Here are some tips.

If you're not sure, go to a nearby Jeweller, and they can measure your ring size accurately for free.
To determine your ring size at home: measure inside diameter of a ring. Check our size chart for accurate measurements. Compare to find perfect size, ensuring a great fit.
Use a strip of paper and wrap it around your ring finger. Make sure it fits over the knuckle.
Use a pen to mark where the paper overlaps.
Measure the length of the paper from the start to the pen mark using a ruler with mm measurement.
Use the chart below to find your correct size. (Circumference)

When in doubt, it's better to order a slightly smaller ring. If it turns out to be too small, it's easier and cheaper to get it resized by a local jeweller.
If the ring is too large, resizing it is not as simple and will probably be more expensive than making it bigger. Some of the metal band has to be cut and soldered again. Most gemstones have to be taken out during this process, and sometimes it's impossible to remove them.