This calculator translates the colour codes on 4- and 5-band resistors into ohms. The value automatically updates as you select colours.
There are lots of other resistor calculators online. I built mine to require the fewest possible clicks before you get the result, and to fit perfectly on a smartphone screen. If you find it useful, please pass on the link on to your friends.
All resistor ratings have a margin of error (indicated by the last colour band). You need a multimeter to check the exact values of resistors. I use a VC99 multimeter. It's a cheap copy of a Fluke 17B but I've found it to be very reliable. Lots of companies have versions of the VC99 - they're all basically the same thing.
Here's a good introduction: