Ring Size Converter
Convert ring sizes between US, UK letter, EU number, and inside diameter in mm.
How to use this tool
- Enter us ring size in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your uk size and the full breakdown beneath it.
Convert US ring sizes to UK letter codes, EU numbers, and inside diameter in millimetres.
Formula
Conversion is performed via a lookup table keyed on US ring size:
{ UK letter, EU number, diameter (mm) } = table[US size]
The inside diameter in mm is related to EU size by: EU ≈ π × diameter (mm), and UK letters advance by roughly 1.25 mm of circumference per letter step.
How it works
Ring sizing systems all ultimately track the inner circumference (or equivalently the inner diameter) of the ring band, but express it differently: the US system uses an arbitrary numerical scale starting near size 0, EU sizing uses inner circumference in mm rounded to a standard step, and UK sizing uses sequential letters of the alphabet. The converter reads the appropriate row from a standardised table for US sizes 5–12.
Common mistake: Ring sizes are highly sensitive to finger swelling — fingers can change by half a size or more during the day, in heat, or after exercise. A size that fits in the morning may be tight in the evening, so most jewellers recommend sizing in the afternoon at a comfortable room temperature rather than first thing in the morning.
Worked example
Convert US ring size 7 to UK, EU, and inside diameter
- Input: US ring size 7.
- Look up size 7 in the ring size table.
- Read off: UK = N, EU = 54, inside diameter = 17.3 mm.
US ring size 7 = UK size N, EU size 54, inside diameter 17.3 mm.
Common mistakes to avoid
- Measuring finger size when cold or early in the morning -- fingers shrink when cold and swell in heat or at the end of the day; size when the finger is at its largest prevents a ring that is too tight.
- Confusing inside diameter with inside circumference -- EU ring sizes are based on circumference in mm, not diameter; a 17 mm diameter finger has a circumference of about 53.4 mm, which corresponds to EU size 53.
- Assuming UK letter sizes increase linearly in equal diameter steps -- they increase in roughly 0.4 mm diameter increments, but the sequence skips some letters in most tables.
Key terms
- US ring size
- An arbitrary numerical scale where each increment represents approximately 0.4 mm of additional inner diameter. Sizes range from 0 (child) upward, with adult sizes typically between 4 and 13.
- EU ring size
- Numerically equals the inner circumference of the ring in millimetres (circumference = π × inner diameter). This direct physical relationship makes EU sizing the most globally unambiguous system.
- UK ring size
- An alphabetical sizing system where each letter step corresponds to an inner circumference increase of approximately 1.25 mm. Sizes run from A (smallest) upward through the alphabet.
- Inside diameter (mm)
- The internal diameter of the ring band in millimetres. It is the most direct physical measurement and can be determined by measuring an existing well-fitting ring across its inner opening.
Frequently asked questions
- How do I measure my ring size?
- Wrap a strip of paper around your finger, mark where it overlaps, and measure the length in mm — that is the inside circumference. Divide by π to get the inside diameter.