Ring Size Converter

Starting value:

Use a measurement when possible; regional charts can differ slightly between sellers.

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Available sizes:

Choose the increment actually offered by the seller before comparing nearby rows.

Between-size choice:

For ordering, the larger option is usually safer when a measurement falls between chart sizes.

Band fit context:

Keep Standard for an ordinary narrow band; wider or stacked rings can feel tighter.

Measurement precision:

Use more decimals when comparing caliper measurements or chart deltas.

System	Converted size	Basis	Copy
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How to use this match

Reference basis: ISO 8653:2016 for ring-size measurement/designation and the Tiffany & Co. US/UK diameter/circumference chart. Always confirm the seller's own chart before ordering.

A ring-size label is only useful when it can be tied back to the opening that must pass over a knuckle and sit comfortably on a finger. US numbers, UK and Australian letters, ISO or European circumference numbers, and several regional scales label that opening differently. The shared physical reference is the ring's inner diameter or inner circumference in millimetres.

An existing round ring is usually easiest to measure across its inside edge. A finger measurement follows the full path around the finger instead, so the tape or strip must be snug without compressing the skin. Temperature, time of day, dominant-hand swelling, and the knuckle can all move a measurement enough to change the nearest retail size.

Ring measurement choices and their common errors
Starting point	Best use	Common error
Inner diameter	A round ring that already fits the intended finger	Measuring the outside width or using an oval ring
Inner circumference	A finger gauge, tape, or carefully marked paper strip	Pulling too tight or leaving the strip loose
Regional label	Comparing a known seller size with another chart	Assuming labels are identical across brands

Band shape changes how the same opening feels. A wide band or several stacked rings covers more skin and often feels tighter than a narrow band. Comfort-fit interiors can pass the knuckle differently from flat interiors. These are fitting questions, not conversion errors, so a chart result should be compared with the seller's own guidance and available increments.

Regional equivalence is approximate because commercial charts can use slightly different diameter and circumference pairs. When a measurement falls between sold sizes, the choice between the smaller and larger option depends on the band, knuckle, swelling, and return policy. A conversion narrows the choice; it cannot replace trying on the actual ring.

How to Use This Tool:

Use a physical measurement when possible, then match it to the sizes the seller actually offers.

Choose Starting value and enter one inner diameter, inner circumference, or supported regional label. Recheck millimetres and use the intended finger.
Select half or whole Available sizes, then choose whether a between-size measurement should go to the nearest, next larger, or next smaller chart row.
Set Band fit context. Wide and stacked bands deserve comparison with the next size up; brand guidance remains more specific than a general chart.
Read the highlighted row in Size match ledger, then check the circumference delta and nearby rows before ordering.

Interpreting Results:

The circumference delta equals the matched chart circumference minus the measured circumference. A positive delta selects a larger opening; a negative delta selects a smaller one. Zero means the source lands exactly on the chosen chart row.

Ring size match status boundaries
Status	Absolute circumference delta	Meaning
Chart match	0 to 0.15 mm inclusive	The selected row is effectively the same as the source measurement.
Close fit	Above 0.15 mm through 0.65 mm inclusive	The selected row is nearby, but band and seller guidance can decide the better option.
Between sizes	Above 0.65 mm	Compare adjacent offered sizes rather than treating one label as exact.

Technical Details:

Every accepted starting value is reduced to inner circumference in millimetres. Physical measurements use circle geometry, supported US sizes can be interpolated between chart rows, UK/AU labels must match a listed row, and the JP/CN and Swiss/Italy/Spain approximations add 40 to the entered regional number.

Formula Core

For a round opening, circumference and diameter are connected by pi:

C = π d d = \frac{C}{π}

C is inner circumference in millimetres and d is inner diameter in millimetres. A 17.3 mm diameter gives about 54.35 mm circumference. The nearest half-size chart row is US 7 / UK O at 54.4 mm, a positive delta of about 0.05 mm.

Lookup Core

The supported chart is bounded from US 3 through US 13. These are the exact rows used for matching:

Ring size lookup chart used for US, UK, diameter, and circumference matching
US	UK / AU	Diameter (mm)	Circumference (mm)
3	F	14.1	44.2
3.5	G 1/2	14.5	45.5
4	H 1/2	14.9	46.8
4.5	I 1/2	15.3	48.0
5	J 1/2	15.7	49.3
5.5	L	16.1	50.6
6	M	16.5	51.9
6.5	N	16.9	53.1
7	O	17.3	54.4
7.5	P	17.7	55.7
8	Q	18.1	57.0
8.5	R	18.5	58.3
9	S	18.9	59.5
9.5	T	19.4	60.8
10	T 1/2	19.8	62.1
10.5	U 1/2	20.2	63.4
11	V 1/2	20.6	64.6
11.5	W 3/4	21.0	65.9
12	Y	21.4	67.2
12.5	Z	21.8	68.5
13	Z+1	22.2	69.7

Whole-size availability removes half-size rows before matching. The larger preference selects the first available row with circumference greater than or equal to the source; the smaller preference selects the last row less than or equal to it. Nearest matching minimizes absolute circumference difference, with an exact tie staying on the earlier, smaller row. ISO/EU output is the matched circumference rounded to the nearest whole millimetre; the JP/CN and Swiss/Italy/Spain outputs round that circumference minus 40.

Accuracy Notes:

Accepted circumferences run from 44.2 through 69.7 mm inclusive. The chart and regional translations are comparison aids, not a guarantee that every maker cuts rings to the same tolerance.

Measure more than once at a representative finger temperature and use the intended finger and hand.
Confirm wide, stacked, and comfort-fit designs against the seller's chart or a jeweller's gauge set.
When a value is between offered sizes, inspect both neighboring rows and the return or resizing policy.

References:

ISO 8653:2016 Jewellery — Ring-sizes — Definition, measurement and designation, International Organization for Standardization, January 2016.
Rings Size Guide, Tiffany & Co.