Back in my engineering days I learned an easy and accurate enough way to convert from millimeters or centimeters to inches and back. I thought I would share it with the forums. Some may already know but for those that don't it's one easy number to remember.
The magic number is 0.03937
(actual number is 0.03937007874)
Convert millimeters to inches, multiply -> mm x 0.03937 = inches
Convert inches to millimeters, divide -> inches / 0.03937 = mm
To convert to or from centimeters simply
move it over one decimal place -> cm x 0.3937 = inches
and of course the inverse is the same -> inches / 0.3937 = cm
Have fun!
TJ
The magic number is 0.03937
(actual number is 0.03937007874)
Convert millimeters to inches, multiply -> mm x 0.03937 = inches
Convert inches to millimeters, divide -> inches / 0.03937 = mm
To convert to or from centimeters simply
move it over one decimal place -> cm x 0.3937 = inches
and of course the inverse is the same -> inches / 0.3937 = cm
Have fun!
TJ
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I have the means to measure both, thank you. Just thought I'd share for others that might be interested. As you know, in the USA we generally still use inches.
For me, 0.03937 (actual number is 0.03937007874) stuck better than 25.4, for whatever reason. We're all wired differently.
I put it out there, do with it as you wish.
Thanks for your feedback!
TJ
For me, 0.03937 (actual number is 0.03937007874) stuck better than 25.4, for whatever reason. We're all wired differently.
I put it out there, do with it as you wish.
Thanks for your feedback!
TJ
Accurate enough for anything anyone here would use it for! What's 0.00000007874 inches between friends?
TJ
All she'll get, in some bedrooms.
No it's not, it's 0.03937007874015748031496
At least according to my calculator
Still easier to remember 25.4...
I got caught out a while back with a PCBCAD footprint.
I converted mm to inches and then laid down a row of 16 SMD pads.
I hadn't realised at the time but the slight error I had in conversion was multiplied by 16 so by the time it got to the 16th pad it was out by half a pad. Luckily, I only paid a couple of $ for 5 pcb's so it wasn't the end of the world.
Some of the fine pitch SMD like 0.55mm aren't an exact multiple of thou so make lining up metric SMD on a imperial CAD snap system a nightmare.
I had to add a couple of Control key functions to my CAD software to backwards and forwards align to the pads on the SMD component. All worked out good in the end.
I converted mm to inches and then laid down a row of 16 SMD pads.
I hadn't realised at the time but the slight error I had in conversion was multiplied by 16 so by the time it got to the 16th pad it was out by half a pad. Luckily, I only paid a couple of $ for 5 pcb's so it wasn't the end of the world.
Some of the fine pitch SMD like 0.55mm aren't an exact multiple of thou so make lining up metric SMD on a imperial CAD snap system a nightmare.
I had to add a couple of Control key functions to my CAD software to backwards and forwards align to the pads on the SMD component. All worked out good in the end.
Machinists use 0.03937 because micrometers measure to 0.0001 inches. That's enough significant digits. One ten-thousandth of an inch isn't very much, considering that the average human hair is 0.003" diameter. Press fits are usually around 0.0002" - 0.0003". Another irritating term for me is when someone says "mils" meaning millionths of an inch. I ask what they use to measure millionths of an inch.
The USA was in the process of converting to the metric system in the 1980s. Whenever we got a metric drawing, we converted to inches because all our measuring tools were in inches. Many machine tools had both English and metric units.
Remember the highway signs in metric and English units? Doesn't it seem funny that the English converted to the metric system years ago and the USA still uses the English system?
The USA was in the process of converting to the metric system in the 1980s. Whenever we got a metric drawing, we converted to inches because all our measuring tools were in inches. Many machine tools had both English and metric units.
Remember the highway signs in metric and English units? Doesn't it seem funny that the English converted to the metric system years ago and the USA still uses the English system?
Exactly! I got it from my days as an automation controls engineer working with cnc machinery/robotics. I can't see any diy application where one is going to need more than 1/100,000ths of an inch accuracy.
I knew the 25.4 method too but for some reason .03937 stuck with me after all these years and made more sense to me. So I got this crazy idea to share it here thinking it might help someone else some day.
mode back on...TJ
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Sorry but it´s not so.
A "Mil" is a thousandth of an inch or 2.54 hundredths of a mm, or a fourth of a tenth of a mm, large enough to be visible with the naked eye, and an unacceptable error in any mechanical piece which must fit somewhere.
In our daily hobby/profession a thin PCB track is 12 Mil (think "Computer PCBs"), a very thin one may be as narrow as 6 Mil, so 1 Mil error there is gross.
All of this showing that 1 Mil is a very significant and useful unit.
Practical example: this is a Canadian PCB company (justly) bragging about what they can offer; all measurements are in Mils (as in .004 means 4 mils and so on):
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