Ring Sizer Compare two ring sizes: how big is the difference?
US 6 is 16.5 mm across the inside, US 7 is 17.2 mm — a full size works out to 0.7 mm of diameter, about 2.2 mm around the finger. That sounds small on paper, but a size 6 and a size 7 side by side feel noticeably different on the knuckle. The Premium Size Comparator lines up two saved measurements at true scale if you want to see it for yourself.
Is one ring size a big difference?
On paper, tiny. On a finger, it's the difference between a ring that clears the knuckle and one that stops dead. US 6 to US 7 adds about 2.2 mm around the finger; a half size, 6.5 to 7, adds roughly 0.9 mm. That's it. That's the whole margin most people are arguing about when a ring 'almost' fits.
| Comparison | Diameters | Diameter difference | Circumference difference | UK letters |
|---|---|---|---|---|
| US 6.5 vs US 7 | 16.9 / 17.2 mm | 0.3 mm | 0.9 mm | N / O |
| US 6 vs US 6.5 | 16.5 / 16.9 mm | 0.4 mm | 1.3 mm | M / N |
| US 6 vs US 7 | 16.5 / 17.2 mm | 0.7 mm | 2.2 mm | M / O |
| US 7 vs US 8 | 17.2 / 18.1 mm | 0.9 mm | 2.8 mm | O / Q |
The steps aren't perfectly even across the table, which is why US 7 to 8 edges out slightly larger than US 6 to 7.
Ring size 6 vs 7 in other systems
In the app, US 6 reads UK M, EU (Swiss) 12, JP 13; US 7 reads UK O, EU (Swiss) 14, JP 14. One US size jumps two UK letters — M to O, skipping N entirely.
When comparing two measurements helps
- Before resizing: compare the ring you own with a fresh measurement of your finger, to see how far off it is.
- Left vs right hand, or morning vs evening, to see how much your own finger varies.
- Two people, for matching wedding bands or a gift.
- A jeweler or seller keeping measurements for several clients.
How to compare ring sizes with the Size Comparator
The Size Comparator needs Premium, and it needs two saved measurements to work with. Free users keep exactly one, so opening this screen on the free plan just shows you the upgrade pitch instead of a comparison.
- Calibrate once in Calibration so both measurements are drawn to real scale.
- Measure the first ring or finger in Ring Sizer or Finger Sizer and tap Save Measurement.
- Measure the second and save it too. If you have fewer than two, the comparator shows Need More Measurements with shortcuts to both sizers.
- Open Comparator from the home screen and pick one measurement on each side.
- Look at the two circles, drawn at true relative size, and at the table of diameter, circumference and US, UK, EU and JP sizes for each.
- Read the Difference box: the gap in diameter (mm) and in US Sizes. It turns orange when the gap is over 1 mm or over one US size.
Without Premium: compare sizes by hand
Comparing by hand costs nothing, just a bit more arithmetic. Measure the first ring, note the diameter, measure the second, subtract, multiply by pi (about 3.14) for the circumference gap. Or skip the math and just scroll the free Size Chart to see two rows stacked against each other.
Questions and answers
How many mm is one ring size?
Why do I need two saved measurements?
Can I compare two people's sizes?
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