Most knitting patterns are pretty clear when it comes to telling you what size knitting needles you will need. That makes sense, because the designer wants you to be able to successfully recreate the project.
Sometimes, however, when you get a pattern from a country other than the one you live in, you might find a number that doesn't make a lot of sense to you. Instead of seeing US size 9, for example, you might see 5.5 mm, or even British size 5 (though the British non-metric sizes are most likely to be found on vintage patterns, since the metric measures have been used for the past 30 years or so).
If a pattern includes all of these numbers you've got no problem, but if the only number listed is one you aren't familiar with, you need a needle conversion chart.
Here is a list of the most common sizes of knitting needles in metric, UK and US measurements. If you find a pattern with a number you don't understand, consult this list and you'll be fine.
Why Does Size Matter?
The size of the needle affects how big your stitches, and thus your finished product, will be. The concept of gauge, or how many stitches fit into an inch of knitting, relies heavily on the size of the needles. In fact, if your gauge doesn't match what the pattern calls for, the way to fix it is to change the size of your needles.
The world would be a simpler place if there were a standard for knitting needle sizes, but in fact there are three. The British and American versions are basically opposites, with the American system starting with low numbers for needles with smaller diameters and working up to larger numbers for larger diameters, while the British system starts with high numbers for low diameters and low numbers for high diameters.
The metric measurements indicate the diameter of the needle in millimeters. The only place the American and British numbering systems agree is at 4.5 mm; both countries call that a size 7.
If you don't think size matters, try knitting a swatch on a size 10 American set of needles and a size 10 British set. You'll get two quite different results!
Knitting Needle Conversion Chart
Source: Yarn Standards