Light Bulb Shape Table
| US code | Metric name | Diameter | Shape family |
|---|---|---|---|
| A19 | A60 | 60 mm = 2.375 in (19/8) | Standard pear |
| CA11 | CA35 | 35 mm = 1.375 in (11/8) | Candle flame |
| BR30 | BR95 | 95 mm = 3.75 in (30/8) | Bulged reflector flood |
| MR16 | MR50 | 50 mm = 2 in (16/8) | Multifaceted reflector |
| Watts (120 V) | Output (lm) | Efficacy (lm/W) |
|---|---|---|
| 5 | 25 | 5 |
| 25 | 200 | 8.0 |
| 40 | 500 | 12.5 |
| 60 | 850 | 14.2 |
| 75 | 1,200 | 16.0 |
| 100 | 1,700 | 17.0 |
| 150 | 2,850 | 19.0 |
| 200 | 3,900 | 19.5 |
Every bulb code is two clues glued together: a letter for the shape family and a number for the diameter in eighths of an inch. A19 is an Arbitrary (pear-shaped) bulb 19/8 of an inch across - 2.375 inches, which the metric world rounds to 60 mm and renames A60; BR30 is a Bulged Reflector 30/8 = 3.75 inches; MR16 is a Multifaceted Reflector 16/8 = 2 inches. The letter-number code is printed on every package and molded into every glass envelope, and it answers the only question that matters at the fixture: will the glass fit the housing?
The suffix after the shape is the base - the part that must match the socket, not the housing: E26 is the standard Edison screw (E12 the candelabra small screw), and GU5.3 is the two-pin push-in of low-voltage track lights. Shape and base are independent purchases: an A19 lamp exists with E26, E12, even GU24 bases, so read the code as shape-plus-base. Brightness is the third axis and the LED era moved it from watts to lumens - the old 60-watt bulb pushed about 850 lumens, which a modern LED delivers at 8-10 watts, so the table's watt rows are now a translation dictionary, not a shopping spec.
How to use
- Measure or read the diameter to pick the shape: the number is eighths of an inch (A19 = 2.375 in, BR30 = 3.75 in, BR40 = 5 in), so a recessed can marked 5-6 inches wants a BR40 or BR30, and a lamp shade harp sized for A19 will pinch an A21.
- Match the base, not the glass: E26 screws into standard North American sockets, E12 is the candelabra size for chandeliers and fans, GU5.3 pins push into low-voltage MR16 sockets - a correct shape with a wrong base is a paperweight.
- Shop by lumens, decode by watts: 450 lm replaces 40 W, 800 lm replaces 60 W, 1,100 lm replaces 75 W, 1,600 lm replaces 100 W - the table's rows give the original incandescent outputs these LED packages target.
Frequently asked questions
What does the number in A19 or BR30 actually measure?
The diameter of the glass at its widest point, in eighths of an inch - nothing else. A19 = 19/8 in = 2.375 in = 60.3 mm, which is why the same bulb is called A60 in IEC metric naming (60 mm, rounded); BR30 = 30/8 = 3.75 in = 95 mm, sold as BR95 in metric markets; MR16 = 16/8 = exactly 2 inches = 50 mm (MR50). The rule is bidirectional and works for every family: G25 globe = 25/8 = 3.125 in, T8 tubular = 1 in, PAR38 = 4.75 in. Length is not encoded - two A19 lamps from different makers can differ in overall height by a quarter inch - so for tight fixtures the diameter rule gets you close, but the final check is physical: hold the old bulb against the new one. The eighths convention is the last surviving inch-era artifact in a bulb aisle that is otherwise metric, and it is why US and EU codes for identical glass never match on paper.
What is the difference between BR30 and PAR38 flood bulbs?
Both are reflector floods, but they are built for different weather and different light. BR (bulged reflector) is a soft-front bulb with the reflective coating inside a gently bulged envelope - cheap, diffuse, and for indoors: recessed cans and track heads where the fixture or ceiling protects it. PAR (parabolic aluminized reflector) is a thick, hard-glass lens with a precisely formed beam - weatherproof, impact-rated, and the choice for outdoor flood fixtures and landscape lighting; PAR38 at 38/8 = 4.75 inches is the classic security-light size. Beam angle separates them further: PAR lamps come in tight spot (under 25 degrees) to wide flood, while BR lamps are inherently wide and soft. The practical substitution rule runs one way only: a PAR can replace a BR in a pinch outdoors, but a BR outdoors will haze and fail its coating in the weather the PAR was engineered for. Fit-wise, both follow the eighths rule - BR30 and PAR30 share a 30/8 diameter, so can size matches either.
How do I convert old bulb watts to LED lumens?
Use lumens, not watts, and keep the old watt column as the dictionary: the incandescent norms were 40 W = about 450 lumens, 60 W = 800-850, 75 W = about 1,100, and 100 W = 1,600-1,700. An LED producing those lumens draws roughly 5, 8-9, 10-11 and 14-17 watts respectively - about a factor of six better, because the old bulbs spent over 90 percent of their watts on heat (the table's efficacy rows: 14-17 lumens per watt for tungsten versus 80-120 for LED). Two cautions: brightness labels on LED boxes quote maximum lumens at turn-on, and cheap LEDs lose 10-20 percent over their first thousand hours, so buy one step up for critical rooms; and for enclosed fixtures, buy lamps rated enclosed-fixture - the old watts were also a cooling spec, and trapped heat is what kills LED drivers early.
Why do some bulbs have pin bases like GU5.3 instead of screws?
Voltage and heat. GU5.3 is the two-pin push-in of the low-voltage (12 V) MR16 halogen system: the lamp runs off a transformer or driver, and a pin base keeps the contact resistance low and the lamp click-in serviceable - screw threads at 12 volts and 50 watts would waste power and loosen with thermal cycling. The GU code itself is a base spec: G = glass pin spacing, U = u-shaped pins without a built-in socket hole, 5.3 = 5.3 mm between pin centers; GU10 is the 10 mm-spaced twist-lock variant that runs line voltage. Everything else in the aisle is an Edison-family screw: E26 in North America (26 mm across the thread), E12 candelabra, E17 intermediate; Europe uses E27 and E14. The base is stamped with the lamp's shape code on most packages precisely because they are independent choices - and the mismatch failure is silent the other way too: a GU5.3 lamp forced into a GU10 socket makes poor contact and flickers.