A light bulbs glass shape is measured in eighths (1/8) of an inch. For example a T12 is 12-eighths (12/8) in diameter or 1 1/2 inches and a A19 would be 19-eighths (19/8) in diameter or equal to 2 3/8 inches.

Below is a clickable menu of lamp types that will take you to a page of common shapes for that category.

This is a list of common glass shapes ATR Lighting Enterprises carries A15 A17 A19 AT19 A21 A23 PS25 PS30 PS35 PS40 PS52 RP11 R12 R14 R16 R20 R25 R30 R40 R52 BR30 BR40 ER30 ER40 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 PG17 T20 S6 S7 S8 S11 S12 S14 F10 F15 F20 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G19 G25 G30 G40 P25 B3 B6 B10 B8 C7 C9 C10 C11 C15 G3 G4 G5 BR38 PAR14 PAR16 PAR20 PAR30 PAR36 PAR38 PAR46 PAR56 PAR64 bent tip, flame tip, blunt tip, torpedo tip, CA10 CRC B17 E17 ED17 ED18 ED23 E25 ED24 ED28 ED37 BT56 BT37 BT28 GEM25 CEN25 CH19 H19.