Eternity Ring Calculator.
Carit's eternity ring calculator works out stone count, spacing and gap for any ring size and stone width - round brilliant, princess, cushion, oval or any other shape. It's the tool bench setters reach for before they lay out a single bead. Inside the Carit app on iOS and Android.
Get the eternity ring calculator in the Carit app.
Precise stone-around-ring math plus visual layout preview. The app version shows the exact bead pattern before you touch a tool. Free tier + trial. Pro at $8.99/month or $69.99/year.
The math behind eternity stone spacing
An eternity ring's stone layout is geometry. The inner circumference of the band is π × inner diameter. Divide that by the total footprint of each stone (stone width + bead gap) and you have the stone count. Round that down (not up - always down, see tip below) and you have the number of stones. The real gap between stones after rounding is (circumference / stone count) − stone width.
Inner circumference = π × inner diameter
Stone count = floor(circumference / (stone width + target gap))
Actual gap = (circumference / stone count) − stone width
Worked example - Size 7 with 2.0 mm rounds
Size 7 ring = 17.3 mm inner diameter = 54.35 mm inner circumference. For 2.0 mm rounds with a target bead gap of 0.3 mm: each stone "uses" 2.3 mm. 54.35 / 2.3 = 23.6, round down to 23 stones. Actual gap: (54.35 / 23) − 2.0 = 0.36 mm. Close to target.
Worked example - Size M (UK) with 1.5 mm rounds
UK Size M = 16.7 mm inner diameter = 52.46 mm inner circumference. 1.5 mm rounds + 0.25 mm gap = 1.75 mm per stone. 52.46 / 1.75 = 29.98, round down to 29 stones. Actual gap: (52.46 / 29) − 1.5 = 0.31 mm. Slightly wider than target - acceptable.
Worked example - Princess-cut eternity, Size 8 with 2.5 mm princess
Size 8 ring = 18.1 mm inner diameter = 56.86 mm inner circumference. Princess stones sit edge-to-edge traditionally (minimal gap), so use 2.55 mm per stone. 56.86 / 2.55 = 22.3, round down to 22 stones. Actual gap: (56.86 / 22) − 2.5 = 0.08 mm - basically edge-to-edge, correct for princess.
Standard bead gaps by stone size
Target gap (before rounding) depends on stone size and setting style. Carit defaults to industry-standard values but lets you override.
| Stone size | Typical gap | Setting style |
|---|---|---|
| 1.0 - 1.2 mm | 0.15 - 0.20 mm | Micro pavé |
| 1.3 - 1.5 mm | 0.20 - 0.25 mm | Standard pavé |
| 1.7 - 2.0 mm | 0.25 - 0.30 mm | Channel / bead-set |
| 2.2 - 2.5 mm | 0.30 - 0.35 mm | Shared-prong / bead-set |
| 2.7 - 3.0 mm | 0.35 - 0.45 mm | Shared-prong eternity |
| 3.5 - 4.0 mm | 0.45 - 0.60 mm | Four-prong eternity |
Stone count reference table
Ballpark stone counts for common ring sizes and stone widths. Exact numbers will shift by ±1 depending on gap preference - use the Carit app for precise calculation.
| Ring size (US) | 1.5 mm stones | 2.0 mm stones | 2.5 mm stones | 3.0 mm stones |
|---|---|---|---|---|
| 5 (L½) | ~27 | ~21 | ~17 | ~14 |
| 6 (L½-M½) | ~28 | ~22 | ~18 | ~15 |
| 7 (N-O) | ~30 | ~23 | ~19 | ~16 |
| 8 (P½-Q) | ~31 | ~24 | ~20 | ~17 |
| 9 (R-R½) | ~32 | ~25 | ~21 | ~18 |
| 10 (T½) | ~34 | ~27 | ~22 | ~18 |
Half-eternity vs full eternity
A full eternity ring has stones all the way around. A half eternity has stones along the top half of the band only (the part that shows face-up), usually about 55-60% of the circumference. For a half eternity, use (circumference × 0.58) as your divisor instead of the full circumference.
Three-quarter eternity is also common - about 75-80% of the circumference - and is a good compromise: it looks like a full eternity from every angle that's visible during wear, but sizes more easily later (the plain section on the palm-side is where a jeweller cuts for a resize).
Why eternity setting is hard without the calculator
The stone count and spacing are the easy parts. What the calculator protects you from is the hard part: picking a stone size and discovering halfway through bench work that it doesn't divide evenly into the band, leaving you with a visibly off-centre last stone. For a client who's paying for an eternity, that's not acceptable. The calculator rules out bad combinations before you cut metal.
It also lets you plan backwards - client wants exactly 25 stones around a Size 7 band; what stone size should you order? Inverse calculation: (circumference / stone count) − typical gap = stone width. 54.35 / 25 − 0.3 = 1.87 mm. So 1.9 mm rounds will give you 25 evenly-spaced stones on a Size 7. Useful for sourcing melee.
Inside the app you see the layout.
Carit's in-app eternity tool doesn't just output numbers - it renders the stone layout as a visual preview so you can see the spacing before you mark out. Free to download, Pro unlocks the full tool.
While you're here
- Diamond setting calculator - all the setting math, including pavé layouts
- Carat weight calculator - TCW for the stones you're about to set
- Ring size calculator - get the inner diameter right before you calculate
- All Carit calculators →