Light Bulb Size Chart: Shapes, Bases & Dimensions 2026

The number is the diameter — in eighths of an inch

Every bulb code is a little formula: the letters name the shape family and the number is the bulb’s diameter in eighths of an inch. An A19 is 19/8 in across — 2.375 in (60.33 mm). A BR30 is 30/8 in — 3.750 in. A PAR38 is 38/8 in — 4.750 in. Once you know the rule you never need to guess a size again. On the defaults — code A19, measured 2.3750 in — the nominal diameter is 2.375 in / 60.33 mm, the family is A-series (standard household), the base is E26, the MOL is 4.38 in, the typical lumens are 450–1,600, the difference is 0.0000 in and the verdict is MATCHES — the nearest code is A19 (ST19 ties).

Light bulb size chart — diameter by code

Every diameter is the exact code value — the code number ÷ 8 in, and the millimetres are the inches × 25.4, half-up — never hand-typed. MOL (maximum overall length), base and typical lumens are nominal manufacturer values: MOL varies ±1/8 in by maker and lumens vary by wattage and maker. At the defaults — code A19, measured 2.3750 in — the nominal diameter is 2.375 in / 60.33 mm, the family is A-series (standard household), the base is E26, the MOL is 4.38 in, the typical lumens are 450–1,600, the difference is 0.0000 in and the verdict is MATCHES. Check the identities: nominal diameter = code number ÷ 8; millimetres = inches × 25.4; difference = |measured − nominal|; MATCHES when the difference is at most 0.0625 in, inclusive; nearest code = the code whose nominal diameter is closest to the measured diameter. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

The worked example, line by line

Setup: code A19, measured 2.3750 in.

Line 1 — the nominal diameter. A19 means 19 eighths of an inch: 19 ÷ 8 = 2.375 in. In millimetres that is 2.375 × 25.4 = 60.33 mm.

Line 2 — the family, base and length. The A stands for the standard household shape — family A-series (standard household) — with an E26 base and a maximum overall length of 4.38 in. Typical lumens are 450–1,600, depending on the wattage.

Line 3 — the match. You measured 2.3750 in and the nominal is 2.375 in, so the difference is |2.3750 − 2.375| = 0.0000 in — inside the 1/16-in tolerance, so the verdict is MATCHES. The nearest code is A19 (ST19 ties).

Reverse: the diameter for PAR38 is 4.75 in / 120.65 mm. The code(s) for a 3.75-in diameter are BR30 & PAR30 (both are 30/8 in — same diameter, different reflector shape). The millimetres for BR40 are 127.00 mm (5.000 in × 25.4). The smallest code(s) with diameter at least 3.00 in are G25 & PS25 (both 3.125 in). The nearest code to a measured 2.50-in diameter is R20 (diff 0.0000 in; A19 is 0.1250 in away). The diameter for T8 is 1.00 in / 25.40 mm.

Every branch, checked

BR30, measured 3.75 in: Nominal 3.750 in / 95.25 mm, MOL 5.38 in, base E26, lumens 650–1,200, difference 0.0000 in, MATCHES, nearest BR30 (PAR30 ties). A 3.75-in bulb in a BR30 socket is exactly on nominal — MATCHES.

PAR38, measured 4.75 in: Nominal 4.750 in / 120.65 mm, MOL 5.31 in, base E26, lumens 900–1,500, difference 0.0000 in, MATCHES, nearest PAR38. The big floodlight is 4.750 in across — MATCHES.

G25, measured 3.125 in: Nominal 3.125 in / 79.38 mm, MOL 4.50 in, base E26, lumens 500–1,100, difference 0.0000 in, MATCHES, nearest G25 (PS25 ties). The globe bulb is 3.125 in across — MATCHES.

MR16, measured 2.0 in: Nominal 2.000 in / 50.80 mm, MOL 2.00 in, base GU10 / GU5.3, lumens 400–800, difference 0.0000 in, MATCHES, nearest MR16. The little reflector is 2.000 in across — MATCHES.

T8, measured 1.0 in: Nominal 1.000 in / 25.40 mm, MOL 48.00 in, base G13, lumens 2,600–2,800, difference 0.0000 in, MATCHES, nearest T8. The tube is 1.000 in across and 48.00 in long — MATCHES.

C7, measured 0.875 in: Nominal 0.875 in / 22.23 mm, MOL 2.25 in, base E12, lumens 25–40, difference 0.0000 in, MATCHES, nearest C7. The night-light is 0.875 in across — MATCHES.

A19, measured 3.75 in: Nominal 2.375 in / 60.33 mm, MOL 4.38 in, base E26, lumens 450–1,600, difference 1.3750 in, NO MATCH, nearest BR30 (PAR30 ties). A 3.75-in bulb is not an A19 — it is 1.3750 in too wide. The nearest code is BR30 (PAR30 ties).

A19, measured 2.50 in: Nominal 2.375 in / 60.33 mm, MOL 4.38 in, base E26, lumens 450–1,600, difference 0.1250 in, NO MATCH, nearest R20. A 2.50-in bulb is 0.1250 in over the A19 nominal — over the 1/16-in tolerance, so NO MATCH. The nearest code is R20.

A19, measured 2.4375 in: Nominal 2.375 in / 60.33 mm, MOL 4.38 in, base E26, lumens 450–1,600, difference 0.0625 in, MATCHES, nearest A19 (R20 & ST19 ties). A bulb 1/16 in over nominal is exactly on the boundary — and the boundary is inclusive, so it still MATCHES.

What the defaults assume

The model assumes the code system: the letter(s) name the shape family and the number is the diameter in eighths of an inch — it does not model the base or the wattage, which the code does not carry. It assumes MOL (maximum overall length), base and typical lumens are nominal manufacturer values: MOL varies ±1/8 in by maker and lumens vary by wattage and maker. It assumes the match tolerance is 1/16 in (0.0625 in), inclusive. With a measured diameter of 0 in the nearest code is C7 (0.875 in, diff 0.875 in) and the verdict is NO MATCH. With a T8 measured at 1.0 in the verdict is MATCHES. With a measured diameter of 10 in the nearest code is BR40 (5.000 in, diff 5.000 in) and the verdict is NO MATCH. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum; the reverses are nearest-value solutions, and the exact unrounded figures (4.75 in, 127.00 mm, 1.00 in) are stated wherever the rounded figures appear.

Output · formula · meaning

Who uses this

• The homeowner replacing a bulb: they learn an A19 is 2.375 in across — and they check the BR30 branch (3.750 in) to see the floodlight size.
• The renter with a 3.75-in bulb: they learn the code is BR30 & PAR30 — same diameter, different reflector shape — and they check the PAR38 branch (4.750 in) for the bigger floodlight.
• The shopper with a 2.50-in bulb: they learn the nearest code is R20 — and they check the 2.4375-in branch (MATCHES) to see the boundary still matches.
• The DIYer measuring a tube: they learn a T8 is 1.00 in across and 48.00 in long — and they check the C7 branch (0.875 in) for the smallest bulb in the chart.

Accuracy & limitations

The math is first-principles code arithmetic: nominal diameter = code number ÷ 8; millimetres = inches × 25.4; difference = |measured − nominal|; MATCHES when the difference is at most 0.0625 in, inclusive; nearest code = the code whose nominal diameter is closest to the measured diameter. Every table cell is derived from the code number — the A19 cell is 19 ÷ 8 = 2.375 in and 2.375 × 25.4 = 60.33 mm, not a hand-typed number. The 4-decimal exact is shown where the rounding matters: the diameter for PAR38 is 4.75 in / 120.65 mm; the millimetres for BR40 are 127.00 mm; the diameter for T8 is 1.00 in / 25.40 mm. What the model cannot do is identify a bulb by itself: the code says nothing about the base (an A19 can be E26 or E12) or the wattage — check the base separately and the wattage/lumens separately — and MOL and lumens are nominal values that vary by maker. For the base, see the light bulb base chart; for the wattage and lumens, see the LED bulb wattage chart; for the wiring that feeds the fixture, see the electrical wire size chart. This is a calculator, not a lighting design.