Convert your US ring size to inner circumference and diameter in millimeters. Useful for international jewelry shopping or measuring an existing ring.
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Your Ring Size (US)
≥ 0.5
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The inner perimeter of the ring, measured in millimeters.
Details
US Ring Size
The US ring size scale is a numerical index that encodes the inner circumference of a ring. Each whole or half size corresponds to a specific inner circumference in millimeters, allowing direct conversion to the EU/ISO system, where the size number equals the inner circumference in millimeters.
How US Ring Sizes Work
The US ring size scale encodes the inner circumference of the ring:
Inner circumference (mm) = US size × 2.55 + 36.55
Inner diameter (mm) = circumference ÷ π (3.14159)
To reverse — find your US size from a measurement:
US size = (circumference mm − 36.55) ÷ 2.55
Quick Reference Size Chart
US Size
Circumference (mm)
Diameter (mm)
4
46.8
14.9
5
49.3
15.7
5.5
50.6
16.1
6
51.9
16.5
6.5
53.1
16.9
7
54.4
17.3
7.5
55.7
17.7
8
57.0
18.1
8.5
58.2
18.5
9
59.5
18.9
10
62.1
19.8
11
64.6
20.6
How to Measure Your Ring Size at Home
Method 1 — paper strip:
Wrap a thin strip of paper snugly around the base of the finger you want to measure. Mark where the end overlaps, then measure the strip length in millimeters. Convert to US size using the formula above.
Method 2 — existing ring:
Place the ring flat on a ruler and measure the inner diameter in millimeters. Multiply by π (3.14159) to get the circumference, then apply the reverse formula.
Tips for accurate measurement:
Measure in the evening when fingers are at their largest.
If your knuckle is wider than the finger base, size for the knuckle so the ring passes over it.
Fingers can be slightly different sizes on left and right hands — measure the actual finger that will wear the ring.
Practical Notes
Most common women's ring size in the US: 6–7 (circumference 51.9–54.4 mm, diameter 16.5–17.3 mm).
Engagement rings are typically sized for the left ring finger, which is often slightly smaller than the right.
Ring sizing varies ±0.5 mm between jewelers; ordering half a size larger online is a safe strategy.
Wide band rings (5 mm or wider) feel tighter — go up half a size for comfort.
Frequently Asked Questions (FAQ)
How do I convert a US ring size to millimeters?
The formula is: inner circumference (mm) = US size × 2.55 + 36.55. Inner diameter = circumference ÷ π. For example, US size 7 gives a circumference of 54.4 mm and a diameter of 17.3 mm. When shopping on international sites, the EU/ISO size equals the inner circumference in mm — so US 7 corresponds to approximately EU/ISO 54.
How do I measure my ring size at home?
Wrap a thin strip of paper around the base of your finger, mark where the ends overlap, and measure the length in millimeters — this is your inner circumference. To find the equivalent US size: US size = (circumference mm − 36.55) ÷ 2.55. Measure in the evening when fingers are at their largest. If your knuckle is wider than the finger base, size to fit over the knuckle.
What is the most common ring size for women in the US?
The most common US ring size for women is 6 to 7, corresponding to an inner circumference of about 51.9–54.4 mm and a diameter of 16.5–17.3 mm. However, ring size varies considerably by hand size and which finger is being measured. Engagement rings are typically sized for the left ring finger, which is often slightly smaller than the right.
Should I size up for a wide band ring?
Yes — wide band rings (5 mm or wider) feel tighter than narrow bands of the same size because more surface area contacts the finger. It is generally recommended to go up half a size for wide bands. When ordering online, sizing half a size larger is also a safe choice given the ±0.5 mm variation between jewelers.
