A US size 5.5 ring measures 16.1 mm across the inside (0.63 inches) and 50.6 mm around. It equals UK K½, EU 51 and India 11. It is a small women’s size, half a size above 5. You can confirm your size in 30 seconds with the ring sizer.
| US / Canada size | 5.5 |
|---|---|
| Inner diameter | 16.1 mm = 0.634 in |
| Inner circumference | 50.6 mm = 1.99 in |
| UK / Australia | K½ |
| EU / ISO / Germany | 51 |
| India / Pakistan | 11 |
| Italy / Spain / Switzerland | 11 |
| Half size smaller | US 5 (15.7 mm) |
| Half size larger | US 6 (16.5 mm) |
On this page (8 sections)
Who wears 5.5?
Size 5.5 sits between the petite 5 and the very common 6. It suits women with slim fingers whose size 6 rings slide around, and it is a frequent size for the middle finger of children and teenagers. Some women wear 5.5 on the right hand and 6 on the left, because the dominant hand is often slightly larger.
Half sizes matter
Half a size is only 0.4 mm of diameter, but on a finger that is noticeable. If a size 6 ring turns on your finger and drops toward the knuckle when your hands are cold, but 5 feels tight in the evening, 5.5 is likely your fit. Quarter sizes (5.25 and 5.75) are also available from many jewellers when made to order.
How to measure for 5.5
- Measure a ring that fits: an inside diameter of 15.9–16.3 mm is about 5.5.
- Or wrap paper around your finger: a circumference of 49.9–51.2 mm points to 5.5.
- For the most reliable answer, match your ring on screen with the online ring sizer after a one-time card calibration.
5.5 vs 5 vs 6
Buying a 5.5 ring
- Not every fashion brand stocks half sizes. If 5.5 is not offered, choose 5 for a thin band and 6 for a wide band.
- For engagement rings, order the exact half size; most fine jewellers make every half size.
- When buying from Europe, look for size 51 (or 50.5 if quarter sizes are listed).
Measuring a small finger accurately
Small sizes are the hardest to measure with string, because a 1 mm error in circumference is almost a full quarter size. For a size around 5.5, use a ring that already fits rather than your finger, and measure it on a calibrated screen or with a caliper. If you only have string, use non-stretch dental floss or a thin paper strip, pull it snug, and repeat three times. Take the middle value. Remember that the knuckle matters too: the ring has to pass over it, so if your knuckle is large, a 5.5 may be right even when the base of the finger measures 5.
Size 5.5 and the little finger
Women who wear 7 on the ring finger often wear about 5.5 on the little finger, which makes 5.5 a popular size for pinky and midi rings as well.
Not sure of your ring size? Measure it on your phone screen in 30 seconds.
Open the free ring sizer →All sizes are on the ring size in mm chart.
Frequently asked questions
What is a size 5.5 ring in mm?
A US 5.5 ring is 16.1 mm in inner diameter and 50.6 mm in inner circumference.
What is US ring size 5.5 in UK?
US 5.5 is UK K½.
What is 5.5 ring size in EU?
EU size 51, which means about 51 mm inner circumference.
Is 5.5 a common ring size?
It is a common small women’s size, though less common than 6 and 7.
Should I choose 5.5 or 6?
If size 6 spins on your finger and 5 is tight in the evening, choose 5.5. For a wide band over 6 mm, choose 6.
Sources and standards
- ISO 8653:2016 – Jewellery – Ring-sizes – Definition, measurement and designation: defines ring size as the inner circumference in mm (EU sizes).
- UK / Australian ring letter scale (customary): based on the withdrawn BS 6820:1987; rings are measured per BS EN ISO 8653.
- NIST – SI units: length: 1 inch = 25.4 mm exactly, used for every inch-to-mm conversion.
The formulas and rounding rules are on our how we measure page.