Ring Size Converter – Online Circumference/Diameter to US, UK, EU

The most reliable home method is the string (or dental floss) method: wrap a non-stretchy string snugly around the base of your finger, mark where it overlaps, then measure the length in millimeters with a ruler. Enter this circumference into our converter. Alternatively, if you have a ring that fits perfectly, measure its internal diameter with calipers and use the diameter input. For best results, measure at room temperature in the evening when fingers are at their largest.

US/Canada: Uses a numeric scale (e.g., size 7) based on internal circumference, with half and quarter increments.
UK/Australia: Uses an alphabetical letter scale (e.g., size N). Each letter represents a specific circumference, with half-sizes denoted by ½ (e.g., N½).
EU/ISO: Uses the internal circumference in millimeters rounded to the nearest whole or half number (e.g., size 54 means approximately 54mm circumference). This is the most intuitive system as the number directly relates to the measurement.

Finger size fluctuates due to temperature, humidity, altitude, and activity level. Cold weather can shrink fingers by up to half a size, while heat, exercise, and salt intake can cause swelling. This is why we recommend measuring in the evening at room temperature. If you're between sizes, most jewelers recommend sizing up for comfort, especially for wider bands.

If your measurement is exactly between two sizes, we recommend choosing the larger size. A ring that's slightly loose is more comfortable and safer than one that's too tight. You can also consider quarter sizes (available from some US jewelers) or UK half-sizes for a more precise fit. Our converter shows the closest standard size; the note beneath the EU result will indicate if you're between sizes.

Yes, the circumference-to-size conversion is universal for all ring types. However, wide bands (6mm+) often require a slightly larger size for comfort because they cover more of the finger. If you're buying a wide band, consider going up about half a US size. Also, stacking multiple thin rings may require sizing up slightly compared to wearing a single ring.

Our converter uses the ISO 8653:2016 standard conversion tables, which are the international benchmark for ring sizing. The mappings between US, UK, and EU sizes are precise to within ±0.2mm. However, slight variations exist between manufacturers, so we always recommend confirming with a professional jeweler for high-value purchases like engagement rings.