Miniature Bulb Numbers

What it is

Miniature bulbs for vehicles and equipment are sold by trade number. The number stands for a full specification: glass shape and size, base, number of filaments, voltage and light output. The number itself is not a code you can decode digit by digit. A 1156 and a 1157 look alike but have one and two filaments. Trade numbers are cross-referenced to ECE codes such as P21W or W5W on imported vehicles. The full cross-reference is on the miniature bulb cross-reference sheet.

How to read it

1. Find the number stamped on the base or glass.
2. Check the glass code. The letter is the shape and the number is the diameter in eighths of an inch. T-3-1/4 is tubular, 3.25/8 in across.
3. Check the base. Bayonet bases have pins; wedge bases have a flat glass or plastic tail.
4. Count the contacts. Single-contact bases have one button; double-contact have two.
5. Read the suffix, if any, for color or long life.

Common trade numbers

Common suffixes

Key points

• Offset pins on BAY15d bases keep a two-filament bulb from going in the wrong way. Do not force them.
• 3157 and 7443 look similar but use different wedge bases. Check the socket.
• Amber lenses usually take a clear bulb. Clear lenses with amber function need an amber bulb.
• LED replacements may hyperflash turn signals or trigger bulb-out warnings without a load resistor or matched flasher.
• Higher wattage bulbs in place of standard numbers can overheat lens housings.

Common mistakes

• Selling a 1156 for a stop/tail socket.
• Assuming similar-looking wedge bulbs fit the same socket.
• Ignoring the NA suffix on turn signal bulbs behind clear lenses.

Governing standards: No single bulb numbering standard; trade numbers follow industry convention. FMVSS 108 governs lamp function and color.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.