Most knitting patterns are pretty clear when it comes to telling you what size knitting needles you will need. This makes sense because the designer wants you to be able to successfully recreate the project. However, knitting needle sizes can change based on what country they're from.
Many patterns offer both the U.S. and metric equivalent numbers so that there will be no confusion as to which size needle to use.
For example, some patterns say to use a pair of size eight U.S. (five mm) knitting needles, so no matter where you live you can find the right needles to complete the project.
However, where things get confusing for most knitters is that the UK also had its own needle sizes at one time, as did Canada. It's very uncommon to run into patterns with Canadian needle sizes. Currently, British non-metric sizes are most likely to be found on older 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'll need a needle conversion chart.
Metric, American, and British Sizes
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 seven.
Here is a list of the most common sizes of knitting needles in metric, U.S., and U.K. 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 1 inch of knitting, relies heavily on the size of the needles. Usually, larger needles will produce a larger gauge. If your gauge doesn't match what the pattern calls for, the way to fix it is to change the size of your needles. Knitting a test swatch before you begin your project is a good way to check the gauge of the yarn and needle combination you've chosen.