Ring sizing at home, and why the answer changes during the day

Ring sizing is the only part of buying jewellery where the correct answer genuinely changes between breakfast and dinner. Fingers are soft tissue with a blood supply, and they swell and shrink by more than the difference between two ring sizes over the course of an ordinary day. That is not a reason to give up on measuring, it is a reason to measure properly, at the right time, and to write down the number rather than the letter.

Measure circumference in millimetres and convert afterwards

Every ring sizing system in the world is a different set of names for the same physical quantity: the inside circumference of the band in millimetres. Get that number and you can convert to anything. Start from a letter or a US number and you have already thrown away precision.

Cut a strip of paper 3 to 5mm wide, which is roughly the width of a band so it behaves like one. Wrap it round the base of the finger, pull it just snug, mark the overlap with a fine pen and lay it flat against a ruler. That length in millimetres is your circumference.

Do not use string or thread. String stretches, bites into the skin and reads one to two millimetres short, which is most of a size. Do not use a strip of sticky tape either, because it will not come off at the same tension it went on. Paper, measured three times on three different occasions, is the whole method.

The conversion, with the numbers that matter

As a working table of inside circumference against the common US sizes: 49.3mm is about a US 5, 51.9mm a US 6, 54.4mm a US 7, 57.0mm a US 8 and 59.5mm a US 9. In UK and Australian letters those fall at roughly J and a half, L and a half, N and a half, P and a half and R and a half.

Continental European sizing usually quotes the inside circumference in millimetres directly, so 54mm is a size 54. Some charts follow the ISO convention and subtract 40, making the same ring a size 14. If a European size looks implausibly small, that is which convention you are reading.

The useful fact buried in that table is the spacing. A full US size is about 2.5mm of circumference and about 0.8mm of diameter. A half size is 1.25mm of circumference. That is a very small distance, and it is smaller than the amount a finger can change in an afternoon.

Why the time of day and the weather change the answer

Fingers are at their smallest first thing in the morning and in cold weather, when surface blood vessels constrict. They are at their largest in the evening, in heat, after exercise, after a salty meal and after a long flight. The swing between a cold morning and a hot evening is commonly most of a size, and for some people more.

So measure in the late afternoon or evening, at normal room temperature, not straight after the gym and not with cold hands. Then measure again on a different day and take the larger reading. A ring that is comfortable at your largest is loose at your smallest, which is survivable. A ring sized to a cold morning will not go on in July.

There are other variables worth knowing about rather than worrying about. Hands swell during pregnancy and at altitude, they change with weight, and for some people they change across a monthly cycle. If you are ordering a ring for an occasion in six months, measure near the time rather than now.

The knuckle problem, which no chart covers

The ring has to pass over the knuckle and then sit at the base of the finger. On most people those two circumferences are within a couple of millimetres of each other and the base measurement is the one to use. On some people the knuckle is substantially larger, and then you have a real conflict with no clean answer.

Measure both. If the knuckle is more than about 2mm bigger, size to the knuckle so the ring goes on, and accept that it will rotate at the base. A ring that spins is annoying. A ring that will not pass the joint is unwearable, and forcing it is a bad idea.

A few mitigations exist. Sizing beads or a sizing bar fitted inside the band by a jeweller hold a slightly large ring upright. A heavier ring spins less than a light one because its own weight keeps the design uppermost. A design with an obvious top, like a figurative ring, shows the rotation more than a plain band does.

Band width, and measuring a ring you already own

A wide band fits tighter than a narrow one at the same nominal size, because it bears on more of the finger and cannot flex past the knuckle as easily. For anything over about 6mm wide, go up a quarter to a half size. For a very narrow band, size as measured.

If you have a ring that fits the right finger, measuring it is more reliable than measuring the finger, with one trap: a ruler across the inside of a band is only good to about half a millimetre, and half a millimetre of diameter is nearly two thirds of a US size. Better to find a cylindrical object that fits snugly inside, a drill bit shank or a dowel, and measure that. Better again to take the ring to a jeweller with a ring stick, which takes thirty seconds and is free.

Printable paper ring sizers work, but only if the printer is told not to scale. Print at 100 per cent with fit to page switched off, then check the sheet against a known dimension before you trust it: a bank card is standardised at 85.6mm long, so measure the card marker on the printout and confirm it matches.

Adjustable designs, and what we can check for you

An open band or a wrap design has some give in it, which is why those shapes are popular for gifts. The give is finite. Every time you flex metal it work hardens slightly, so repeatedly opening and closing an open ring will eventually crack it at the point of maximum bend. Adjust once, carefully, with your thumbs rather than pliers, and then leave it.

We do not publish a size range on a page unless the supplier gives us one, so if you are looking at a ring like The Two Headed Snake or The Moth Ring and you need to know what sizes it comes in or whether it adjusts, email us and we will read the supplier's own figures for that piece.

That matters more with us than it would elsewhere, because we do not accept returns for a size chosen incorrectly. There is no warehouse to send a ring back to. Five minutes with a strip of paper, or one email, is the difference between a ring you wear and a ring in a drawer.

Questions people actually ask

What time of day should I measure my ring size?

Late afternoon or evening, at normal room temperature, and not straight after exercise. Fingers are smallest in the morning and in the cold, and largest in heat and after activity. Measure on two or three separate days and take the larger reading.

How do I work out my ring size from millimetres?

Measure the inside circumference in millimetres and convert. Roughly, 49.3mm is a US 5, 51.9mm a US 6, 54.4mm a US 7, 57.0mm a US 8 and 59.5mm a US 9. A full US size is about 2.5mm of circumference, so a half size is only 1.25mm.

Should I size up for a wide ring band?

Yes, by about a quarter to a half size for bands over roughly 6mm wide. A wide band contacts more of the finger and cannot flex over the knuckle as easily, so it feels tighter than a narrow band of the same nominal size.