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Ring Size Converter

Convert ring sizes between US, EU, UK, Japan and diameter in millimeters.

Ring Size Chart
US / CanadaEU / ISOUK / AU / NZJapanDiameter (mm)
344F½413.97
3.545G½514.36
446H614.76
4.547I715.09
549J½815.7
5.550K½916.1
651L1016.51
6.552L½1116.92
754M½1217.35
7.555N1317.75
856O1418.19
8.557O½1518.53
959P½1619.05
9.560Q1719.41
1061R1819.84
10.562R½1920.2
1163S½2020.68
11.565T2121.08
1266U2221.49
12.567V2321.89
1368W2422.33

About this tool

The Ring Size Converter translates a ring size between the US, EU, UK, and Japan sizing standards and shows the corresponding inner diameter in millimeters. Because each country measures ring sizes on its own scale, a US size means nothing to a UK or Japanese jeweler without conversion, which is exactly the gap this tool closes.

To use it, enter a size you already know in any one standard, or type in the inner diameter in millimeters, and read off the equivalents in every other system at once. It is handy when buying rings abroad or online, choosing a gift, or working out the right size before a resizing.

As a tip, measuring the inner diameter of a ring that already fits is the most reliable starting point, since it avoids guesswork from charts. Everything runs locally in your browser, so nothing you enter is sent anywhere.

Frequently Asked Questions

Code Implementation

import math

# Ring size conversion tables
# Circumference (mm) → US size
CIRCUMFERENCE_TO_US: list[tuple[float, float]] = [
    (44.2,  3.0), (45.5,  3.5), (46.8,  4.0), (48.0,  4.5),
    (49.3,  5.0), (50.6,  5.5), (51.9,  6.0), (53.1,  6.5),
    (54.4,  7.0), (55.7,  7.5), (57.0,  8.0), (58.3,  8.5),
    (59.5,  9.0), (60.8,  9.5), (62.1, 10.0), (63.4, 10.5),
    (64.6, 11.0), (65.9, 11.5), (67.2, 12.0), (68.5, 12.5),
    (69.7, 13.0),
]

# EU size = circumference rounded to nearest integer (mm)
def circumference_to_eu(mm: float) -> int:
    return round(mm)

# UK letter sizes A–Z (size A = 37mm circumference, each step ≈ 1.25mm)
def circumference_to_uk(mm: float) -> str:
    index = round((mm - 37.0) / 1.25)
    index = max(0, min(index, 25))
    return chr(ord('A') + index)

def diameter_to_circumference(diameter_mm: float) -> float:
    return diameter_mm * math.pi

def circumference_to_diameter(circumference_mm: float) -> float:
    return circumference_mm / math.pi

def circumference_to_us(mm: float) -> float | None:
    """Find closest US ring size for given circumference."""
    best = min(CIRCUMFERENCE_TO_US, key=lambda x: abs(x[0] - mm))
    if abs(best[0] - mm) <= 0.7:  # within ~0.7mm tolerance
        return best[1]
    return None

# Example: convert diameter to all sizes
diameter_mm = 16.5
circumference = diameter_to_circumference(diameter_mm)

print(f"Diameter:       {diameter_mm:.1f} mm")
print(f"Circumference:  {circumference:.1f} mm")
print(f"US size:        {circumference_to_us(circumference)}")
print(f"EU size:        {circumference_to_eu(circumference)}")
print(f"UK size:        {circumference_to_uk(circumference)}")
# Diameter:       16.5 mm
# Circumference:  51.8 mm
# US size:        5.5
# EU size:        52
# UK size:        M

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