Ring Size Converter

Getting ring sizing right for online and international jewellery shopping

Ring sizing is unusually precise compared to clothing or even shoe sizing — a difference of even half a size can make a ring uncomfortably tight or prone to slipping off. Like shoe sizing, ring size isn't governed by a single global standard: the US uses a numeric scale, many European countries size rings directly by internal circumference in millimetres, and other regions use their own letter or numeric systems. This tool focuses on the most universally useful and precise approach — converting between US ring size, internal diameter, and internal circumference in millimetres — since millimetre measurements are the most reliable, brand-independent reference point available.

Why millimetre measurements are the most trustworthy anchor

Unlike a shirt size or a shoe size, which can vary meaningfully between brands due to differences in cut or last, a ring's internal diameter and circumference are direct physical measurements with no room for brand-specific interpretation — 18mm of internal diameter is 18mm of internal diameter, regardless of which jeweller made the ring. This is why, whenever possible, working from an actual measured diameter or circumference (either of an existing ring that fits well, or of your finger using a proper sizing tool) and converting from there gives a more reliable result than converting from a remembered size number in an unfamiliar system.

How to measure at home

If you have a ring that already fits the intended finger well, measuring its internal diameter with a ruler (edge to edge, not including the metal band's thickness) gives you a diameter figure you can enter directly into this tool. Alternatively, wrapping a strip of paper or string snugly around the base of the finger, marking where it overlaps, and measuring that length gives you a circumference figure. Both approaches are more reliable than guessing a size based on a different ring you own from a different jewellery line, since even rings from the same brand can vary slightly between styles depending on band width and design.

A note on band width and finger size fluctuation

Wider bands generally need to be sized slightly larger than a thin band would for the same finger, since a wider band contacts more of the finger's surface and can feel tighter at the same internal diameter — if you're switching from a thin ring to a noticeably wider style, consider sizing up slightly from a pure mathematical conversion. It's also worth knowing that finger size naturally fluctuates with temperature and time of day, typically being slightly larger in warm weather or in the evening, so for a purchase that needs to be precise, measure at a few different times if possible.

Where this tool is useful

Online jewellery shoppers converting a US size into the millimetre measurements many international and boutique jewellers require, gift-givers trying to determine a partner's ring size discreetly using a borrowed ring's internal diameter, and anyone resizing or replacing a ring who wants to double-check a jeweller's quoted measurement against a size they're more familiar with all benefit from this converter.