Size guides › Will it fit?
Fit check
Will this diamond fit my ring setting?
Buying a new setting? Good news: most are made to fit the diamond you choose, in any style. Already have the ring, or eyeing a ready-made mounting? Then its head is a fixed size — and the carat weight alone won't tell you whether your diamond goes in.
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Two kinds of setting
Every prong setting has a head: the little basket of prongs that holds the stone. Bought new, the head is made to fit your diamond. In a ring you already have, it's a fixed size — too small and the stone sits loose; too big and it simply won't drop in.
Why "fits 1.5 ct" isn't enough
Ready-made settings are often sold as "fits 1.00–1.50 ct", and that range is almost always for a round diamond. Other shapes of the same weight are different widths: at 1.5 ct, a marquise is 5.88 mm wide and a round 7.37 mm.
Even two rounds of the same weight can differ by a third of a millimeter, depending on how deep they're cut — and a fixed head takes a stone only a tenth or so over its size. So the only reliable check is millimeters against millimeters.
How IfTheRingFits checks it
Pick a diamond — by shape and carat, or by pasting the measurements off its certificate — and a setting style. By default the answer is for a new setting made to fit, with the head size to ask for. Have the ring already? Tap "Already have the ring? Check its head size" and type the size from its listing.
A ring you already have
- FitsThe stone is within a tenth of a millimeter of the head's size.
- Only just fitsUp to 0.4 mm over: it goes in, but a setter has to thin the metal.
- Smaller than it's made forMore than 0.3 mm under: the prongs close a long way to hold it.
- Too wideIt won't go in. The tool says what head size it needs instead.
A new setting is simpler: every style can be made for your diamond. Past 8 ct, a jeweler builds the head.
Quick check: popular diamonds, popular settings
The head to ask for, and which styles are also sold ready-made in that size. Every style here can be made to order for every one of these diamonds. Tap a diamond for its full guide.
| Diamond | Head to ask for | Solitaire | Halo | Three stone | Bezel | Cathedral | East–west |
|---|---|---|---|---|---|---|---|
| 1 ct round | 6.4 mm | order | |||||
| 1.5 ct round | 7.4 mm | order | |||||
| 1.5 ct oval | 9.1×6.5 mm | order | order | order | order | order | order |
| 2 ct oval | 10×7.1 mm | order | order | order | order | order | order |
| 2 ct cushion | 7.4×7 mm | order | order | order | order | order | order |
| 2 ct emerald | 9.1×6.5 mm | order | order | order | order | order | order |
| 2.5 ct pear | 11.5×7.4 mm | order | order | order | order | order | order |
| 2.5 ct round | 8.7 mm | order | order | order | order | order | order |
comes ready-made in that size. order means choosing a setting made to fit, which most online retailers sell anyway.
Before you buy
Ask for the head size in millimeters
It's the one number that settles the question. Retailers often list it as "center stone size" or "head size" on the product page.
Check the certificate, not just the carat
A grading report lists the stone's length, width and depth. Those are what have to fit, so paste them into the tool for an exact answer.
Some styles measure differently
An east–west setting holds an elongated stone across the finger, so its length is what has to fit. A three-stone setting's side stones tighten the room lengthwise.
A head can often be changed
Many settings can be ordered with a different head, and a jeweler can often swap one. Ask before you give up on a ring you love.
Where these numbers come from. Ready-made head sizes are read from RIVA Precision and Ivy & Rose listings; the cathedral range is still an estimate. How far a stone can stray from a head's size comes from Ivy & Rose's bezel listings and Brian Gavin's notes on ready-made heads, and the made-to-order range from Blue Nile. Each is linked on setting styles. Use the figure from the actual product page whenever you have it.
Check your diamond and setting
Pick the stone, pick the style, and get a straight answer — then see it at true size on your own hand.
Questions
Do I need to check the fit if I'm buying a new setting?
Usually not. Most settings sold online are made to fit the diamond you choose — Blue Nile lists its settings for 0.20 to 8.00 carats in every shape. The fit matters for a ring you already have, or a ready-made mounting sold with a fixed head.
What is a head size?
The head is the small basket of prongs (or the metal rim, on a bezel) that holds the center stone. A head is sold by the size of stone it holds: a "6.5 mm round" head takes a 6.5 mm round diamond. Listings call it the center stone size or head size.
Does a setting for a 1 carat diamond fit any 1 carat diamond?
Not a ready-made one. A head quoted for "1 ct" is really sized for a 1 ct round, about 6.44 mm across. A 1 ct oval is 5.65 mm wide and 7.91 mm long, a 1 ct princess 5.49 mm square — and two stones of the same weight can differ by a few tenths of a millimeter depending on how deep they're cut.
Can I put a bigger diamond in my setting?
Only if its head can take it. A head fits a stone up to about a tenth of a millimeter over its size, and up to four tenths only by thinning the metal. Enter your ring's head size in the tool and it tells you. Past that, a jeweler can often replace the head.
What if the diamond is too small for the setting?
More than about 0.3 mm under the head's size, the prongs have to be bent a long way to reach it, which can leave it loose or look pinched. Ask for the head in the size the stone needs.