You do not need their ring size
People delay proposals over this. They should not.
If you are buying a surprise
Any one of these gets you close. Resizing afterwards is routine.
Borrow a ring they wear
We can read its size. Note which finger.
Trace one on paper
Less exact, but a good place to start.
Ask someone who knows
A sibling or close friend has usually been told.
Or plan to resize it
Plenty of people propose with a ring resized after.
Things that change the answer
Fingers are not a fixed size, which is why an online chart is only ever a guess. Two of these are about the hand and two are about the ring.
The hand
It changes through the day
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Heat and humidity
Fingers swell in heat and humidity, and Maui has both. A ring measured at home in winter can feel different here.
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Time of day
Hands swell through the day. Late afternoon is a more accurate measurement than first thing.
The ring
And the same number fits differently
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Band width
A wide band sits tighter than a thin one at the same nominal size.
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Knuckles
If the knuckle is larger than the base of the finger, the size has to be a compromise between the two.
An online chart is only ever a guess. A ring that already fits is the better guide.