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.
The Many Types of Needle Sizes
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.
This star stitch cuff, to pick an example, says it uses a pair of size 8 U.S. (5 mm) knitting needles, so no matter where you live you can find the right needles to complete the project.
The UK also had its own needle sizes at one time, as did Canada. Currently, 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'll need a needle conversion chart.
Metric, American and British Knitting Needle 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 7.
Knitting Needle Conversion Chart
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.
Source: Yarn Standards
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. If your gauge doesn't match what the pattern calls for, the way to fix it is to change the size of your needles.