If you’re planning on popping the big question this Valentine’s Day – make sure you get the right size.

You want your proposal to come as a surprise, yet you also want the carefully chosen engagement ring to fit. So what do you do?

If you’ve picked out the perfect band and planned the most romantic proposal, you don’t want to blow it by your new fiancée, or fiancé, being unable to wear the ring because it’s too big or too small.

To avoid catastrophe, this is one occasion where it’s actually okay to go behind your other half’s back.

Here are 3 sneaky tricks to help you size up your loved one’s hands.

How to size an engagement ring

Borrow another ring

One way is to borrow a spare ring from your other half’s jewellery box and use that as a guide.

Obviously, fingers aren’t all the same size so if you get a ring which fits their middle finger, you will likely need to size down for it to fit their ring finger.

The dominant hand also tends to be slightly bigger so if you borrow a ring they wear on their right hand you will need to size up or down accordingly – so if they are righthanded, size down, if they are lefthanded, size up.

Create a cover story

If you can come up with a scenario where they actually put a ring on their engagement finger you could end up with an accurate idea of their ring size.

However, it may also mean you need to enlist the help of a trusted third party so make sure it’s definitely someone who won’t spill the beans.

As an example, get someone wearing a special piece of hand candy, such as their own engagement ring or a family heirloom and get them to suggest your other half tries it on to see what it looks like on them – specifically on their engagement finger.

Measure while sleeping

This will take some guts – and a steady hand.

When they are deeply asleep, have a ring measuring guide at the ready and very carefully get an accurate measurement.

If you don’t have a ring measuring guide, use a blank piece of paper, or a piece of string, wrap it around the correct finger and make a mark where the two ends meet. You can then measure this to find out the ring circumference.

Top tip: It is easier to size down rather than up so if in doubt, go for the larger size.

UK ring sizes

Categorized from A to Z, UK ring diameters range from 12.04mm to 21.79mm with circumferences from 37.8mm to 68.5mm.

Most jewellers will have a ring size guide on their website or you could enlist their help in store too.