Ring Size Converter

Features

• Six systems converted both ways: US, UK / AU letter, EU, JP, inner diameter in mm, inner circumference in mm
• US sizes use the standard formula, exact in both directions, with half a size equal to 0.4064 mm
• EU size per ISO 8653 — the inner circumference in millimetres; JP size is that minus 40
• ⚠️ The result states that UK letter charts disagree and that you should order by circumference
• Lists the size above and below — choosing between two neighbouring sizes is the real question when buying
• Measure your finger with callipers and type the diameter or circumference; it snaps to the nearest stock size and says so
• Sizes outside the usual stock range (½ to 16) are flagged as likely custom orders
• Diameter and circumference to 0.1 mm, so 16.5 is never rounded to 17
• Runs entirely in your browser

How to use

1. Pick which system your number is under “what the input is”
2. Enter it (US sizes take decimals like 6.5, UK sizes take a letter like L)
3. The result gives all six systems plus the two neighbouring sizes

FAQ

Why the special warning about UK letter sizes?

Because the British and Australian letter scales were never strictly unified. Different jewellers and different decades put a given letter one size apart — so an M measured at one shop may be a different size when you buy an M at another. That is not this table being wrong; it is the system. So the mapping used is stated outright (one letter per half US size) and the advice is repeated: order by the inner circumference, a physical quantity every shop measures the same way.

Why not just multiply the US size by 2.5 for EU?

Because the systems are not in a linear ratio — they grew up independently. The EU size is itself a physical quantity (the inner circumference in millimetres) while the US size is an arbitrary sequence; the two can only be related by looking up what the same physical size is called in each, which is a table. A multiplier looks acceptable at small sizes and drifts half a size or more by the large ones, which is after the order has been placed.

Should I measure the diameter or the circumference?

Either — both are reported. With no ring to hand, wrap a non-stretchy string around the base of the finger and measure the string: that is the circumference. With a ring that fits, measure the inside diameter with a ruler; that is more accurate, because string pulls tight or slack. ⚠️ A finger is about half a size thicker in the evening than in the morning, and thicker in heat, so measure in the evening.

Why go up a size when in between?

Because too tight cannot be fixed and too loose can. A ring that is too small will not go on, and forcing it affects circulation; one that is slightly large can take a sizing insert or be resized, and it will not slip off when fingers thin out in winter. So the tool reports the nearest size — an honest reading of the measurement — while the buying advice is to go up one.

Where does the JP size come from?

The inner circumference minus 40. Japan uses a plain number roughly from 1 to 30, which sits 40 below the European number — so a 53 mm circumference is EU 53 and JP 13. ⚠️ JP sizes vary between sources too, so as with letters, order by the circumference.