Smart ring sizing

1. Order the kit for your exact generation. Ring 4 kit for Ring 4 (white plastic sizers, sizes 4–15); Gen 3 kit for Gen 3 (black sizers, sizes 6–13). A kit for another generation, or another brand, does not size this ring.
2. Wear the test ring on the finger you will actually wear the ring on. Oura recommends the index, middle or ring finger. Wear it with the size number visible on top of the hand, as the kit instructs.
3. Wear it for at least 24 hours before deciding. Fingers swell and shrink through the day and with heat and activity. A size that feels right for ten minutes in the morning can feel tight by evening — and a smart ring that is too loose gives worse sensor readings, not just a worse fit.
4. Confirm the size, then order that size. Do not adjust the kit result toward your jewellery size. If the kit says 8 and your jewellery size is 7, order 8: the kit, worn for a day, is the measurement for this ring.

• Ordering your jewellery size as your Oura size. The numbers look the same; the rings are not. Oura explicitly advises against sizing from another ring.
• Reusing a Gen 3 size for a Gen 4 ring (or the reverse). Different interiors, different fit — re-run the kit for the generation you are buying.
• Deciding after a few minutes. A smart ring is worn all day and night. The 24-hour test exists because a finger at 9 am is not the same finger at 9 pm.
• Sizing on the wrong finger. Index, middle and ring fingers differ by whole sizes. Size the finger the ring will live on, not the finger that is easiest to test.
• Trusting an approximate conversion table. Some sites publish Oura-to-UK approximations. Oura does not officially convert, so any such table is guidance at best — and a kit test makes it unnecessary.

The same principle applies to every smart ring brand: each brand sizes on its own scale, with its own kit or fit test, and none of them converts reliably from jewellery sizes. If a brand offers a sizing kit, use it; if it offers a printable sizer or an in-app fit check, follow that brand's instructions for that brand's ring. A millimetre measurement of your finger is still useful background — it tells you roughly where you sit — but it is not a substitute for the brand's own sizing method.

This guide gives no smart-ring conversion table, deliberately: no reliable conversion from jewellery sizes to Oura sizes exists, and any table claiming one would be inventing it. Oura's own guidance — use the generation-correct sizing kit — is the only method this page recommends. Other smart-ring brands have their own scales; their kits, not this page, are the reference for those rings.