Mil to mm Converter for PCB Design
Convert mils (thousandths of an inch) to millimetres and back, with inches and micrometres alongside, at full precision: 1 mil is exactly 0.0254 mm. For the dimension you enter it also shows the nearest metric drill size, the nearest round imperial value, and where it sits against typical fab minimums for trace, space and drill. A copper weight block turns oz per square foot into thickness. Every input lives in the URL, so a copied link reproduces the conversion.
1 mil = 0.001 in = 0.0254 mm = 25.4 µm
1 mm = 39.3701 mil
- Millimetres
- 0.29972 mm
- 1 mil = 0.0254 mm exactly
- Mils (thou)
- 11.8 mil
- thousandths of an inch
- Inches
- 0.0118 in
- 1 in = 25.4 mm exactly
- Micrometres
- 299.72 µm
- 1 mil = 25.4 µm
- Nearest metric drill
- 0.3 mm
- 11.811 mil, +0.00028 mm from your value
- Nearest common imperial value
- 12 mil
- 0.3048 mm, +0.2 mil from your value
1 oz/ft² copper is nominally 34.8 µm thick (0.0348 mm, 1.37 mil).
As a drilled hole, 0.29972 mm (11.8 mil) is at the typical minimum for standard mechanical drilling, about 0.2 to 0.3 mm (8 to 12 mil). Some fabs treat anything under 0.3 mm as an extra. Typical figures; check your fab's capability sheet.
The link reproduces these exact inputs - paste it in a design review or a fab query.
Bring the stackup, your fab's capability sheet and the parts that worry you. We will go through the design rules with you (trace and space, via sizes, copper weight) before the board goes to fab. If you need the whole board done, see PCB design and layout services.
Common PCB dimensions in mil and mm
The values a layout meets every day, each in the unit it is normally quoted in and converted to the others. The notes on trace, space and drill minimums are typical ranges that vary by fab; check your fab's capability sheet.
| Quoted as | mil | mm | inch | Where you meet it |
|---|---|---|---|---|
| Trace width and spacing | ||||
| 3 mil | 3 | 0.0762 | 0.003 | typical advanced minimum |
| 4 mil | 4 | 0.1016 | 0.004 | typical advanced minimum |
| 5 mil | 5 | 0.127 | 0.005 | typical standard minimum |
| 6 mil | 6 | 0.1524 | 0.006 | typical standard minimum |
| 8 mil | 8 | 0.2032 | 0.008 | |
| 10 mil | 10 | 0.254 | 0.01 | |
| 12 mil | 12 | 0.3048 | 0.012 | |
| 20 mil | 20 | 0.508 | 0.02 | |
| 50 mil | 50 | 1.27 | 0.05 | |
| Board thickness | ||||
| 0.8 mm | 31.496 | 0.8 | 0.031496 | the 31 mil (1/32 in) board |
| 1 mm | 39.37 | 1 | 0.03937 | |
| 1.6 mm | 62.992 | 1.6 | 0.062992 | the 62 mil (1/16 in) board |
| 2 mm | 78.74 | 2 | 0.07874 | |
| Pin and ball pitch | ||||
| 0.4 mm | 15.748 | 0.4 | 0.015748 | fine-pitch BGA, CSP, QFN |
| 0.5 mm | 19.685 | 0.5 | 0.019685 | QFN, QFP, FPC connectors |
| 0.65 mm | 25.591 | 0.65 | 0.025591 | TSSOP, SSOP |
| 0.8 mm | 31.496 | 0.8 | 0.031496 | BGA, QFP |
| 1 mm | 39.37 | 1 | 0.03937 | BGA |
| 1.27 mm | 50 | 1.27 | 0.05 | SOIC, 50 mil exactly |
| 2.54 mm | 100 | 2.54 | 0.1 | pin headers and DIP, 100 mil exactly |
| Via drill | ||||
| 0.2 mm | 7.874 | 0.2 | 0.007874 | low end of the typical standard minimum |
| 0.3 mm | 11.811 | 0.3 | 0.011811 | high end of the typical standard minimum |
| 0.4 mm | 15.748 | 0.4 | 0.015748 | |
Copper weight to thickness
Nominal thickness at 34.8 µm per oz/ft². IPC standards allow the finished copper to come in below nominal after processing, and outer layers gain plating on top of the base foil, so the finished figure on your fab's stackup is the one to design to.
| Copper weight | µm | mm | mil |
|---|---|---|---|
| 0.5 oz/ft² | 17.4 | 0.0174 | 0.685 |
| 1 oz/ft² | 34.8 | 0.0348 | 1.37 |
| 2 oz/ft² | 69.6 | 0.0696 | 2.74 |
| 3 oz/ft² | 104.4 | 0.1044 | 4.11 |
| 4 oz/ft² | 139.2 | 0.1392 | 5.48 |
The conversion
A mil is one thousandth of an inch, 0.001 in. Machinists call the same unit a thou. The inch is defined as exactly 25.4 mm, so 1 mil is exactly 0.0254 mm, or 25.4 µm, with no rounding anywhere in the chain. Multiply mils by 0.0254 to get millimetres; divide millimetres by 0.0254 to get mils, which makes 1 mm equal to 39.3701 mil. A mil is not a millimetre, even though in everyday speech in some countries the word is short for exactly that. On a fab drawing the two differ by a factor of about 39: 6 mil is 0.1524 mm, and 6 mm is 236.22 mil. Write the unit next to every number.
Why PCB design still mixes mils and millimetres
Electronics packaging grew up on an inch grid. DIP packages and pin headers sit on a 100 mil pitch, DIP rows are 300 mil apart, and SOIC leads are 50 mil apart. Those parts are still on almost every board, and their footprints are round numbers only in mils: 100 mil is exactly 2.54 mm and 50 mil is exactly 1.27 mm. Newer packages were defined in metric from the start: QFN and QFP parts at 0.5 and 0.65 mm, BGAs at 1.0, 0.8, 0.5 and 0.4 mm.
The fab side is just as mixed. Drill sizes step in metric, usually 0.05 mm at a time. Board thickness is quoted in millimetres but descends from fractions of an inch: the standard 1.6 mm board is the old 1/16 in board (62.5 mil, 1.5875 mm), still called 62 mil on many drawings. Copper is specified in ounces per square foot. Trace and space rules are quoted in mils by many fabs and in millimetres by others. So one layout routinely holds a 100 mil header, a 0.5 mm QFN, a 6 mil trace and a 0.3 mm drill, and the designer converts between them all day. Gerber and drill files declare their own unit as well; read in the wrong one, a board comes out 25.4 times too large or too small.
Grid and rounding traps
The dangerous conversions are the ones that look round. A 0.5 mm pitch is 19.685 mil, not 20 mil; 20 mil is 0.508 mm. The 0.008 mm difference is invisible on one pad, but it accumulates: draw a 40-pin 0.5 mm connector at a 20 mil pitch and the error between the first pin and the last is 39 × 0.008 = 0.312 mm, more than half a pitch, so the end pins no longer line up with their pads. A 0.65 mm pitch is 25.591 mil. Entered as 25.6 mil it is off by 0.24 µm per pin, which no process can see. Rounded to 25 mil it is 0.015 mm short per pin, and 0.195 mm short across the 14 leads on one side of a TSSOP-28.
Grids set the same trap. Parts at 50 and 100 mil are exact in both systems (1.27 and 2.54 mm), so they can be entered either way. A 0.5, 0.65 or 0.8 mm part does not land on a 5 mil (0.127 mm) grid, and snapping its pads to one moves them. Draw each footprint in the unit its datasheet uses, enter the pitch exactly, and place the part by its origin instead of dragging pads onto the board grid. Convert for reading, not for re-entering.
Copper weight, and why trace current capacity depends on it
Copper thickness is the one PCB dimension given as a weight: ounces of copper per square foot of board. 1 oz/ft² is nominally 34.8 µm thick, which is 0.0348 mm or 1.37 mil, and thickness scales in proportion, so 2 oz is 69.6 µm (2.74 mil). Many tables round 1 oz to 35 µm, which is 1.378 mil; both describe the same foil. These are nominal figures: IPC standards allow the finished copper to come in below nominal after processing, so ask your fab what finished thickness it guarantees if the number matters.
It matters for current. A trace heats according to its cross-section, width times thickness, so the same current needs half the width in 2 oz copper that it needs in 1 oz. The PCB trace width calculator takes copper weight as an input and returns the width for a given current and temperature rise. The 11.8 mil this page opens with is its answer for 1 A on an outer layer of 1 oz copper with a 10 °C rise, one reason that odd-looking number turns up so often; it is also 0.3 mm to within 0.28 µm. Heavier copper is harder to etch finely too, so a fab's minimum trace and space go up with copper weight, and the capability sheet lists them per weight.
Frequently asked questions
How many mm is 1 mil?
1 mil is exactly 0.0254 mm, or 25.4 µm. A mil is one thousandth of an inch (0.001 in), and the inch is defined as exactly 25.4 mm, so there is no rounding anywhere in the conversion. Going the other way, 1 mm is 39.3701 mil. Multiply mils by 0.0254 to get millimetres, and divide millimetres by 0.0254 to get mils.
What is 11.8 mil in mm?
11.8 mil is 0.29972 mm, which is 299.72 µm or 0.0118 inch. It is the imperial spelling of 0.3 mm: 0.3 mm is 11.811 mil, and the two differ by 0.28 µm, far less than any PCB process can resolve. In a metric layout tool enter it as 0.3 mm; the nearest standard metric drill is also 0.3 mm.
Is a mil the same as a millimetre?
No. A mil is a thousandth of an inch, also called a thou, and equals 0.0254 mm. A millimetre is 39.3701 mil, so confusing the two is an error of about 39 times. It is the classic PCB mix-up, helped by the fact that in everyday speech in some countries mil is short for millimetre: a 6 mil trace is 0.1524 mm wide, while a 6 mm trace is 236.22 mil. Write the unit next to every number on a fab drawing.
How thick is 1 oz copper in mm and mils?
1 oz copper is nominally 34.8 µm thick, which is 0.0348 mm or 1.37 mil. The name means one ounce of copper spread over one square foot, and thickness scales with weight: 0.5 oz is 17.4 µm (0.685 mil) and 2 oz is 69.6 µm (2.74 mil). These are nominal values. IPC standards allow the finished copper to come in below nominal after processing, and outer layers gain plating on top of the base foil, so ask your fab what finished thickness it guarantees.
What is 6 mil trace width in mm?
A 6 mil trace is 0.1524 mm wide (152.4 µm). 6 mil trace and space is a typical standard capability: many fabs go down to 5 mil (0.127 mm) on a standard service, and to 3 or 4 mil (0.0762 or 0.1016 mm) on an advanced one. Those ranges are typical, not a rule, so check your fab's capability sheet before routing at its minimum. Metric-first tools often carry the same rule as 0.15 mm, which is 5.9055 mil.
If the board is the product
A unit conversion is a ten-second job. Choosing the trace, space, via and copper rules a board can be built to, at a price that fits the product, is not, and that is our day job: PCB design and layout explains how we work and what a board costs in engineering hours. For the numbers behind the rules, the PCB trace width calculator, the microstrip impedance calculator and the PCB cost calculator are next door. If a number here decides something real, a free 30-minute technical call is the fastest way to check it against someone who has shipped the board.