Method 1:

Do you currently have a ring that fits the finger you are buying a ring for? If so we can get the size from that with a simple photograph like the photos below (with this type of photograph we are looking for the internal circumference of the ring

We will use that information to see where your finger lines up on the following chart (this method hasn't failed us yet)

Method 2:

If you do not have a ring that fits the intended finger ... DON'T Guess! The best thing to do is visit a local jewelry store and simply ask for help, they are usually very happy to help.

Method 3:

If you are nowhere close to a jewelry store the next best thing is to order a ring size kit from Amazon (at the time we're writing this the average price was $6 - so no worries. 

 ... also:

If the ring you are ordering is 1/2 inch or more wide on the palm side, we may want to add a 1/4 or 1/2 size to your measurement. Ordering a larger size allows the ring to fit over the knuckle.