Late afternoon or early evening, at room temperature, on an ordinary day. Here is the protocol, and what to do if you cannot wait.
Take three readings and enter the average.
Your size
Late afternoon or early evening, at normal room temperature, on a day you have not done anything unusual. That is the short answer to when you should measure finger size for a ring.
The rest of this page is the protocol — what to avoid, how many readings to take, and how to tell whether your measurement is trustworthy. The reasoning behind it is on why finger size changes.
Fingers are at their smallest when you wake. You have been horizontal for hours, so fluid has distributed evenly instead of settling into your hands, and you have not used them yet.
Measure then and you will size a ring to the smallest your finger ever is. It will be comfortable for perhaps two hours and increasingly tight for the rest of the day. This is the most common timing error and it produces a genuinely unwearable ring rather than a slightly imperfect one.
Late evening runs the opposite risk, especially after a large or salty dinner, a drink, or a long day on your feet. Size to your most swollen state and the ring spins for most of the day and may come off.
Late afternoon to early evening is the sweet spot: your hands are at their normal working size, which is what the ring needs to fit for the majority of its life.
Roughly 4pm to 8pm, on an ordinary day, indoors, at a comfortable temperature, having been settled for at least twenty minutes.
Sometimes you need a number now. Measure anyway, but adjust your interpretation:
If a ring needs to fit on a particular day — a wedding in high summer, for instance — measure at the time of day the event happens and in conditions as close to it as you can manage. A ring sized on a cool February evening may be uncomfortable at an August reception.
Some people see much more than half a size of daily variation, whether from circulation, medication, or a health condition. If your readings differ by a full size between morning and evening, size toward the larger end and expect to want the ring a little loose at its smallest.
Persistent or one-sided swelling that does not settle overnight is worth mentioning to a doctor rather than working around as a sizing problem.
Enough to matter. A half size is only about 1.25 mm of circumference, and strip tension, temperature and time of day each move a reading by a comparable amount. Take several readings rather than trusting one, and for anything expensive have a jeweller confirm the size on a metal sizer before you order.
Late afternoon to early evening, roughly 4pm to 8pm, indoors at a comfortable temperature and after being settled for twenty minutes. That is when your hands are at their normal working size, which is what the ring needs to fit for most of its life.
Fingers are at their smallest on waking, because you have been horizontal overnight and fluid has not settled into your hands. Sizing to that reading gives you a ring that is comfortable for a couple of hours and tight for the rest of the day.
A couple of hours. Blood flow to the hands increases during sustained activity and fluid shifts into the tissues, so a reading taken soon afterwards will overstate your size. The same applies after a hot bath or shower.
Yes, and more than people expect. Cold makes the blood vessels in your hands constrict, and a reading taken with cold hands can be most of a size small. Warm up indoors for twenty minutes first.
Three readings on each of three ordinary days, taken at a similar time. If your readings sit within about half a millimetre of each other, the measurement is stable and you can order from it. A wider spread usually means your strip tension is varying.
The physiology behind the timing advice on this page.
The measuring method itselfOnce you have the right moment, here is how to take the reading.
Other measuring errorsTiming is one of eleven things that commonly skew a result.
When readings straddle a sizeHow the time you measured should influence which way you round.