km/hour = km/Stunde
Can I say km/S in German to mean km/hour?
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The international standard, at least for the SI system, is m/s and this is the standard followed by the German. But km/h is also frequent, especially in non-scientific contexts. In spoken German we often say "kmh" while meaning Kilometer pro Stunde or Kilometer je Stunde. I've never seen km/S as the SI unit for time is s for seconds but sometimes Stunde is abreviated std or st. But none of these are in use in this context.
So to sum up you can use km/h but not any other form which i have treated here.
Can you do it? Yes, you can! And you already did it.
But language is not only a matter of the sender. It's also a matter of the receiver. So the much more important question is:
Will anybody understand what you mean?
And here the answer definitely is:
Most people will not understand you, and those who understand you will be confused.
Language in all of its features is a big (and not always logical) collection of general agreements, and one of this agreement is about the units of speed.
There is a scientific standard which is »meters per second« or »Meter pro Sekunde« in Geman, and you write it as »m/s«. But you rarely use it in daily life.
In daily life you measure speed when you drive a car, and in a car you are not interested in how far you can travel within just one second. You are thinking in minutes and hours. But when you use minutes, then you get distances of about 1000 meters or 1 kilometer, but this gives unwieldy numbers of meters or kilometers.
So you measure the distance that you can travel within one hour. Unit-symbols are abbreviations of latin names, and the hour is »hora« in latin. This is why »h« is the scientific symbol for »hour«.
When you use the metric system, you count kilometers per hours, and the symbol for this unit is everywhere on this planet »km/h«.
If you use the angloamerican system, then you use miles per hour (mph), but here the h is not derived from latin »hora«, but from english »hour«. This is because mph is not a scientific unit, and because it is only used in a small number of countries, where everyone speaks English.
Here are the symbols and names in Latin, English and German:
In the last example you can see, that the unit symbols are not derived from english names, but from latin names.
»pars minuta« is in english »small part«. It means a small part (1/60) of an hour.
»pars minuta secunda« means »2nd small part«. It means that the small part (minute) a second time is divided into even smaller parts.
When you use the unit »km/h« in a German sentence, then you pronounce it in one of this ways:
The last one (Stundenkilometer) is wrong if you measure it by means of science, but language sometimes doesn't care about scientific rules. Language is a collection of common agreements, and many people agree, that Stundenkilometer is a valid term for km/h, so they use it. (Some other people disagree and don't use it, but they still understand it. Some of them also begin a discussion and tell you that it's scientifically wrong, but they still understand it.)
You can not use km/S for kilometres per hour in German in no context.
The abbreviations for time durations are:
Sekunde: s, Sek.
Minute: min, Min.
Stunde: h, Std.
Tag: d, –/–
Jahr: a, –/–
There is always one scientific one which agrees to the worldwide scientific standard (even if the SI units only contain the second as a defined unit). These are taught at school and especially those for second, minute and hour are in common usage. There are also commonly used abbreviations of the German words. They are sometimes also used. Note how there is no abbreviation
S for hour. It would confuse people as it is not expected.
Now you could always say ‘okay, then I’ll just define it at the beginning of my text and it’s good.’ Well yes, you could, but bear in mind that km/S is very, very similar to km/s. I’ll admit, the latter is probably not used much outside astronomic contexts (and I do have a slight interest in astronomy, so I probably come across it more often) but it just suggests itself so much more to anybody reading the sequence of letters. And 1 km/s is about 80,000 km/h, you really don’t want to mix them up.
So do yourself a favour and stick with the established abbreviations, km/h, Kilometer pro Stunde or Stundenkilometer (the last one only in common speech/nonscientific texts).