How to measure your number plate size
Plate Guides · 1 September 2026 · 5 min read
Two measurements and a character count are all you need. Here's how to size a plate for a recess, a holder or a narrow bumper, and how to know when a short plate can still be road legal.
Start with what's on the car now
If the current plate fits well, measure that. Width across, height top to bottom, in millimetres. Most cars carry the standard size, and you'll get 520 x 111 mm. Don't measure a plate that's already the wrong size and copy it.
If there's a recess or a holder, measure the inside of that instead, and take 3 mm off each dimension so the new plate drops in without forcing.
Match it to a size we make
| Size | Width x height | Typically for |
|---|---|---|
| Standard | 520 x 111 mm | Almost every car |
| 21" x 6" | 533 x 152 mm | 4x4s, vans and imports with a square rear recess |
| 18.5" | 470 x 111 mm | Cars with a slightly narrow bumper |
| 16" | 406 x 111 mm | Older and smaller cars |
| 14" | 356 x 111 mm | Classic and kit cars with a very short mount |
| Hex, full width | 520 x 111 mm | Standard footprint with angled corners |
| Hex, compact | Follows the registration | Show and short-reg cars |
If your measurement is between two sizes, take the smaller one. A plate that overhangs a recess looks worse than one with a few millimetres to spare.
Count the characters before you go short
This is the step people skip. A road-legal registration has fixed letter sizes and gaps: 50 mm per character (14 mm for a 1 or an I), 11 mm between characters, 33 mm between the two groups and 11 mm clear at each end. That adds up.
A seven-character registration like AB12 CDE needs about 424 mm of lettering plus margins. It fits a standard plate and an 18.5" plate at full size. On a 16" or 14" plate it can only be made with smaller letters, and smaller letters mean a show plate, not a road-legal one.
Shorter registrations, or ones with a 1 or an I in them, fit shorter plates. The short plates guide goes through who can legally run one.
You don't have to do the arithmetic. Put the registration into the builder, pick a size, and it will tell you whether the letters stay legal or the plate has to become a show plate.
Front and rear can be different sizes
A 21" x 6" rear with a standard front is common on 4x4s. A short front with a standard rear happens on cars with a slim grille. Each plate is judged on its own, so mix as the car needs. More on that in do front and rear plates have to match.
Holders and clips
If you're using a plastic clip holder rather than pads, measure the holder's grip height as well. Standard holders expect a 111 mm tall plate with square corners. Hex and short plates don't clip reliably, and a 21" x 6" won't fit at all. For anything other than a standard rectangle, fit with pads.
If you're still unsure
Send us a photo of the bumper with a tape measure across it and we'll tell you which size to order. It takes us thirty seconds and saves a plate going back.