Measure it with your phone and a bank card
Every other calculator asks for a measurement you probably do not have. This one takes the measurement for you: photograph your ring beside any bank card, mark the card, and get its true size in millimetres — then a ring size, a carat weight, or a metal value.
1. Take the photo
Lay the ring flat on a table next to any bank, credit or debit card — they are all exactly the same size by international standard. Shoot from directly above, with both fully in frame. Good light, no flash glare.
Your photo is processed entirely inside your browser and is never uploaded.
2. Mark the card
Pick one and drag the two points across it
Why a bank card
Every bank, credit and debit card in the world conforms to ISO/IEC 7810 ID-1 and measures exactly 85.60 × 53.98 mm. It is the one precisely-dimensioned object almost everybody already carries, which makes it a free and perfectly accurate ruler.
Why the angle of the photo does not matter
A phone is never exactly square to the table. If you simply compared pixel counts, a photo taken at a modest angle would be several percent out — and because a diamond's weight rises with the cube of its diameter, a 2% error in millimetres becomes roughly a 7% error in carats. On a $5,000 stone that is a few hundred dollars.
So this tool does not compare pixels. Marking the card's four corners fully determines the projective transform between the photo and the real table top, and every later measurement is mapped through it. The angle is cancelled out rather than ignored. The tool tells you how oblique your shot was, and asks you to retake it if it is too extreme to trust.
Nothing is uploaded
The photograph is read into your browser and measured there. It is never sent to us, never stored, and disappears when you close the tab. Our privacy policy explains the rest.