There is nothing to calculate here: one mil is exactly one thou. The two are the same unit under two names, so any figure in mils can be rewritten in thou unchanged.
One thousandth of an inch, the standard thickness unit for plastic film and PCB traces. British engineering shorthand for one thousandth of an inch — the same size as the American mil.
The reason both names survive is that they come from different standards bodies and different trades, and each is entrenched in its own literature. If you are reading a data sheet, use whichever unit it uses; if you are writing one, use the unit your audience expects. The number itself never changes.
The formula
mils- Your figure in mils
1- The factor between the two names — exactly one
thou- The same figure, relabelled
How it works, step by step
- Enter your value in mils.
- Nothing is multiplied: the units are identical in size.
- Read the same number off as thou.
- Use whichever name the document you are working from uses.
Worked examples
A published figure in mils
A data sheet quoting 46 mil is quoting 46 thou. No arithmetic is involved, and any tool that returns something other than 46 has confused this with a different quantity.
When the numbers should not match
If a conversion between these two names changes the number, the source has almost certainly mixed up length with a neighbouring quantity. That is worth checking before trusting the figure, because the neighbouring conversion usually needs a material property this one does not.
How to read your score
Frequently asked questions
Is 1 mil the same as 1 thou?
Yes, exactly. They are two names for the same size of unit, not an approximation that happens to be close.
Why are there two names for the same unit?
Different standards traditions. One thousandth of an inch, the standard thickness unit for plastic film and PCB traces. British engineering shorthand for one thousandth of an inch — the same size as the American mil.
Which one should I use?
Match the document you are working from. Standards bodies increasingly prefer thou, but mil remains what most trade literature prints, and switching mid-document is how errors get introduced.
Does the conversion depend on temperature or material?
No. Both units measure length, and the equality between them is a matter of definition rather than measurement.
What is a mil?
One thousandth of an inch, the standard thickness unit for plastic film and PCB traces.
What is a thou?
British engineering shorthand for one thousandth of an inch — the same size as the American mil.
mil and thou at a glance
| Figure | mil | thou |
|---|---|---|
| 0.5 | 0.5 mil | 0.5 thou |
| 1 | 1 mil | 1 thou |
| 2 | 2 mil | 2 thou |
| 3 | 3 mil | 3 thou |
| 5 | 5 mil | 5 thou |
| 10 | 10 mil | 10 thou |
| 20 | 20 mil | 20 thou |
| 25 | 25 mil | 25 thou |
| 50 | 50 mil | 50 thou |
| 75 | 75 mil | 75 thou |
| 100 | 100 mil | 100 thou |
| 250 | 250 mil | 250 thou |
| 500 | 500 mil | 500 thou |
| 1000 | 1000 mil | 1000 thou |
| 5000 | 5000 mil | 5000 thou |
The columns are identical by definition — that is the point of the page.