What is a unit?

A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.

What does this mean in practice? That you may measure for example the quantity length of an object with the SI standardised metre (U.S. English metre) to be of magnitude 5 metre. But if you happen to live in Chicago you might more likely use the U.S. custom unit yard and find the object to be 5.468 yards. A Swedish viking from the 10th century would have come to the conclusion that the object was 2.81 famnar and yet other metrologists would use other units.

How can we bring order in this multitude of units that have been and still are around? Are there some standards and when should we use them? Why are the common units chosen to be what they are both in terms of their names and magnitudes. Why is a kilogram just that particular amount of mass? Could there be even better ways to set up a general system of measurement to make the magnitudes of the units less arbitrary?

This web site tries to go into depth of the interesting subject of units.

Convert units

First of all, if you want to convert between different kinds of units, try to find the unit of interest in the menu of unit categories. If you are unsure of what quantity the unit is measuring try to search for it.


Some popular categories are to convert masses and distances.

All units are categorized in what family of measurement system they belong to. We have put effort into adding as much interesting facts as possible about the individual units to give some context to their history and when they are useful to deploy.

Articles on units

If you find yourself interested in the history of units we have a few in depth articles on that covering the history of measurement and the etymology of many of the names of the units. You may guess why a foot is about the size of the lower extremity of the leg below the ankle but why is the abbreviation of the English pound lb and much more?

Units are standardised in the SI system which is used in science, international trade and commerce. Do you know all the SI-prefixes like micro, milli, kilo, mega, giga, tera? There are more SI-prefixes than you might think. Why is the second an SI base unit of the international system, but the volt a derived SI unit? This question opens up even more interesting questions concerning the relationship of the quantities we measure in physics. When analysing the definition of units in terms of more basic units the understanding of the quantities they measure may become more clear as well.

When we are on the topic of physics, great thinkers of our time like Max Planck put a great deal of thought into units and suggested a completely revised system of measurements based on natural units (Planck units). See why this is a good idea and what shortcomings of the SI system could be ridded by defining physical quantities in terms of natural basic constants that eliminates cumbersome arbitrary constants like the speed of light c being equal to the magic number 299 792 458 m/s or the gravitational constant G equaling 6.67384×10-11m3kg-1s-2. If we ever need to discuss engineering with an alien civilization, this is probably the unit system we both would agree on.

Unit cheat sheets

Finally we should mention that we have some handy unit conversions in more colloquial areas like shoe size conversions between European, American and other systems. Make sure to use the food and recipie converters if you are stuck in the kitchen, take a look at the number converters when dribbling between binary, octal and hexadecimal representations of your number systems and much more.


If you find any factual mistakes in the articles or in the conversions please be sure to let us know and if you know of any unrepresented units, we would be very happy to hear from you. The more information we can add about these topics the better.

Welcome to all you need to know about units!