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mm to mil converter

Convert millimetres to mils and mils to millimetres, with inches and micrometres alongside, plus copper weight in ounces to thickness. The charts below cover the trace widths, drills, pitches and board thicknesses PCB work actually uses.

Type in any box; the others update.

Millimetres

Mils (thou)

mil

1 mil = 1 thou = 0.001 inch. Not a millimetre.

Inches

Micrometres (microns)

µm

0.2 mm in mils

7.874 mil

Rounded: 1 mil is 0.0254 mm, so most metric values are not a whole number of mils.

More digits: 7.874015748 mil
Nearest whole mil: 8 mil = 0.2032 mm (+1.6 %)
Conversion factor: 1 mm = 39.3701 mil

Copper weight to thickness

PCB copper is sold by weight per square foot. Fabs count 1 oz as 35 µm.

Copper weight

Thickness

µm

Thickness

mil

1 oz/ft² copper

35 µm

1.378 mil · 0.035 mm

Fab convention: 35 µm per oz
From the weight alone (8.96 g/cm³): 34.1 µm (1.34 mil)

Fabs quote finished copper weights, and inner layers are often thinner than outer ones: JLCPCB's default inner copper is 0.5 oz (17.5 µm).

How many mils are in a millimetre?

1 mm = 39.3701 mil, and 1 mil = 0.0254 mm exactly, because a mil is a thousandth of an inch and an inch is defined as 25.4 mm. To convert mm to mil, divide by 0.0254 (or multiply by 39.37). To convert mil to mm, multiply by 0.0254. So 0.2 mm is 7.874 mil, and a 6 mil trace is 0.1524 mm.

How to use the mm to mil converter

Type a value into any of the four boxes: millimetres, mils, inches or micrometres.
The other three update as you type, and the panel shows the mm to mil (or mil to mm) answer.
Check the nearest whole mil and nearest 0.01 mm if you need a round number for a design rule.
For copper, type an ounce weight or a thickness in the copper section underneath.

The converter tells you whether a result is exact or rounded, and warns when a number looks like it was typed in the wrong unit, such as a 6 mil trace entered as 6 mm.

How to convert mm to mil and mil to mm

A mil is 0.001 inch, and since 1959 the inch has been exactly 25.4 mm. So 1 mil = 0.0254 mm exactly, and 1 mm = 1 ÷ 0.0254 = 39.37007874… mil.

The two directions behave differently. 0.0254 is 127 ÷ 5000, so multiplying by it always gives a finite decimal: every mil value has an exact millimetre equivalent. Dividing by it leaves the 127 in the denominator, so a millimetre value almost never has an exact mil equivalent. That is why fab tables round their mil figures and why the converter marks mm to mil results as rounded.

Worked example 1: a 0.2 mm trace in mil

0.2 ÷ 0.0254 = 7.874015748… mil. Rounded, that is 7.874 mil. If a mil-based tool needs a whole number, 8 mil is 0.2032 mm, 1.6 % wider.

Worked example 2: a 6 mil clearance in mm

6 × 0.0254 = 0.1524 mm, exactly. In a metric tool you can enter 0.1524 mm, or round up to 0.16 mm. Rounding down to 0.15 mm would undercut a 6 mil rule by 1.6 %.

Worked example 3: a 0.5 mm pitch part on a mil grid

0.5 mm is 19.685 mil. Place a 16-pin row of a 0.5 mm QFN on a 5 mil grid and the worst pin lands 0.063 mm (2.48 mil) off its true position; on a 25 mil grid it is 0.31 mm off, more than half a pad width. Metric parts belong on a metric grid.

Mil vs mm: why the names get confused

“Mil” is short for milli-inch, a thousandth of an inch, but it sounds like millimetre and the two get swapped all the time. The difference is a factor of about 39: a 10 mil trace is 0.254 mm, while a 10 mm trace would be a power bus. If a number seems far too large or far too small for what it describes, the unit is the first thing to check.

Mil and thou are the same unit. Plastic film, coatings and flooring wear layers are also sold in mils, and the conversion is identical: a 20 mil layer is 0.508 mm. The mil used for gun sights and artillery is a different thing entirely: an angle close to a milliradian, with nothing to do with length.

mm to mil chart for PCB values

Rounded to four significant figures. Values that are exact in both units are noted.

Millimetres	Mils	Inches	Where you meet it
0.05 mm	1.969 mil	0.001969 in	Fine grid step for metric parts
0.09 mm	3.543 mil	0.003543 in	JLCPCB multilayer minimum trace and space
0.1 mm	3.937 mil	0.003937 in	Common 1–2 layer minimum trace and space
0.127 mm	5 mil	0.005 in	Exactly 5 mil
0.15 mm	5.906 mil	0.005906 in	Comfortable signal trace; smallest common drill
0.2 mm	7.874 mil	0.007874 in	General signal trace
0.25 mm	9.843 mil	0.009843 in	Wider signal trace or clearance
0.3 mm	11.81 mil	0.01181 in	Typical via drill
0.4 mm	15.75 mil	0.01575 in	Fine pitch (some QFN and WLCSP)
0.5 mm	19.69 mil	0.01969 in	QFN and TQFP pitch
0.6 mm	23.62 mil	0.02362 in	Via pad for a 0.3 mm drill
0.8 mm	31.5 mil	0.0315 in	BGA pitch; thin board
1 mm	39.37 mil	0.03937 in	Through-hole pin drill
1.27 mm	50 mil	0.05 in	Exactly 50 mil (SOIC pitch)
1.6 mm	62.99 mil	0.06299 in	Standard board thickness
2.54 mm	100 mil	0.1 in	Exactly 100 mil (header pitch)
3.2 mm	126 mil	0.126 in	M3 mounting hole
10 mm	393.7 mil	0.3937 in	1 cm

mil to mm conversion chart

Every value here is exact. Common trace widths and clearances (4 to 12 mil), board thickness (62 mil) and pin pitch (100 mil) are included.

Mils	Millimetres	Micrometres	Inches
1 mil	0.0254 mm	25.4 µm	0.001 in
2 mil	0.0508 mm	50.8 µm	0.002 in
3 mil	0.0762 mm	76.2 µm	0.003 in
3.5 mil	0.0889 mm	88.9 µm	0.0035 in
4 mil	0.1016 mm	101.6 µm	0.004 in
5 mil	0.127 mm	127 µm	0.005 in
6 mil	0.1524 mm	152.4 µm	0.006 in
8 mil	0.2032 mm	203.2 µm	0.008 in
10 mil	0.254 mm	254 µm	0.01 in
12 mil	0.3048 mm	304.8 µm	0.012 in
15 mil	0.381 mm	381 µm	0.015 in
20 mil	0.508 mm	508 µm	0.02 in
22 mil	0.5588 mm	558.8 µm	0.022 in
25 mil	0.635 mm	635 µm	0.025 in
30 mil	0.762 mm	762 µm	0.03 in
40 mil	1.016 mm	1016 µm	0.04 in
45 mil	1.143 mm	1143 µm	0.045 in
50 mil	1.27 mm	1270 µm	0.05 in
62 mil	1.5748 mm	1574.8 µm	0.062 in
100 mil	2.54 mm	2540 µm	0.1 in
125 mil	3.175 mm	3175 µm	0.125 in
250 mil	6.35 mm	6350 µm	0.25 in
500 mil	12.7 mm	12700 µm	0.5 in
1000 mil	25.4 mm	25400 µm	1 in

PCB board thickness in mm, mil and inches

JLCPCB's FR-4 thicknesses run from 0.4 to 2.0 mm; PCBWay also offers 2.4 and 3.2 mm. Both quote ±10 % tolerance on boards 1 mm and thicker and ±0.1 mm on thinner ones.

Thickness	Mils	Inches	Tolerance range
0.4 mm	15.75 mil	0.0157 in	0.3 to 0.5 mm
0.6 mm	23.62 mil	0.0236 in	0.5 to 0.7 mm
0.8 mm	31.5 mil	0.0315 in	0.7 to 0.9 mm
1 mm	39.37 mil	0.0394 in	0.9 to 1.1 mm
1.2 mm	47.24 mil	0.0472 in	1.08 to 1.32 mm
1.6 mm	62.99 mil	0.063 in	1.44 to 1.76 mm
2 mm	78.74 mil	0.0787 in	1.8 to 2.2 mm
2.4 mm	94.49 mil	0.0945 in	2.16 to 2.64 mm
3.2 mm	126 mil	0.126 in	2.88 to 3.52 mm

PCB drill sizes in mm and mil

PCB fabs drill in metric steps; the smallest standard mechanical drill at JLCPCB and PCBWay is 0.15 mm. Fractional-inch sizes still appear in older footprints and hardware.

Metric drill	Mils	Inches
0.15 mm	5.906 mil	0.005906 in
0.2 mm	7.874 mil	0.007874 in
0.25 mm	9.843 mil	0.009843 in
0.3 mm	11.81 mil	0.01181 in
0.35 mm	13.78 mil	0.01378 in
0.4 mm	15.75 mil	0.01575 in
0.45 mm	17.72 mil	0.01772 in
0.5 mm	19.69 mil	0.01969 in
0.6 mm	23.62 mil	0.02362 in
0.7 mm	27.56 mil	0.02756 in
0.8 mm	31.5 mil	0.0315 in
0.9 mm	35.43 mil	0.03543 in
1 mm	39.37 mil	0.03937 in
1.2 mm	47.24 mil	0.04724 in

Fractional inch	Mils	Millimetres
1/64″	15.625 mil	0.396875 mm
1/32″	31.25 mil	0.79375 mm
3/64″	46.875 mil	1.19063 mm
1/16″	62.5 mil	1.5875 mm
5/64″	78.125 mil	1.98438 mm
3/32″	93.75 mil	2.38125 mm
1/8″	125 mil	3.175 mm

Copper weight to thickness (oz to µm and mil)

Fabs use 35 µm per ounce. The last column is the thickness the weight alone implies for copper at 8.96 g/cm³.

Copper weight	Thickness	Mils	Millimetres	From weight alone
0.5 oz/ft²	17.5 µm	0.689 mil	0.0175 mm	17 µm
1 oz/ft²	35 µm	1.38 mil	0.035 mm	34.1 µm
1.5 oz/ft²	52.5 µm	2.07 mil	0.0525 mm	51.1 µm
2 oz/ft²	70 µm	2.76 mil	0.07 mm	68.1 µm
3 oz/ft²	105 µm	4.13 mil	0.105 mm	102 µm
4 oz/ft²	140 µm	5.51 mil	0.14 mm	136 µm

Component pitch and grid conversions

Older packages were drawn in inches, newer ones in millimetres. Only the inch-based pitches come out as round mils.

Pitch	Mils	Packages
0.4 mm	15.748 mil	Fine-pitch QFN, wafer-level packages
0.5 mm	19.685 mil	QFN, TQFP, USB-C receptacles
0.635 mm	25 mil	QSOP (exactly 25 mil)
0.65 mm	25.591 mil	TSSOP, MSOP and many SSOP
0.8 mm	31.496 mil	BGA
1 mm	39.37 mil	BGA, some connectors
1.27 mm	50 mil	SOIC (exactly 50 mil)
2 mm	78.74 mil	2 mm headers and wire-to-board connectors
2.54 mm	100 mil	DIP and 0.1″ headers (exactly 100 mil)

0402, 0603 and 0805: imperial vs metric footprint codes

Imperial codes give length and width roughly in hundredths of an inch, metric codes in tenths of a millimetre. Nominal body sizes; use the datasheet for the land pattern.

Imperial code	Metric code	Body (mm)	Body (mil)
01005	0402	0.4 × 0.2	16 × 8
0201	0603	0.6 × 0.3	24 × 12
0402	1005	1 × 0.5	40 × 20
0603	1608	1.6 × 0.8	60 × 30
0805	2012	2 × 1.25	80 × 50
1206	3216	3.2 × 1.6	120 × 60
1210	3225	3.2 × 2.5	120 × 100
2512	6332	6.3 × 3.2	250 × 120

Fab minimums in mil: what the rounding hides

Capability pages print a rounded mil value next to the millimetre figure. The exact conversion shows which way each was rounded.

Rule	Published	Listed mil	Exact mil
Min trace and space, 1–2 layer, 1 oz (JLCPCB)	0.1 mm	4 mil	3.94 mil
Min trace and space, multilayer, 1 oz (JLCPCB)	0.09 mm	3.5 mil	3.54 mil
Min trace and space, 2-layer, 2 oz (JLCPCB)	0.16 mm	6.5 mil	6.3 mil
Min trace and space (PCBWay)	0.1 mm	4 mil	3.94 mil
Min via hole (0.25 mm via diameter) (JLCPCB)	0.15 mm	not listed	5.91 mil
Min annular ring (PCBWay)	0.15 mm	6 mil	5.91 mil

Common PCB conversions

Scenario	Metric	Imperial
Designing to a 6/6 mil rule A classic conservative trace and space. In a metric tool enter 0.1524 mm, or round up to 0.16 mm.	0.1524 mm	6 mil
JLCPCB 2-layer minimum trace Listed as 4 mil. A 4 mil rule in a mil-based tool is 0.1016 mm, just above the minimum, so it passes.	0.10 mm	3.94 mil
0.5 mm pitch QFN pads TI recommends pads no wider than 0.28 mm on 0.5 mm pitch to avoid solder bridges.	0.5 mm pitch, 0.28 mm pad	19.685 mil pitch, 11.02 mil pad
0.8 mm pitch BGA Never a round number of mils. Lay BGAs out on a metric grid.	0.8 mm	31.5 mil
0402 resistor body Imperial 0402 is metric 1005. Metric 0402 is a far smaller imperial 01005.	1.0 × 0.5 mm	39.4 × 19.7 mil
0.3 mm via on a 0.6 mm pad A common default via that any fab can drill; the annular ring is 0.15 mm (5.9 mil).	0.3 / 0.6 mm	11.8 / 23.6 mil
0.1″ pin header Exact in both units, because the pitch was defined in inches.	2.54 mm	100 mil
1 oz copper The fab convention. Half-ounce inner layers are 17.5 µm (0.69 mil).	35 µm	1.38 mil
1.6 mm board versus 62 mil The two “standard” thicknesses differ by 1.6 %, well inside the ±10 % tolerance fabs quote.	1.6 mm / 1.5748 mm	62.99 mil / 62 mil

Imperial and metric in PCB fabs and EDA tools

Never round a minimum down. If a fab says 0.1 mm, 4 mil (0.1016 mm) is safe and 3.9 mil is not. Round clearances and trace widths up when you convert.
Use the fab's native unit for rules. JLCPCB's and PCBWay's capability pages are metric first, with mil in brackets. Enter the millimetre value so nothing is lost to their rounding.
Match the grid to the part. Inch-pitch parts (SOIC, DIP, headers) sit on 25, 50 or 100 mil grids; metric parts (QFN, BGA, TSSOP) sit on 0.05, 0.1 or 0.25 mm grids. Switch grids when you move between them.
Say which system a case code uses. “0402” on a BOM is assumed to be imperial; a metric 0402 is an imperial 01005, which is a very different part to place.
Switch display units instead of converting by hand. KiCad toggles with Ctrl+U and accepts unit expressions such as 1in + 1mm in any field; Altium's PCB editor toggles with Q.
Know your board thickness in both units. 1.6 mm and 62 mil differ by 1.6 %, well inside the ±10 % tolerance JLCPCB and PCBWay quote, but a mechanical drawing in inches should still say 0.063 in if it means 1.6 mm.

For the rules themselves, the PCB design rules guide covers trace, space, drill and annular-ring limits, and the trace width calculator gives current-carrying widths in both units.

How exact is this converter?

The factors are definitions, not measurements: 1 in = 25.4 mm and 1 mil = 0.0254 mm, as listed by NIST. The converter multiplies by whole numbers of nanometres, so mil to mm, inch to mm and mil to inch results are exact and shown with up to 12 significant figures. Metric to imperial results are rounded to six significant figures, far finer than any fab tolerance. The only judgement call is copper weight, where fabs use 35 µm per ounce while the weight definition gives 34.1 µm.

Do this automatically in ProtoFlow

The ProtoFlow desktop app keeps every board in millimetres and lets you choose how lengths are shown: pick mm, mil or inches in the PCB settings and the editor's readouts and numeric entry follow. In Board Setup you can type a rule with its unit, so 6mil, 0.15mm and 35um all land correctly whichever unit is on screen, and the stackup accepts copper as an ounce weight or a thickness. Fab presets for JLCPCB, PCBWay, OSH Park and AISLER load those fabs' published minimums, and DRC checks the board against them. When you open an Altium PcbDoc, the importer converts Altium's mil-based units to millimetres for you, and KiCad and EAGLE designs import directly too. Download ProtoFlow free for macOS, Windows or Linux.

Glossary

Mil: One thousandth of an inch, exactly 0.0254 mm. Short for milli-inch.
Thou: Another name for a mil, 0.001 inch, more common in British usage.
Millimetre (mm): One thousandth of a metre, equal to 39.37 mil.
Micrometre (µm): One thousandth of a millimetre, often called a micron. 1 mil is 25.4 µm.
Inch: Exactly 25.4 mm since 1959, and 1000 mil.
Copper weight: Copper foil thickness given as ounces per square foot. 1 oz/ft² is about 35 µm.
Pitch: The centre-to-centre distance between neighbouring pins or pads.
Grid: The snap spacing in an EDA tool, for example 50 mil or 0.5 mm.
Finished hole size: A plated hole’s diameter after plating, which is what drill files usually state.
Square mil: The area of a 1 mil × 1 mil square, 0.00064516 mm². Used for trace cross-sections.
Circular mil: The area of a circle 1 mil in diameter, about 0.000507 mm². Used for wire sizes.
Imperial and metric case codes: Two four-digit schemes for chip parts: hundredths of an inch versus tenths of a millimetre.

Related guides

PCB trace width calculator — trace width for a current, shown in mm and mil.
Via current calculator — current per via from drill size and plating.
Footprints and land patterns — how pad sizes and pitches turn into footprints.
PCB design rules — fab minimums for trace, space, drills and rings.
KiCad vs Altium — how the two most common EDA tools compare.
PCB impedance calculator — trace widths for 50 Ω and differential pairs.

Frequently asked questions

How many mils are in a mm?

There are 39.37008 mils in 1 mm, usually rounded to 39.37. It is exactly 1 ÷ 0.0254, a decimal that never ends, so any mil value converted from millimetres is rounded somewhere.

How do I convert mm to mils?

Divide the millimetres by 0.0254, or multiply by 39.37. For example 0.2 mm ÷ 0.0254 = 7.874 mil and 1.6 mm ÷ 0.0254 = 62.99 mil.

How do I convert mils to mm?

Multiply the mils by 0.0254. The answer is always exact: 6 mil × 0.0254 = 0.1524 mm, 20 mil = 0.508 mm and 100 mil = 2.54 mm.

Is a mil the same as a mm?

No. A mil is one thousandth of an inch (0.0254 mm); a millimetre is about 39 times bigger. The names sound alike because "mil" comes from milli-inch. A 6 mil trace is 0.15 mm wide, while a 6 mm trace would be a wide power track.

What is the difference between a mil and a thou?

None. Mil and thou are two names for 0.001 inch; thou is more common in British usage. Neither has anything to do with the angular mil used in optics and artillery.

What is 1 mil in mm and µm?

1 mil is exactly 0.0254 mm, which is 25.4 µm. That exactness comes from the inch, defined in 1959 as exactly 25.4 mm.

What is 0.5 mm in mil?

0.5 mm is 19.685 mil, about 19.7 mil. It is the most common fine pitch for QFN and TQFP packages, which is why those parts never sit neatly on a mil grid.

How thick is a 1.6 mm PCB in mils and inches?

1.6 mm is 62.99 mil, or 0.063 in. The traditional US thickness of 0.062 in is 1.575 mm. The 1.6 % difference is well inside the ±10 % thickness tolerance JLCPCB and PCBWay quote.

How thick is 1 oz copper in mm and mil?

Fabs count 1 oz/ft² as 35 µm (0.035 mm, 1.38 mil). The weight definition gives 34.1 µm for pure copper, so you will also see figures from 34 to 35 µm (1.34 to 1.38 mil). 2 oz is 70 µm.

How do I change mil to mm in Altium or KiCad?

In the Altium PCB editor press Q to toggle between imperial and metric; in the schematic editor use View » Toggle Units (V, U). In KiCad press Ctrl+U or use the unit buttons on the left toolbar. KiCad fields also accept a unit, so typing 6mil converts it for you.

How do I convert square mils or circular mils to mm²?

One square mil is exactly 0.00064516 mm². A circular mil, the area of a circle 1 mil across, is π/4 of that, 0.0005067075 mm². IPC-2221 trace current formulas use square mils.

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