So you've decided to pop the big question and you want to surprise her with the perfect engagement ring that she'll love forever. But wait... you don't know her finger size! What do you do? Of course it's best if you've visited a few jewelry stores together to look at rings and have had her finger sized, but we get it, you want to surprise her. No worries there are other options.
Depending on the design of the ring it will be more or less important to get the size right. If the ring has a plain shank (the part that goes around the finger) then it should be relatively easy to change the size later. If the shank is covered in diamonds or intricate decoration it makes it more difficult to change the size so it's more important to get close the first time.
Also a wide ring needs to be a slightly larger size than a narrow ring. This is just another part of the equation to factor in when figuring out her finger size. Your jeweler will know what to do.
To get your spouse-to-be's ring size, ask her best girlfriend, or her mother. Trust me, she's been thinking about the design of her engagement ring since before she met you, so she knows her ring size. And I guarantee she's shared all of this with her BFF or her mom, especially if you have been dating for some time. Ask them. They'll enjoy helping you and being in on the surprise. They can even be helpful in planning the surprise if you need some pointers.
You can also sneak a ring out of her jewelry box and have it measured by a reputable jeweler. Of course you need to know which finger she wears that ring on. It's best if she wears it on the ring finger of either hand. As a general rule the right hand ring finger is half a size larger than the left hand ring finger. This will get you a size close enough until she actually tries it on (hopefully accompanied with a resounding 'yes').
My last bit of advice is, when in doubt, go a little large rather than too small. You want to know that when you get down on bended knee to propose that she'll be able to get the ring on her finger. If it slips on too easily that's much better than not being able to get it on at all. If she can't get the ring on she'll be SO disappointed. Not to mention it will make her feel fat and that is the LAST thing you want to do. If the ring is too large she can always wrap tape or yarn around the shank like she did in high school and have the size adjusted later.
Remember, when in doubt, bigger is better. And ladies, try not to be so hard on them if the size isn't perfect. It's the thought that counts, right?