In Denmark and most of Europe, ring size is given as the ring's inner circumference measured in millimetres. A ring in size 54 therefore has an inner circumference of about 54 mm. This makes the system easy to work with, because the number you measure around your finger is exactly the number you need.

Some measurement charts instead use the ring's inner diameter - that is, the distance across the hole in the ring. The diameter is smaller than the circumference, so it's important to know which of the two you have measured before you look up the size. A ring with an inner diameter of 17.2 mm corresponds, for example, to size 54.

There are several ways to arrive at the right size. The two most commonly used are based on the finger and on a ring that already fits, respectively.

Cut a narrow strip of paper about 5-10 mm wide, or find a piece of string without any elastic. Wrap the strip around the finger the ring is meant to sit on and tighten it so it fits snugly without pinching. Mark with a pen exactly where the strip meets itself. Then straighten the strip out and measure the distance from the end to the mark with a ruler. The number in millimetres is your ring size.

Paper is usually more accurate than string, because string can stretch or twist while you measure. Measure two or three times and use the average - small variations of a single millimetre can shift you half a size.

If you already have a ring that fits well on the right finger, you can measure its inner diameter instead. Place the ring on a table and measure across the hole from inner edge to inner edge with a ruler that has millimetre markings. Measure at the widest point, that is, through the centre, and read it off as precisely as you can.

This method requires you to be able to read millimetres accurately, so use a ruler with clear markings or a calliper if you have one. Afterwards you translate the diameter into ring size using the table further down.

The most accurate method is still to have the finger measured with a set of professional ring sizers - a series of metal rings in fixed sizes that are tried on the finger until one fits properly. If the ring is meant to be a surprise, you can instead bring in one of the recipient's existing rings and have it measured.

If you have measured the ring's inner diameter rather than the finger's circumference, you can use this table to find the corresponding ring size:

If your measurement falls between two values, it's usually best to round up to the nearest size. A ring that sits slightly loose can be adjusted, whereas a ring that can't get over the knuckle can't be used.

Fingers change size throughout the day and over the course of the year, and this can easily create a difference of half to a whole size. Heat causes fingers to expand, while cold causes them to contract. In the morning fingers are often a little swollen, and the same applies after exercise, after salty food or late in the day.

That's why measuring in the late afternoon or in the evening gives the most accurate result, when the fingers are at a normal temperature and are neither cold nor swollen. Measure on different days and see whether you get the same number.

The knuckle also plays a role. If you have a prominent knuckle, the ring has to be able to pass over it, so you should measure both around the widest part of the finger at the knuckle and around the base of the finger itself. Choose a size that can get over the knuckle but won't turn freely once it's in place.

Also remember that the right and left hands are rarely the same. Most people have slightly larger fingers on the hand they use most, so always measure on the finger the ring will actually sit on.

A wide ring takes up more space on the finger than a narrow one and therefore feels tighter in the same size. If you're going for a wide ring, it may be necessary to go half to a whole size up compared to a thin ring.

This applies among others to wide designs such as a bold statement ribbed ring and an oval wide band ring, where the band covers a larger part of the finger. Narrower models such as a tiny eternity ring with a band of around 1 mm, on the other hand, sit closer to your measured size.

The shape of the inner side matters too. A ring with a rounded inner side slides over the knuckle more easily than a ring with a flat inner side, and in practice this can feel like a slight difference in size, even though the number is the same.

If the ring is to be worn together with other rings on the same finger, it's worth trying them on together if possible. Two rings side by side can together feel tighter than each on its own.

An engagement ring with a centre stone often sits a little differently than a plain ring, because the weight of the stone can cause the ring to turn if it's too loose. That's why it's extra important to get the size exactly right when it comes to a solitaire ring such as a round solitaire ring or an oval solitaire ring.

If the ring is meant to be a surprise, the most discreet method is to borrow a ring the recipient already wears on the right finger and have it measured. Alternatively, you can place the ring on a piece of paper, trace the outline of the inner side and measure the diameter afterwards. Be aware that rings from other fingers - for example the index finger or thumb - are typically several sizes larger than the ring for the ring finger.

If you choose both an engagement ring and a wedding ring, the two rings need to sit comfortably together. It's becoming more and more common to choose two rings, and here it's an advantage to think about the combined width from the start. A narrow small half eternity ring or a medium eternity ring takes up less space next to an engagement ring than a wide model.

A ring fits when it can be pushed over the knuckle with light pressure and stays in place without sliding around on its own. It should be possible to turn it with your fingers, but not to rotate freely when the hand hangs down.

When removing it, the ring should offer a little resistance at the knuckle - that's a sign that it won't fall off by accident. If you can pull it off without any resistance at all, it's probably too big. If, on the other hand, you need soap or force to get it off, it's too small.

Try the ring on several times over the course of a day if you're in doubt. If it sits comfortably both morning and evening, you've got it right.

Most simple rings can be adjusted up or down by a jeweller by removing or adding material in the band. How much a ring can be altered, however, depends on the design.

Rings with diamonds all the way around - such as an eternity ring - can rarely be adjusted, because the change would interfere with the settings. The same applies to rings with continuous pavé details or engraved patterns all the way around. A round pavé ring has settings on part of the band, while a large eternity ring has stones all the way around.

If you're in doubt, it's therefore better to spend a little extra time on the measurement from the start than to count on being able to correct it afterwards. This is especially true for rings that are to be worn every day for many years.

Ring size naturally applies only to rings. If you need to find the right size for other jewellery, other measurements are relevant.

For necklaces, the length of the chain itself is measured, and here 42 cm is a common length that sits right around the collarbone. Pendants such as a solitaire pendant or a heart cut necklace therefore hang differently depending on the length of the chain and on how they are worn together with other chains.

Stud earrings require no size measurement - here it's solely about the size of the stone and the look in the ear. A pair of prong studs fit everyone, regardless of ring size.

Measure at the end of the day, when your fingers are at a normal temperature. Use a strip of paper rather than elastic string, and measure at least twice to check the result. Remember to measure over the knuckle if it's prominent, and choose the size the ring can pass over. Take the ring's width into account - wide rings often require half to a whole size more. And always measure on the finger and the hand the ring will actually sit on.

With these precautions, you'll get it right the first time in the vast majority of cases.