Secret Ring SizeAny finger
Thumb, pinky and index finger ring size
Every finger has its own size, so measure the one the ring is for. Measure around the base and around the knuckle: where the knuckle is wider, as it usually is on a thumb, a size between the two slides on and stays put.
Two measurements, in mm
Wrap a thin paper strip, mark where it meets, and measure it flat. The example values are a large thumb.
Right on a size.
Screens, paper and string can be off by a little. Before you order, tell the jeweler how you measured and ask about resizing.
Why the knuckle matters
A ring has to pass the knuckle to reach the base. Sized for the base alone, it may not go on; sized for the knuckle alone, it spins and can slip off. A size between the two is the usual choice for thumbs and for anyone with large knuckles.
Worked example: a thumb 60 mm around the base and 66 mm around the knuckle. The base alone is US 9½ and the knuckle alone US 11½, so the tool gives the size for 63 mm halfway between: US 10½.
Thumb rings are often a few sizes larger than ring-finger rings, and a pinky ring a few sizes smaller, but fingers vary too much to convert from one to another. Measure the finger itself. For the ring finger, see string or paper, or the size chart for every size.
Questions
How do I measure my thumb for a ring?
Measure around the base and around the knuckle with a thin paper strip, then use a size between the two so the ring passes the knuckle without spinning.
Is pinky ring size smaller than ring finger size?
Usually by a few sizes, but it varies from person to person. Measure the pinky itself rather than converting.
Does an index finger take the same size as the ring finger?
Often not; the index finger is usually a little larger. Measure it.
What if the ring will not go over the knuckle?
Choose a larger size, closer to the knuckle measurement. Some jewelers can also fit sizing beads inside a band to keep a larger ring from spinning.