Are you purchasing without her knowledge? This is actually easier than it sounds. Why not download our
'Ring Size Ruler' (PDF format) which you can print out to help you in your quest. You will need to get hold of one of her existing rings.
Alternatively, simply get hold of one of her existing rings and, holding the ring side on, take the INTERNAL diameter measurement (ie the left to right measurement of the hole) and equate it to the chart opposite.
Are you purchasing with her knowledge? This makes the whole process a lot easier. However, it is always worth getting her to double check her size before you place your order.
If she knows about your planned purchase of a diamond ring but doesn't know her ring size, why not download our
'Ring Size Tape' (PDF format) which will help you determine the ring size.