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Reading a datasheet that mixes mil and mm

A fab's design rules come in mm, a component footprint in mil, and copper weight in oz. Three unit systems on one drawing is an ordinary day in PCB design, and rounding at the wrong point means a rule violation or a board that won't take the part.

Last updated: 2026-10-28 Unit conversionmilPCB designFootprints

A mil is 1/1000 inch

A mil is 1/1000 inch: 1mil = 0.0254mm, and 1mm = 39.37mil. Long used in US PCB design, it survives in CAD defaults, older footprints, and US manufacturers' datasheets.

"Mil" and "millimeter" are easily confused in speech, a 40x error, so confirm which one is meant. Micrometers (um) get mixed in too: since 1mil = 25.4um, a 35um copper foil is about 1.4mil.

Common mil-to-mm figures in PCB design
milmmWhere it shows up
3mil0.076mmnear the limit for fine trace/clearance
4mil0.102mmthe "4/4 rule" width/clearance
5mil0.127mmcommon minimum width/clearance
6mil0.152mmsame, with margin added
8mil0.203mmtypical general signal trace width
10mil0.254mmpower traces, mask opening margin
12mil0.305mmsmall-via drill diameter (0.3mm)
20mil0.508mm0.5mm-pitch QFP/BGA
50mil1.27mmSOIC pin pitch
100mil2.54mmDIP/header pin pitch
A drawing and measuring tools using different units
A drawing and measuring tools using different units. AI-generated conceptual illustration; see the article’s diagrams and text for circuit details and conditions.

Is it fine to round "4mil = 0.1mm"?

4mil is exactly 0.1016mm. If a fab's minimum trace width is "0.1mm" and you draw at 4mil, 0.1016mm ≥ 0.1mm meets the rule. If the fab says "4mil" and you draw at 0.1mm, 0.1mm < 0.1016mm is 1.6um short, a marginal violation.

A fab won't mind 1.6um, but a DRC (design rule check) judges mechanically: with a CAD rule of 4mil, a trace drawn at 0.1mm is an error.

The fix is to enter the fab's rule into CAD in the unit the fab gave it in. Convert and round it yourself and you drift off in one direction or the other.

5mil is 0.127mm. A fab table reading "0.127mm" is mil-based underneath; "0.125mm" is mm-based.

Rounding 4mil to 0.1mmTwo cases with the trace width greatly stretched. With a 0.1mm fab rule, a 4mil (0.1016mm) trace passes; with a 4mil fab rule, a 0.1mm trace falls 1.6um short. Rounding the fab's unit drifts you off in one direction or the other 4mil = 0.1016mm. The scale is stretched far out of proportion Fab rule 0.1mm, trace drawn at 4mil Min 0.1mm 4mil = 0.1016mm 0.1016 ≥ 0.1: passes Rule met Fab rule 4mil, trace drawn at 0.1mm Min 4mil (0.1016mm) 0.1mm 0.1 < 0.1016: violation DRC flags an error 1.6um short Enter the rule into CAD in the fab's own unit. Converting and rounding drifts it one way.
Figure 1: a 4mil trace (0.1016mm) meets a 0.1mm rule, but a 0.1mm trace falls 1.6um short of a 4mil rule.

Which unit system does a component's pitch belong to

An SOIC's 1.27mm pitch is 50mil and a DIP's 2.54mm is 100mil: inch-family parts. QFP pitches of 0.5mm, 0.65mm, 0.8mm and BGA pitches of 0.8mm, 1.0mm are metric, and convert to awkward fractions like 19.69mil or 25.59mil.

The trouble comes when you convert a pitch, round it, and use it on a part with many pins. Treat 0.5mm as 20mil (0.508mm) and each pin drifts 0.008mm, so a row of 50 pins is off by about 0.4mm at the end. Against a 0.25mm pin width, 0.4mm puts the land completely off the pin.

Build the footprint in the component drawing's unit system and specify the exact pitch through your CAD's pin-array feature. For a reused library part, check the accumulated dimension from the origin.

  • Grid. Inch-family parts on a 0.635mm (25mil) grid and metric parts on a 0.5mm grid never share a grid. Use a fine routing grid (0.05mm or 0.025mm).
  • Hole size. Drills come in 0.05mm steps (0.3, 0.35, 0.4...) or inch number drills (#60 = 1.016mm, etc.); an awkward size is swapped for the nearest one.
  • Board thickness. 1.6mm is 0.063inch (63mil), 0.8mm is 31mil, 1.0mm is 39mil. US card-edge connectors are built for 0.062inch, which 1.6mm matches closely enough.
Accumulated land offset from rounding a pitchComparison across 25 pins between lands placed at the exact 0.5mm pitch and lands placed at a pitch rounded to 20mil. The rounded row drifts further off the pin toward the ends. Rounding a converted pitch builds up offset toward the ends A 0.5mm-pitch part drawn at 20mil (0.508mm). Offset exaggerated 5x Lands at the exact 0.5mm pitch Lands rounded to 20mil (0.508mm) 0.19mm by pin 25, 0.4mm by pin 50 Gray: component pin. Outline: PCB land The land offset (0.4mm) exceeds the pin width (0.25mm) of a 0.5mm-pitch part. Build footprints in the component drawing's own unit system, and place pitch at its exact value rather than by accumulation.
Figure 2: treating a 0.5mm pitch as 20mil (0.508mm) accumulates 0.008mm per pin, reaching 0.19mm by pin 25 and 0.4mm by pin 50 (offset exaggerated 5x).

oz expresses thickness as a weight

Copper weight in oz is the weight of copper per square foot. 1oz (28.35g) spread over one square foot (929cm²) is 34.8um thick, so 1oz ≈ 35um ≈ 1.4mil; 2oz is 70um, 0.5oz is 18um.

Outer-layer copper gets thicker from plating: 1oz foil plus 20–25um of through-hole plating finishes at 50–60um. Use the fab's "finished copper thickness" for current-capacity and trace-width calculations. Inner layers have no plating, so 1oz stays 35um.

Cross-section of 1oz copper (outer and inner layers)Two cross-sections of 35um (1oz) foil. On the outer layer, 20–25um of through-hole plating is added for a finished 50–60um; the inner layer has no plating and stays 35um. The outer layer thickens with plating; the inner layer stays at foil thickness Starting from 1oz foil (thickness drawn to a common scale) Outer layer 1oz foil: 35um Plating 20–25um Inner layer 1oz foil: 35um Board (dielectric) Board (dielectric) Finished 50–60um Use for current and width calcs Stays 35um
Figure 3: a 1oz outer layer gains 20–25um of plating and finishes at 50–60um. An inner layer stays 35um.

Practical rules for avoiding the mix-up

  • Set the CAD display unit to mm, but enter each rule in the unit it arrived in.
  • When writing a converted value on a drawing, keep the original alongside it, such as "1.27 (50mil)", so the next reader can see where the rounding came from.
  • Specify to the fab in mm, and stay off values right at the rule limit. Against a 0.1mm minimum, drawing at 0.12mm or 0.15mm passes whether the rule is read in mil or mm.
  • Ask for copper thickness as a "finished" figure.

Frequently asked questions

How many mm is 1mil?
0.0254mm; 1mm is 39.37mil.
Can I design at 4mil against a fab's "0.1mm minimum trace width"?
Yes. 4mil = 0.1016mm satisfies a 0.1mm rule. If the fab states 4mil and you draw at 0.1mm, you fall 1.6um short and get a DRC error.
What happens if I draw a 0.5mm-pitch part using mil?
0.5mm is 19.685mil, and rounding to 20mil offsets each pin by 0.008mm, reaching 0.4mm across 50 pins, enough to put lands off the pins. Build the footprint in the component drawing's unit, mm here.
I calculated trace width from 1oz copper and the resistance does not match the real board.
Outer layers finish at 50–60um, so the real resistance is 30–40% lower than a calculation on bare foil. Recalculate with the fab's finished copper thickness.

Standards and references

  • IPC-2221B — General PCB design standard: conductor width and spacing requirements
  • IPC-7351B — Land pattern standard for surface-mount components (metric)
  • JEDEC MS-012 / MO-153, and others — Package outline drawings for SOIC, TSSOP, etc.; pitch definitions
  • IPC-4562 — Specification for copper foil for printed boards: oz-to-thickness correspondence

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