Inplico ring sizing kits are the most reliable way to get the correct ring size. The kits are a £5 deposit plus postage (£2 applied at checkout). Inside you will get a blue plastic ring gauge and a ring sizing guide.
You can choose to keep the ring gauge or you can return the ring gauge in its original packaging within 14 days for a full refund (minus the £2 postage costs) so it can be re-used. Simply e-mail firstname.lastname@example.org for return address and details. Once the ring gauge is returned, you will get your £5 refund within 14 days.