Understanding Light Bulb Shapes and Sizes

Choosing the right light bulb becomes much simpler when you can identify their shapes and sizes. Whether you’re upgrading your lighting or switching to LEDs for their benefits—like energy efficiency, longer lifespan, and better control—understanding light bulb codes can make all the difference.

Light bulbs are labeled with a combination of letters and numbers known as light bulb codes, which indicate their shape, features, and size. The letters typically describe the shape or a specific characteristic, while the numbers denote size, measured in millimeters or fractions of an inch. By decoding these codes, you’ll know if a bulb will fit your fixture and meet your lighting needs.

For instance, each lighting application often requires a specific bulb shape to achieve the best results. Familiarity with these codes allows you to quickly determine which bulb fits your fixture. Alternatively, tools like a Universal Bulb Finder can simplify the selection process.

Here’s an overview of common bulb groups, their codes, and typical uses:

A Group: A15, A19, A21, A25

Standard or Arbitrary (A) bulbs are the most common for household lighting, ideal for fixtures like ceiling lights, lamps, and porch lights.

A15: Diameter of 1-7/8″ (15/8 inches)
A19: Diameter of 2-3/8″ (19/8 inches)
A21: Diameter of 2-5/8″ (21/8 inches)
A25: Diameter of 3-1/8″ (25/8 inches)

G Group: G11, G14, G16/G50, G60, G25/G80, G30

Globe (G) bulbs have a round shape and are often used in chandeliers, kitchens, or bathroom vanities.

G11: Diameter of 1-3/8″ (11/8 inches)
G14: Diameter of 1-3/4″ (14/8 inches)
G16/G50: Diameter of 2″ (16/8 inches or 50mm)
G60: Diameter of 2-3/8″ (60mm)
G25/G80: Diameter of 3-9/64″ (80mm)
G30: Diameter of 3-3/4″ (30/8 inches)

B and C Groups: B10, C7, C9, C15, CA10

These bulbs mimic the shape of a candle flame. Variants include blunt-tip (B), conical (C), and conical angular (CA) bulbs, making them ideal for chandeliers, decorative strands, and holiday lights.

B10/CA10: Diameter of 1-1/4″ (10/8 inches)
C7: Diameter of 7/8″
C9: Diameter of 1-1/8″
C15: Diameter of 1-7/8″

BR Group: BR20/R20, BR30, BR40

Bulged Reflector (BR) bulbs, traditionally made with reflective interiors, diffuse light to minimize shadows. LED BR bulbs mimic this effect without requiring a reflector. They’re commonly used in recessed or track lighting.

BR20/R20: Diameter of 2-1/2″ (20/8 inches)
BR30: Diameter of 3-3/4″
BR40: Diameter of 5″

PAR Group: PAR16, PAR20, PAR30, PAR36/AR111, PAR38

Parabolic Aluminized Reflector (PAR) bulbs focus light into spot or flood beams, making them perfect for outdoor lighting, track lights, and recessed fixtures. To adjust the color of your light, you can use one of our lens filters, such as the KastLite Clear Acrylic Filter, available here.

This connects the PAR bulbs to the lens filters seamlessly while also promoting the product.

PAR16: Diameter of 2″ (16/8 inches)
PAR20: Diameter of 2-1/2″
PAR30: Diameter of 3-3/4″
PAR36/AR111: Diameter of 4-1/2″ (36/8 inches or 111mm)
PAR38: Diameter of 4-3/4″

MR Group: MR11 and MR16

Multifaceted Reflector (MR) bulbs are small but powerful, often used in track or display lighting. Their faceted design creates precise, focused lighting. Customize your lighting color using one of our lens filters, such as the KastLite Clear Acrylic Filter, which you can find here.

MR11: Diameter of 1-3/8″ (11/8 inches)
MR16: Diameter of 2″

T Group: T7, T8, T10, T14

Tubular (T) bulbs are elongated and work in a variety of fixtures, from chandeliers to garage lights.

T7: Diameter of 7/8″
T8: Diameter of 1″
T10: Diameter of 1-1/4″
T14: Diameter of 1-3/4″

By understanding these shapes and sizes, you can confidently select the right light bulb for any project, ensuring both compatibility and optimal lighting performance.