What ring size is 18 mm?
A 18 mm inner diameter is a US 8 — UK P½, EU 57, Japan 16.
Measure again at a different time. If the result stays between sizes, compare band width, knuckle clearance, and the retailer's chart before choosing. A wider band can feel tighter than a narrow one at the same labelled size.
- US
- 8
- UK
- P½
- EU
- 57
- Japan
- 16
- Inner diameter, millimetres
- 18
- Circ mm
- 56.5
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Frequently asked questions
Is 18 mm a diameter or circumference?
On this page 18 mm is the inner diameter (the width across the inside of the ring). The matching inner circumference is about 56.5 mm.
I am between sizes — should I size up or down?
Measure again at a different time. If the result stays between sizes, compare band width, knuckle clearance, and the retailer's chart before choosing. A wider band can feel tighter than a narrow one at the same labelled size.
How should I confirm a converted ring size before ordering?
Measure the same finger or a ring that fits with a second method. For an expensive, wide, or non-resizable ring, check the retailer's chart and confirm with a physical sizer or jeweller.
Values computed from the US jewellers’ diameter scale and cross-checked against ISO 8653 — methodology last reviewed 2026-08-27. Methodology