How accurate is home measurement?

+/-1 size for string-and-ruler or existing-ring methods, done carefully. Best practice for expensive rings: have a jeweler measure with a professional mandrel (free at most jewelry stores). For modest-priced rings or gift sizing, home methods are usually sufficient. Always size at end of day when fingers are slightly larger from blood pooling.

Should I size up or size down if between sizes?

Size up for normal fit. Reasons: knuckles are larger than the base of the finger, fingers swell in heat and after meals, getting a too-tight ring stuck on the knuckle is more painful than a slightly loose ring. Size DOWN if the finger is unusually skinny at the base relative to knuckle. For wide bands (6mm+), always size up half to full size.

Why are sizes inconsistent across brands?

While the major systems (US, UK, EU) are standardized, individual brands sometimes use slightly off-spec sizing for fashion-fit reasons. Tiffany's rings tend to run slightly larger than Cartier; both run differently than department-store brands. For boutique purchases, always check the brand's own size chart and prefer their stocked sizes over conversions.

Can I resize a ring later?

Most can be resized within 2 sizes either direction. Plain bands resize easily. Eternity bands, channel-set diamonds, tension settings, or some unusual designs can't be resized at all (the structure breaks). Resizing typically costs $20-100 depending on metal (silver/gold cheap, platinum more) and complexity. For online purchases, confirm resize-ability before buying.

What's circumference vs diameter?

Circumference is the inside perimeter of the ring (~54.4mm for US 7). Diameter is the inside straight-across measurement (~17.3mm for US 7). Diameter is what you measure if you have an existing ring on a flat surface; circumference is what string-and-finger gives you. Both convert to size; pick whichever your measurement method gives.

Do men's and women's rings size differently?

No — the underlying measurement systems are gender-neutral. But average sizes differ. US women average size 6-7; US men average size 9-10. So “size 7” on a women's ring isn't different from “size 7” on a men's ring; it's just less common in men's collections. Wide men's bands often size up half to compensate for thicker contact area.

File & Format Converters · Free tool

Ring Size Converter US UK EU JP mm

Convert ring sizes between US, UK, EU, and Japan, plus see circumference in mm. Includes home measurement tips, all free with no registration.

Updated June 2026

Input typeSource countrySize

Your ring size

Circumference

17.35 mm

Inner diameter

5.52 mm

How to measure at home

Wrap a thin strip of paper or string around the base of your finger.
Mark where it overlaps, then measure the length in millimeters — that’s your circumference.
Measure at the end of the day when fingers are largest; avoid right after exercise.
If you’re between sizes, round up — knuckles need room to pass through.
Wider bands (>6mm) fit tighter — go up a half size.

US	UK	EU	JP	mm
3	F	44	4	14.05
3.5	G	45	6	14.46
4	H	46	7	14.86
4.5	I	47	8	15.27
5	J	49	9	15.70
5.5	K	50	10	16.10
6	L	51	11	16.51
6.5	M	52	12	16.92
7	N	54	13	17.35
7.5	O	55	14	17.75
8	P	57	16	18.19
8.5	Q	58	17	18.53
9	R	59	18	18.89
9.5	S	61	19	19.41
10	T	62	20	19.84
10.5	U	63	22	20.20
11	V	64	23	20.68
11.5	W	66	24	21.08
12	X	67	25	21.49
12.5	Y	68	26	21.89
13	Z	70	27	22.33

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What it does

Ring sizing is one of the few measurement systems where five major regional standards coexist with no clean conversion formula between them. The US system uses numeric half-sizes (3, 3.5, 4, … 13.5); the UK uses alphabetic letters (A through Z and beyond, where larger letter = larger size); the EU uses internal circumference in millimeters (50, 51, 52, etc., where the number is the actual circumference in mm); Japan uses its own numeric scale (1-30); India uses a 12-point scale (1-12, sometimes A-Z). The conversions among them aren't linear — US 7 maps to UK N, EU 54, JP 14, India 14 — so a Japanese site selling jewelry to an American customer needs a real conversion table, not arithmetic.

The converter takes a size in any of the five major systems and returns equivalents in all the others, plus the actual internal circumference in millimeters (the underlying physical measurement that all systems encode differently). For US 7: UK N, EU 54, JP 14, circumference 54.4mm, diameter 17.3mm. The diameter and circumference numbers are useful for measuring an existing ring at home — wrap a piece of string around the inside of the ring once, mark where it overlaps, measure the string in mm, and look up the size from circumference.

Home measurement methods (when you don't have access to a jeweler's mandrel set): (1) String-and-ruler — wrap string around finger, mark, measure, look up size. Best done at end of day when fingers are slightly larger from blood flow. (2) Existing-ring method — measure a ring that fits the target finger using the diameter (inner edge to inner edge) and look up by mm. (3) Printable ring sizer — print a paper template with a known scale, cut out the slot, slide your finger through. Each method is +/-1 size accurate. For an engagement ring or anything expensive, visit a jeweler for professional mandrel sizing — free in most jewelry stores.

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How to use it

Enter your size in your country's system, OR enter the inner circumference in mm.
The tool returns equivalents in US, UK, EU, JP, India sizing.
Cross-reference with the seller's sizing chart before ordering — some boutique brands use proprietary scales.
For surprise gift sizing without asking, “borrow” an existing ring and measure inner diameter.
When in doubt, size up by half — easier to resize down than up.

When to use this tool

Buying jewelry from international sellers (US buyer + UK Etsy, EU customer + US brand).
Ordering an engagement ring online without an in-person jeweler visit.
Surprise gift sizing using an existing ring.
Resizing planning — converting your size to mm so the jeweler can match exactly.
Researching whether a vintage ring (often sized to non-standard local systems) will fit.

When not to use it

When the seller specifies a unique size system not covered (some Asian boutique brands use bespoke scales).
For fitted bands where finger shape matters (knuckles larger than the base joint require comfort-fit sizing, not standard).
When the ring is for someone with significantly fluctuating finger size (pregnancy, weight changes, arthritis).
Wide bands (over 6mm) — these typically require sizing up by half or full size due to extra contact area.

Common use cases

Quick conversion during a typical workday
Pre-decision sanity-check on inputs and outputs
Educational use — demonstrating the underlying concept
Onboarding a colleague who needs the same calculation/conversion

Frequently asked questions

How accurate is home measurement?: +/-1 size for string-and-ruler or existing-ring methods, done carefully. Best practice for expensive rings: have a jeweler measure with a professional mandrel (free at most jewelry stores). For modest-priced rings or gift sizing, home methods are usually sufficient. Always size at end of day when fingers are slightly larger from blood pooling.
Should I size up or size down if between sizes?: Size up for normal fit. Reasons: knuckles are larger than the base of the finger, fingers swell in heat and after meals, getting a too-tight ring stuck on the knuckle is more painful than a slightly loose ring. Size DOWN if the finger is unusually skinny at the base relative to knuckle. For wide bands (6mm+), always size up half to full size.
Why are sizes inconsistent across brands?: While the major systems (US, UK, EU) are standardized, individual brands sometimes use slightly off-spec sizing for fashion-fit reasons. Tiffany's rings tend to run slightly larger than Cartier; both run differently than department-store brands. For boutique purchases, always check the brand's own size chart and prefer their stocked sizes over conversions.
Can I resize a ring later?: Most can be resized within 2 sizes either direction. Plain bands resize easily. Eternity bands, channel-set diamonds, tension settings, or some unusual designs can't be resized at all (the structure breaks). Resizing typically costs $20-100 depending on metal (silver/gold cheap, platinum more) and complexity. For online purchases, confirm resize-ability before buying.
What's circumference vs diameter?: Circumference is the inside perimeter of the ring (~54.4mm for US 7). Diameter is the inside straight-across measurement (~17.3mm for US 7). Diameter is what you measure if you have an existing ring on a flat surface; circumference is what string-and-finger gives you. Both convert to size; pick whichever your measurement method gives.
Do men's and women's rings size differently?: No — the underlying measurement systems are gender-neutral. But average sizes differ. US women average size 6-7; US men average size 9-10. So “size 7” on a women's ring isn't different from “size 7” on a men's ring; it's just less common in men's collections. Wide men's bands often size up half to compensate for thicker contact area.

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