Staying Within The Lines: The Strict Regulatory Geometry of Your United States Registration-Number

July 2026

When it’s time to refresh your aircraft’s paint, choosing a custom livery is one of the most exciting parts of the process. However, before you fall in love with an ultra-modern font or a highly stylized design for your registration markings, remember that your registration number is heavily regulated under 14 CFR Part 45. While it isn’t a part of the aircraft's type design data, failing to meet strict FAA geometric standards could ground your aircraft.

To keep your aircraft compliant, the following outlines several non-negotiable dimensions and rules:

• Height and Stroke Proportion: Characters must be a minimum of 12 inches tall. Additionally, they must be formed by solid lines that are exactly one-sixth as thick as the character is high.
• The Two-Thirds Rule: Characters must be two-thirds as wide as they are high. The only exceptions are the number "1" (one-sixth wide) and the letters "M" and "W" (which can be as wide as they are high).
• Character Spacing: To ensure clear legibility, the space between each character cannot be less than one-fourth of the character width. For a standard 12-inch character, this requires a minimum of 2 inches of clearance between the closest points of adjacent letters or shadows.
• Slant Limits: If you want a dynamic, slanted look, your registration number can be slanted, but only up to +/-30 degrees from vertical.
• No Ornamentation: The FAA requires capital Roman-style block or round letters with completely consistent stroke thickness. Standard, unmodified Arial font is actually non-compliant because its stroke thickness varies. Embellishments, flourishes, or multi-colored characters are prohibited unless you provide written approval documentation from your local FSDO. FAA Guidance allows shading or outlining of the marks as long as it does not inhibit the ease of seeing the Registration Marks.

Incorrect

• Contrast: The Registration Number must contrast with the background base color.

The 500-Foot Test

Ultimately, the FAA considers a 12-inch marking legible and contrasting if it can be easily read from 500 feet away in daylight without binoculars. If you choose a complex, highly decorated background (like checkering) or a color-shifting metallic/pearl paint, the number might disappear when viewed from the nose or tail. If there is any ambiguity, the local FAA Inspector or your local Chief Inspector will make the final determination.