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If they leave a ring lying around, make an imprint of it into some 'Blu Tack', plasticine, clay or even a bar of soap! Then just take this to a jeweller who will be able to take a reliable measurement of the ring size for you.
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Trace the inside of the ring onto a piece of paper. This is not 100% accurate but is more so when you use a ring of a similar width to the one you are planning on buying.
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Put the ring onto one of your fingers, push it down as far as it will go and mark the position with a pen. Then you can either go to a jeweller or use our free ring-sizer.
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Another way would be to ask a mutual friend to find out-perhaps during a shopping trip they could try on some rings and discuss ring-sizes.