Ring size calculator
Enter a size or a measurement, get the other.
How do I find my ring size?
Wrap a thin strip of paper around the base of your finger, mark where it overlaps and measure the length in millimeters. Enter it as the circumference. Or measure the inside diameter of a ring that fits.
What size is a 17.3 mm ring?
US size 7. Its inside diameter is about 17.3 mm (0.682 in) and its inside circumference about 54.4 mm.
How much bigger is each size?
Each whole US size adds 0.032 inch (0.8128 mm) to the inside diameter — about 2.55 mm around. Half and quarter sizes split the step.
What are ISO ring sizes?
Under ISO 8653, the ring size is simply the inside circumference in millimeters. A US 7 is about ISO 54.
Are all ring size charts the same?
Not exactly — there is no single US standard, and charts differ by a few hundredths of a millimeter. Check with your jeweler before ordering an expensive ring.
Worked example: from size 9 to millimeters, and from a paper strip to a size
The calculator uses d = 11.63 + 0.8128 × size, in millimeters. For size 9 that is 11.63 + 0.8128 × 9 = 18.95 mm of inside diameter. The inside circumference is π × 18.95 = 59.5 mm, so on the ISO scale the ring is about 60. In the other direction, a paper strip that measures 58 mm gives a diameter of 58 ÷ π = 18.46 mm, and (18.46 − 11.63) ÷ 0.8128 = size 8.4. That falls between sizes 8 and 8.5, so ask a jeweler about the nearer half size.
Why are diameter and circumference easy to mix up?
They differ by a factor of π, about 3.14. A strip of paper measures the circumference; a ring laid on a ruler shows the diameter. Entering the 58 mm strip length in the diameter field would give size 57, far outside the range of real rings, so a result like that signals the wrong field. Under ISO 8653 the size number is the circumference itself, which is why an ISO size is roughly 3.14 times the diameter in millimeters.
Common mistakes when sizing a ring
Measuring the outside of a ring: a band 2 mm thick adds 4 mm to the outer diameter, so a US 7 with an inside diameter of 17.3 mm measures 21.3 mm across the outside. Entering inches in a millimeter field. Stretching a paper strip or string while measuring, which makes the finger look larger. Assuming sizes from other countries match the US scale: convert through the inside circumference or diameter. And ordering a costly ring on a single estimate: charts differ slightly, so confirm with the jeweler.
US sizes with their measurements
From the relation d = 11.63 + 0.8128 × size, rounded to the nearest tenth of a millimeter:
- US 4: inside diameter 14.9 mm (0.586 in), inside circumference 46.8 mm (ISO about 47)
- US 5: inside diameter 15.7 mm (0.618 in), inside circumference 49.3 mm (ISO about 49)
- US 6: inside diameter 16.5 mm (0.650 in), inside circumference 51.9 mm (ISO about 52)
- US 7: inside diameter 17.3 mm (0.682 in), inside circumference 54.4 mm (ISO about 54)
- US 8: inside diameter 18.1 mm (0.714 in), inside circumference 57.0 mm (ISO about 57)
- US 9: inside diameter 18.9 mm (0.746 in), inside circumference 59.5 mm (ISO about 60)
- US 10: inside diameter 19.8 mm (0.778 in), inside circumference 62.1 mm (ISO about 62)
- US 11: inside diameter 20.6 mm (0.810 in), inside circumference 64.6 mm (ISO about 65)
- US 12: inside diameter 21.4 mm (0.842 in), inside circumference 67.2 mm (ISO about 67)
A whole size changes the circumference by 2.55 mm and a half size by 1.28 mm, so a half size is a difference of little more than a millimeter around the finger. That is why a strip of paper measured to the nearest millimeter is good enough to pick a whole size but not always a half size.
Measuring at home
Use a strip of non-stretching paper or a thin string, wrap it once around the base of the finger where the ring will sit, mark the point where it overlaps and measure the length against a ruler in millimeters. Repeat two or three times and use the average. Size the finger it is for: the ring finger on the left and right hand can differ. Measure when the hand is at a normal temperature; fingers can look larger when warm and smaller when cold. A ring has to pass over the knuckle, so if the knuckle is much larger than the base of the finger, pick a size that fits over the knuckle and ask the jeweler about sizing beads or an adjustable band.
Sources and related calculators
Sources: ISO 8653, jewellery ring sizes (size defined by inside circumference in millimeters); the US size relation d = 11.63 + 0.8128 × size used by the calculator; circumference = π × diameter. Related: length converter, circle calculator, inches to mm converter and mm to inches converter.