Every diamond size guide shows you a stone on a stock hand. This one shows it on yours, at true millimeter scale, and tells you whether it physically fits the setting you are looking at.
Diamonds are sold by carat, which is a weight. What you see on a hand is a size — and the two come apart, because depth is free to vary. Two stones of the same weight can differ by a third of a millimeter across the face, and two stones the same size across the face can be a fifth of a carat apart in weight.
So "1.5 ct" does not tell you how big a ring will look, and a millimeter figure on a product page does not tell you how big it will look on your finger. This site closes that gap: photograph your hand, set the scale from something whose real size is known, and see the stone rendered at the size it actually is.
It also answers the question almost nothing else does — whether the stone fits the head. A setting is built around a seat of a particular diameter. Clearances run a tenth to half a millimeter, so a stone can be exactly the size you wanted and still not sit in the ring you picked.
IfTheRingFits is built and run by Steven, working independently. It is not a retailer and is not owned by one. Nobody pays to appear here, no retailer has any say in what the tool reports, and the fit verdict is arithmetic on millimeters — it could not be influenced by money even if someone tried.
It is free, carries no advertising, and is intended to be funded by affiliate commissions on links out to retailers. As of the date above, links to Blue Nile and Rare Carat are affiliate links and may earn a commission. With Clarity pays this site nothing. The disclosure page is generated from the code that builds those links, so it changes in the same deploy the first paid link does — it cannot quietly fall out of date.
Your photo never leaves your device. It is decoded, measured and rendered in your own browser, and no server ever receives it. The hand detection and the stone-removal tool are downloaded and run locally too. The privacy page sets out the detail, including the one request the site makes to anything other than this domain.
Honest about its limits, which is the point. The scale is only as good as what you anchor it to: a bank card in the shot is the most accurate, and your ring size is quick but good to about half a size. The app says which one it used and warns when two methods disagree.
One correction the tool does make. The scale is measured across the finger, and a finger’s widest point is halfway down its side — so that scale describes a plane running through the middle of it. The top of the finger is nearer the camera than that, and the stone the ring holds above the top is nearer still, and a camera makes nearer things larger. The stone is drawn about 4% bigger to allow for it, assuming the photograph was taken at arm’s length. Applying no correction would amount to assuming the camera was infinitely far away, which is the one distance it certainly was not.
The largest single source of error is that fingers are oval rather than round, so the conversion from ring size to finger width is an approximation. That is stated in the interface rather than buried here. A new setting is usually made to fit the diamond, so the fit check names the head size to ask for; for a ring you already have, enter its head size and the check is against that. The ready-made head sizes per style are read from named retailer listings, cited on the setting styles guide — still, use the figure from the product page whenever you have one.
Corrections, questions, and reports of anything that looks wrong are all welcome, and the third of those most of all. Write to hello@iftheringfits.com.