
• Split Backspace / split Shift / stepped Caps — covered by a kit you are buying?
• Enough quantity of 1u/1.25u for ortho/40% boards (40s kit)?
• For sculpted sets: every physical row matched, not just every width.
• You have identified which extension kits, not just the base, you need — before paying.

The honest verdict

Kitting is just bookkeeping: units measure width, the base kit covers a standard board, extension kits cover everything non-standard, and the render assumes you bought it all. Do the five-minute key-by-key arithmetic against the kit breakdown images before you commit, and the single most common, most frustrating keycap regret — a months-late set with blank keys — simply does not happen to you. If you own unusual boards and do not want to think about any of this, a uniform profile turns kitting from a puzzle into a non-issue. Plan coverage deliberately; it is the cheapest insurance in the hobby.