The prefixes in the SI system are intended to create appropriately-sized units so the number is easily read and understood. For instance, consider the value
1234567890 wattsEven if spaces (or commas) are put into the actual number it is still not easy to grasp its size at a glance. How much better to see it as
1.234567890 gigawattsor, more sensibly, to 4 significant figures (and using symbols) as
1.235 GWThe main purpose of this calculator is for when a figure is given for a unit with one prefix in place, and it is desired to see if another prefix might not be more appropriate. Once the given number is keyed in, and the given prefix is selected, then the suitably adjusted value is shown for all the other prefixes. Also shown (at the bottom) is the value in (bare) units if NO prefix is used at all.
No basic unit may have more than one prefix attached!
For instance MkJ for 'mega+kilo+joules' is wrong. In this case it would have to be gigajoules or GJ.
Take care to distinguish between capital and lower-case symbols.
The general principle is that, if the prefix makes the unit bigger then the symbol for the prefix is a CAPITAL letter, with just three exceptions: kilo[k], hecto[h] and deca[da]. All the other prefixes (which make the unit smaller) are written in lower case. The table above makes this clear. The only 'awkward' one is that for 'micro' which is the Greek letter 'mu'.
Similarly, if the unit is named after a person (like Watt, Newton, Joule etc.) the symbol for the basic unit is a CAPITAL letter (like W, N, J, etc.), but if the unit is written in full then a lower-case letter is used (like 18 watts, fifty-six newtons, 1045 joules, etc.).
All other units (like metre, are, tonne, candela, etc) use lower-case for their symbols (like m, a, t, c, etc.).
Of course there is an exception! The litre. The CGPM (don't ask!) ruled in 1979 that the symbol for the litre could be either l (lower-case ell) OR L (capital ell) to avoid confusion with 1 (one). And that ruling still stands.