#1 Look in her jewellery box
If your girl already wears rings on a daily basis, this makes your job a whole lot easier - simply "borrow" one of her rings and bring it to the jeweller for a day (and pray she doesn't discover that it's missing.) Alternatively, you can trace the outer and inner rims of the ring on paper, or press it into Blu Tack or clay.
Just be sure you're swiping one with a size that's closest to the ring finger on her left hand - the optimal choice would be same finger on her right hand, but do note that her dominant hand can be larger than the other. When in doubt, buy a size up - it's easier to resize a bigger ring than a smaller one.
#2 Recruit her friends
We're sure they'd be more than happy to be in on the secret! Three ways they can do it, if they don't already know:
- If one (or better, two!) already has an engagement ring, she can bring up the topic of rings, then suggest swapping and trying each others' on. That way, they can roughly gauge how big your girlfriend's finger is.
- Get one of them to go ring shopping (for the friend).
- Ask her directly, and say something along the lines of, "Well he's going to propose in the future anyway, so if he does come to me, I can give him your size."
#3 Pretend you're getting a gift
It'll be more convincing if there's a special occasion coming up, like if it's your mother's birthday. When you're shopping for a ring, get her to try it on. From here, you can also get a rough idea of how the size of a stone will look on her finger (too small and it makes her finger look big, or if it's too big for her slender digits).
#4 Go shopping with her
And make it a point to walk by an accessory store, or one of those pop-up stores or bazaars with plenty of costume jewellery rings. Many girls will stop to try on some of them or you can talk her into trying one on. Keep your eyes peeled here; the sizes might be listed on the ring tray. Or simply buy her the ring (They're affordable, and you'll score bonus points! Plus, you can snag it for reference - refer to point 1).
#5 Ask for it yourself
Okay, so this might ruin the surprise a little. But hey, your proposal date, and how you're popping the question is still hush-hush, yes?