Hi, this is a tricky one as these rings are crafted especially for you and as such no room for error. We can send you a little ring size tool but then she will know to expect something. How about you pretend its for a cheap ring and surprise her with this beautiful ring? Failing that, if there is another ring she wears on the same finger, you could take it to a jeweller to measure a size from. If we can help at all, please do not hesitate to contact us. Thanks.