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How to Convert Miles to Kilometres? What about cm and mm?

Updated: Feb 26

In 1998, NASA launched the Mars Climate Orbiter in search of evidence of water and to piece together Mars’ history. But, the spacecraft never even started its mission. Less than a year after launch, the spacecraft drifted off course and and hurtled into the Martian atmosphere, causing the spacecraft to disintegrate. Why? The engineers made errors with converting between the imperial system of measurement and the more standard metric system. This article gives you a quick and easy method to convert between imperial unit of miles and the metric units of kilometres, metres, centimetres and millimetres. After all, nobody wants to blow up their spacecraft.


An artist’s rendering of the Mars Climate Orbiter

NASA Jet Propulsion Laboratory via Wikimedia Commons


If somebody asks how tall you are, they might be expecting an answer in feet and inches. Driving around in the car, the road signs you pass give the distances in miles and the speed limits in miles per hour. These are the imperial units of measurement. If you keep driving south, through the channel tunnel and pop out in France, you’ll see all the road signs are suddenly in kilometres. This is the metric system. As scientists, we have to be able to discuss our ideas with people from all over the world and so it’s essential to use a standardised system of measurement. When it comes to measuring distances, the metre, and its bigger cousin the kilometre, is the unit of choice.



Miles and Kilometres


A mile is slightly longer than a kilometre with there being approximately 1.609 kilometres in a mile. Next time you pass a road sign, try multiplying the distance by 1.609 to convert the sign into kilometres. This is the way the sign would be written in most other countries in the world. It’s also the way that scientists would usually talk about the distance.


If you want to go the other way and take something that is written in kilometres and turn it into miles, just reverse the process and divide by 1.609. The band The Proclaimers walked 804.5 km but that’s not very catchy. Divide 804.5 by 1.609 and it turns out they walked 500 miles. Now that’s songwriting material!


Signs pointing in different directions

Gilles Rolland-Monet via Unsplash



Metres and Kilometres


The handy thing about the metric system is how easy it is to talk about tiny little objects and vast distances, all using the same type of unit. The metre is your bog-standard, pick-it-up-off-the-shelf sort of unit. But if you want to talk about something huge, just stick the word kilo in there and you’ve got a nifty new unit. So you might say that it’s about two thousand metres from Edinburgh castle to the Scottish Parliament, or you could just say that it’s two kilometres. You can do this because kilo means a thousand in Greek. If you want an even quicker way of writing it, you could just write km which stands for kilometres.


Edinburgh castle

Photo by Jörg Angeli on Unsplash


Millimetres and Centimetres


You can go the other way too. If something is a thousand times smaller than a metre, you can use millimetres instead (with the symbol mm). Milli means a thousand in Latin. Think about a millennium having a thousand years or a millipede looking like it might have a thousand legs. Millimetres can be really useful for talking about tiny things. For example, its much easier to say that a 5p coin is 1.7 mm thick than it is to say 0.0017 metres!


Now if the metre is a bit too big and the millimetre is slightly too small, then the centimetre might just be your Goldilocks unit! This measurement is 100 times smaller than a metre because centi means one hundred in Latin. This time you can think about a century with its 100 years or perhaps a centipede having 100 legs. It’s also good to know that there are ten millimetres in every centimetre. So, if you have a length in centimetres and want it to be in millimetres, just multiply by 10. On the other hand, if you want to find out how many centimetres something is and you already know the length in millimetres, just divide by 10. That means that the 5p coin we mentioned earlier is 0.17 cm thick.


Coins in a jar

Nick Fewings via Unsplash



Scientific Notation


If you think this has been easy-peasy so far, we can take it up a notch by using scientific notation, also known as standard form. If you have never seen this before, then don’t worry about this section but if you are used to using scientific notation then it’s useful to know that kilo means × 10³, centi means × 10⁻² and milli means × 10⁻³. For example, a plane that is flying at an altitude of 9 × 10³ metres would be 9 km above the Earth.


Cockpit above a city

Austin Neill via Unsplash


Top Tips

All these units can be confusing at first, but once you get the hang of it, it’s really straightforward. You can easily practice this skill by finding the length of something, using the calculation methods in this article to convert to a different unit and then searching for an online unit converter. These online tools are great for checking your answer but remember you won’t have access to these during school tests. Plus, it’s much faster to work it out yourself once you know how.


To finish with, here’s a top tip: if you’re ever doing a calculation that uses distances and you get it wrong and don't know why, check the units. Maybe you’re trying to work out the speed of something in miles per hour, but you’re using the number of kilometres in your calculation. You would need to do a quick conversion to miles first. I’m sure the Jet Propulsion Lab at NASA wish they had checked their units before sending them to their spacecraft.


Written by Ishbel Dyke

Ishbel Dyke is a third year chemistry student at the University of Edinburgh and the News Editor at EuSci Magazine. She hopes to pursue a career in Science Communication.



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