Converting Inches to Milimeters

From

http://www.ex.ac.uk/cimt/dictunit/dictunit.htm#length

It exactly 25.4

or inch dimension / divided by .03937 = millimeters

and inch dimension *times 25.4= millimeters

Take a look at this for more info from NIST. It appears that 1ft = .3048m => 1ft/12 = 0.0254m. 1ft = (1200/3937)m => 1ft/12 = 0.0254000508m.

ft: International foot

ft: US Survey foot

1 inch equals 25.399999999988697000000005029835 mm

Personally I think 25.4 is close enough.

The meter has a universally accepted definition. The Inch is a bit more flexible.

Just out of curiosity , is the 'little program' self build ? I use my own routine because Excel bails out with big numbers (and I don't trust it anyway).

no....not my own.

I'm not that smart...and I'm just lazy.

There is a free outstanding program at:

http://joshmadison.com/

It converts loads of units.

I use it every day.

looks something like this:

This conversion is wrong so what is the use of a site that can't even get this simple calculation right?

To be fair to Convert [FreeWare] -

If one asks Inches to mm (as the original question was and not mm to Inches)

it gives 25.4

Thanks Capt Psycho , I'll check it out .

There is also a convenient unit converter called YAUC (for Yet Another Unit Converter) available for Palms.

Dude, what's an inch?

Nice one!

It's exactly 25.4. That's how the (international) inch is defined.

Inches to Millimeters = Multiply by 25.4

Milllimeters to Inches = Multiply by .03937

Good luck,

Deborah

In South Africa 1" = 25.4mm by definition.

The meter is defined as the length of a wave for a specific frequency while the international ft is defined as the distance between 2 markings on a piece of material kept at some institute. (or at least was a few years ago).

We seldom have to uses accurate conversions, even precision parts are made to tolerances exceeding the real conversion error.

Where real accuracy is required conversions will not be used.

Sometimes accurate conversions are absurd.. In 1961 when we changed to the metric system a reporter at an accident scene interviewed a witness and submitted a report stating something like "the 220lb victim landed about 20ft from the point of impact". The editor applied 6 to 10 decimal places conversion factors and the following were published "The 100.003456 kg victim landed about 6.0002345123 m from the point of impact" (I cant remember the exact factors used)

VIVA METRIC

I believe in going metric, every inch of the way!

does bounce to the ounce become wham to the gram?

That'll never stand up in court.

Good Point! If your Inch is in South Africa and your Millimetre is on the North Pole (santa's Toy Factory) then 1" will not Be 25.4mm!

It is 25.3807 mm it gets rounded up for ease of conversion.

This site gives 2 different values that it says are both correct, and gives the reason why! go figure. http://www.wsdot.wa.gov/reference/metrics/factors.htm

US Congress defined an inch 39.37th part of a meter in the 19th century; Canada defined it as 25.40000 mm. Uk had a master in their Board of Trade office, which defined a yard (I believe); in UK 1 inch equals 25.399999999988697000000005029835 mm.

The difference did not matter in the 19th century, when manufacturing & measurement errors were higher than the difference. After the I World WAR the manufacturers of inch slip gauges marked the definition on the box.

I believe, ISO have defined an inch as 25.4000000 mm

Yeah, and I recall a time when the Indiana (USA) state government debated a law that the value of pi would be rounded to 3 to make calculations easier.

Maybe somebody from France knows the exact conversion, they invented the metric system....

Wasn't that a deliberate act to show that the legislator was an ass?

I think this whole thing is one big conspiracy by the Brits after we put the hurt on them back in 76'. They gave us the foot knowing that in future generations some fool would question the size difference between a woman's foot and a mans foot and soon there wold develop friction between the sexes over proper reporting. That is a woman reporter would report an accident as occurring 60wf (60 women feet) from some point and a man reporter would say 40 mf. Well soon this would get serious and boil over to the origin of the inch (from a man's digit) and we wold develop the wi and mi. Soon some fool would start in on the mile - and well now you can see why the Brits quietly went metric and avoided this future conflict!!!

Moral - use 25.4 or create another world problem!!!

Criky! I thought I was a professional cynic!

http://www.straightdope.com/mailbag/mmetric.html http://en.wikipedia.org/wiki/Foot_(unit_of_length) These explain the origins of metres and feet respectively. You can figure it out from there.

For all you supposed purist out there that keep quoting Wikipedia- go back and read the disclaimers-Wikipedia is a glorified blog that is only as good as the people posting on it, so again don't believe everything you read on the Internet including this response

True, Wikipedia isn't always accurate, but most of the posters are serious in their efforts and it's always good fun, if nothing else!

On a tangential topic, does anybody know why the US still uses the English system? Beyond the cost of the changeover I'm not sure what the issues are.

The USA congress passed a law that one inch equals 25.4 mm!

The definition of the inch has varied over time and from country to country. At one time the inch was different in Australia and Canada (where it was defined as 25.4mm) from the inch in the UK and different again from the US inch. The difference made little difference for most day to day work but for very accurate work the differences mattered.

The inch is now DEFINED as 25.4mm in all countries.

[The back-conversion 39.37 inch ≈ one metre is an approximation. To a better approximation, one metre ≈ 39.370078 74inch = 1000/25.4]

Greg

Greg , this may seem pedantic , but ;

The 25.4 mm Inch was agreed in 1958 by the USA and Commonwealth Countries.

Anything made or drawn before (or arguably after ) that date can not be assumed as 1 Inch = 25.4 mm . I can't think of a cover-all maxim , except to suggest we are all mindful of the inch having historically different meaning . eg if you were asked to supply a replacement cylinder for a vintage engine and were given dimensions in Inches , you'd probably want clarification in metric .

Kris

Kris,

That depends. Here (in my automotive machine shop) all of our equipment is in inches. We just convert mm to Inches down to 1/10's of a thousand, usually rounding up the 1/10's 1 point. Example: 88mm bore = 3.464566929134 but we just round it to 3.4646 inches which = 88.00084 mm.

Close enough for the engine blocks we work on. :)

Ken

Thanks Ken - I take your point . It's unlikely that the % error in a conversion would amout to much significant in the real world. I was grasping for an example where somebody might look at an old technical drawing and convert dimensions that gave a significant error. I couldn't think of a real life case ( though I suspect it must have happened to someone at sometime ). Maybe someone tried to copy an old dimensioned drawing for a clock mechanism by converting some old type of Inch and got messed up ! It would be interesting to know if something along those lines has happened since metric became internationally used. Maybe an astronomer my know a case (unless Inches etc is still used a lot. As you point out , plenty of machine shops are tooled for Inches still ). Cheers , Kris

I had several professors who would flunk you if you dared to give an answer using more than a couple of significant decimal points -- especially if the most accurate number in a calculation was less than the answer's number of significant decimal points!

Why would you need so many decimal points accuracy? Are you converting the length of a highway from miles (as in 102.0139876534 miles) to millimeters?

Good point.

That must be why our wheels roll 1.3% further.

The standard used by U. S. Surveyors is held to be exactly 25.4 mm = 1", and 1 meter = approx. 3.2808 U. S. Survey feet. Do not confuse with S.I. or the international foot.

Always use 25.4 when converting. NEVER multiply 0.0393 to get to inches. Always divide by 25.4. Try and see the different. I've rejected parts due to such rounding errors. Any calculator will do division easily and correctly so using the exact 25.4 will always give you the right answer.

Pineapple

For official information check the following links:

http://ts.nist.gov/WeightsAndMeasures/upload/AppendC_07_HB44_Final.pdf

http://physics.nist.gov/Pubs/SP811/appenB8.html#I

http://ts.nist.gov/WeightsAndMeasures/Metric/pub814.cfm

As seen these links are part of our national metrology institute, we all know as NIST, National Institute of Standards and Technology, the single national authority in the field of measurement science. We need to remember that the "inch" value is defined using metric measurement units, simply because the metric system, today called SI, is the only one in existence worlwide. So even two countries in the world (USA is one), still using the inch-pound system, the traceability of this system units is defined using metric units, so practically SI is unique and global.

As for the number of decimals, we need to stop at the decimal for which we are able to measure; anything behind that does not have any metrological sense. If your micrometer smallest value of division is 0.001 inch you should stop at the 3rd digit with your reading, if is 0.01 mm you should stop at the second digit. In one instance, when the measurement uncertainty value is calculated, the number of digits is one rank higher than the value of division of the measuring device.

Handy links, good post.

..."If your micrometer smallest value of division is 0.001 inch you should stop at the 3rd digit with your reading"...

That is just about funny. That isn't the way it works in the "real" world where us people using mics everyday in our job make decisions on sizes. Without making "educated" guesses by many, many, many people throughout the ages, well, geeeez, the USA would probably still be in the horse and buggy days.

Anyone (after a little practice on seeing what they are looking at) should be able to come within a 1/10th on a mic that reads to .001, just by looking where the line is in relation to the next thousandths mark.

.001 difference is a BIG number at my automotive machine shop. Most of our measurements require 1/10ths. Full floating wrist pins for example need to be 1.5-2/10ths on most automotive piston/pin clearances, pin = .9375 thou, then the hole can only be .9375², because if it is .9375⁵ it will not give the customer a quality job and won't last long because of to much clearance, causing rapid wear.

Even though my employees can use a .001 mic and come really close on the 1/10ths, I don't make them do that, they get to use 1/10ths mics (.000¹) and the rod machine has a $1,500.00 1/10ths mic on it so they can turn out quality work.

Thanks for making me smile. Have a good evening Metrolog.

Ken

How about this. In a tunnel under construction you monitor convergence with a tape extensometer. Tunnel is , say , 10 m across. You have seconds to get a reading before some truck or train squashes you or the tape. The dial gauge attached can read to .001 cm. It's possible to read consistently to the nearest .01 cm accurately. All down to 'feel' and practice. You can get a convergence plot as smooth as a baby's bum (with readings taken several times a day by different crew). Not bad considering the temperature variation, dust, lighting, speed etc.

The length of the foot should change to the size of the current president of the USAs' foot :) It would open up a whole new world of speculative trading..

Buy length when the foot is large and sell when it is short :)

PS: spell checker corrected the title.