Gauge to mm Calculator

One Word, Two Different Rulers

A gauge number looks like a measurement. It is not. It is a row number in a table, and there are several tables, each written for a different product.

Wire follows American Wire Gauge, also called the Brown and Sharpe gauge. The numbers run from 0000 up to 40, they step in a fixed geometric ratio, and a bigger number means thinner wire. Sheet metal follows a family of steel gauges that also run backwards, from about 3 to 38, but the thickness behind a number changes with the material and the coating.

The two systems do not agree. 16 AWG wire is 1.2908 mm across. 16 gauge sheet steel is 1.5875 mm thick on the federal table and 1.5189 mm on the table most uncoated coil is sold against. One number, three answers, and a part cut to the wrong one is either too thin for the load or too thick for the slot.

The calculator on this page switches between them. Pick wire and the number is treated as an AWG size, with diameter, area and a copper ampacity hint. Pick a sheet standard and the same number is treated as a gauge of steel, galvanized steel or aluminum. The rest of this page explains where each number comes from and how to check it with a caliper.

The AWG Equation, Step by Step

American Wire Gauge is defined by one equation, and that equation is exact. Every published AWG table is a rounding of it.

d(mm) = 0.127 x 92^((36-n)/39)

Read it in three pieces. 36 AWG is 0.127 mm by definition, which is 0.005 inch. 0000 AWG is 92 times that, or 11.684 mm. Between those two sizes sit 39 equal ratio steps, which is where the 39 in the exponent comes from. The 92 is the ratio between the largest and smallest size, not a fudge factor.

Area follows from the diameter, because a wire is a circle:

area(mm2) = 3.1416 x d x d / 4

Run 12 AWG through both steps. The exponent is (36 - 12) / 39, which is 0.6154, and 92 raised to that power is 16.162. Multiply by 0.127 and the diameter is 2.0525 mm. Square it, multiply by 3.1416 and divide by 4, and the area is 3.3088 mm2. That matches the 3.309 mm2 printed in every wire table, because the table is the same equation rounded to four digits.

Now 24 AWG, which is 12 steps smaller. The exponent becomes 0.3077, the diameter is 0.5106 mm, and the area is 0.2047 mm2. A phone charger cable and a branch circuit conductor are the same geometry at wildly different scales.

Inches come from the millimeter answer divided by 25.4, because the international inch is exactly 25.4 mm under the SI rules set out by NIST Special Publication 811. For 12 AWG that is 2.0525 / 25.4 = 0.0808 in. The old trade unit, the mil, is a thousandth of an inch, so 12 AWG is 80.8 mils and 6,530 circular mils.

AWG Sizes in Millimeters, Inches and Square Millimeters

The table below covers the sizes a homeowner or a panel builder is likely to hold in their hand. Diameter and area come straight from the equation above. The ampacity column is the 60 degrees C copper column of NFPA 70 Table 310.16, which is the base number before any adjustment for ambient temperature, bundling or conduit fill.

Two entries in that table are worth a second look. 12 AWG carries 20 A at 60 degrees C, which is why it is the standard kitchen circuit conductor, and 14 AWG carries 15 A, which is why it is the standard lighting conductor. The ampacity does not scale with area in a straight line: 12 AWG has 1.59 times the copper of 14 AWG and only 1.33 times the ampacity, because heat has to leave the surface of the insulation and a fatter wire has less surface per unit of copper.

Sizes 16 and smaller are not in Table 310.16 at all. They show up as fixture wire under Article 402, as control cable, or as low voltage cable, and each of those has its own rules. A 16 AWG wire is not a small branch circuit conductor and should not be treated as one.

Three Steps Doubles the Copper

Because the gauge series is geometric, useful rules fall out of it without a table.

One step is a ratio of 92^(1/39), which is 1.1229. Three steps multiply the diameter by 1.1229 cubed, or 1.416, and the area by the square of that, or 2.005. Six steps double the diameter. Twelve steps multiply the area by 16.16.

• 12 AWG to 9 AWG: area goes from 3.3088 mm2 to 6.6342 mm2, which is 2.005 times the copper.
• 12 AWG to 6 AWG: diameter goes from 2.0525 mm to 4.1154 mm, which is 2.005 times as wide.
• 12 AWG to 24 AWG: area goes from 3.3088 mm2 down to 0.2047 mm2, a drop of 16.16 times.

The rules matter for two jobs. Voltage drop is proportional to current times resistance, and resistance is inversely proportional to area, so three gauge steps down halves the drop on the same current. Bending stiffness of a sheet is proportional to the cube of thickness, so a 4.3 percent thinner sheet is about 12 percent less stiff, which is why a gauge substitution on a panel is not a cosmetic change.

The same logic explains ampacity tables. Going from 14 AWG to 12 AWG adds 59 percent more copper and buys 5 amps. Going from 12 AWG to 10 AWG adds another 59 percent and buys 10 amps. The returns improve as the wire gets bigger, and that is why long runs are upsized rather than doubled.

Sheet Steel Gauge Is Written Into Federal Law

For sheet and plate iron and steel there is an actual statute. 15 U.S.C. 206 establishes "the only standard gauge for sheet and plate iron and steel in the United States" and prints the whole table from 0000000 down to 38, with fractions of an inch, decimal inches, millimeters and weight per square foot.

The numbers in that table are not a rounded trade convention. They are fractions: 16 gauge is 1/16 inch, 20 gauge is 3/80 inch, 24 gauge is 1/40 inch. Multiply by 25.4 and the millimeter column follows: 0.0625 x 25.4 = 1.5875, 0.0375 x 25.4 = 0.9525, 0.025 x 25.4 = 0.635.

One caution on the statute's weight column. The weights per square foot were computed with a density of 480 pounds per cubic foot, so 16 gauge shows 2.5 pounds per square foot even though modern steel at 0.2836 pounds per cubic inch works out slightly heavier. Use the thickness columns and ignore the weight columns.

That table is the legal definition of the U.S. Standard Gauge. It is not the table that a coil of cold rolled sheet is sold against today, and that gap is the next section.

Why 16 Gauge Is Four Different Thicknesses

The sheet you buy at a service center is usually specified on the Manufacturers' Standard Gauge, sometimes written MSG. The numbers are close to the federal table but not identical, because MSG was built from weight per square foot rather than from fractions of an inch. 16 gauge MSG is 0.0598 inch, not 0.0625 inch.

Galvanized sheet adds a zinc coating on top of the base steel, and the published gauge includes the coating, so 16 gauge galvanized is 0.0635 inch. Stainless follows its own series at 0.0625 inch. Aluminum sheet in the United States is sold on the Brown and Sharpe gauge, which is the same series as AWG, and 16 gauge aluminum is 0.0508 inch.

The spread at 16 gauge runs from 1.2903 mm to 1.6129 mm, which is 25 percent between the thinnest and the thickest. Even within steel the gap is real: MSG 16 gauge is 0.0686 mm thinner than the federal table, or 4.3 percent less metal, and the coating on galvanized adds 0.0037 inch back on top.

Those small percentages decide whether a job fits. A bend allowance calculated for 1.5875 mm steel will be off when the press brake runs 1.5189 mm stock, and the error compounds over several bends. A weld schedule set for 16 gauge steel will burn through 16 gauge aluminum. If a drawing says 16 ga and nothing else, the drawing is incomplete, and the question to ask is which standard and which material.

Measure Before You Cut

A caliper settles the argument in ten seconds. Close the jaws on the sheet, away from a sheared edge and away from a bend, and read the number. Then compare it with both tables and see which one the stock matches.

• Measure in three places. Sheet thickness varies across a coil, and the tolerance is part of the material spec, not the gauge number.
• Know what the coating adds. Galvanized and painted stock measure thicker than the base steel. If you need the steel core, subtract the coating or ask for the base metal thickness.
• Use the right tool. A tape measure is useless below 1 mm. A digital caliper at $25 reads to 0.01 mm, which is enough to tell MSG from the federal table at 16 gauge.
• Check the wire with a gauge, not a ruler. Stranded wire measures larger than its nominal size, and the AWG number refers to the equivalent solid area, not to the outside diameter of the bundle.
• Do not mix the families. A wire gauge number and a sheet gauge number are unrelated. 12 AWG is 2.0525 mm and 12 gauge sheet steel is 2.778 mm.

Conversions in the other direction are just as easy to get wrong. One millimeter is 0.03937 inch, so a 1.5 mm sheet is 0.0591 inch, which lands between 16 gauge MSG at 0.0598 and 17 gauge at 0.0563. Metric sheet is usually specified in millimeters and toleranced under ISO 9445 or EN 10131, and forcing it into a gauge number loses information. If the drawing is metric, buy metric stock.

For the wire side, the NIST Handbook 44 Appendix C is the reference for the exact inch and the mil, and it is where the 25.4 in these conversions comes from. Nothing on this page depends on a rounded constant.