How do we figure carats from mm size and stone?
Count first. We start with the stone's typical strand weight from our buying notes, then scale by the beaded length you plan. When we lack a direct match, we estimate with bead-per-inch benchmarks and IGS estimating formulas that multiply dimensions by specific gravity. This gives repeatable totals when you are swapping sizes, mixing shapes, or trimming a strand to a bracelet length without guessing at the leftover weight.
For rondelles, measure diameter and height. The rondelle shape page shows how we record size as diameter by height. For ovals, take length by width as on our oval shape page, then record depth. Write it down.
- Beads per inch benchmarks we use: 6 mm about 4.2, 8 mm about 3.2, 10 mm about 2.5.
- Standard Jaipur-cut strands are about 13 to 16 inches; faceted rondelles are often 13 inches.
How many carats are 3 mm rondelle strands by stone?
Use these. These are the working weights we use when buying and stringing 3 mm rondelles. Each figure refers to a single continuous run of beads. Multiply by length as needed.
- Sapphire: assume about 20 carats per 8 inches at 3 mm (stone guide).
- Tourmaline: assume about 17 carats per 8 inches at 3 mm (stone guide).
- Ruby: assume about 8 to 15 carats per 8 inches of commercial faceted rondelles (stone guide).
- Emerald: assume about 10 to 15 carats per 8 inches of faceted rondelles (stone guide).
- Rhodolite garnet: assume about 35 carats per 16 inches of 3 mm rondelles (stone guide).
- Peridot: assume about 30 carats per 16 inches of 3 mm rondelles (stone guide).
- Apatite: assume about 28 carats per 16 inches of 3 mm rondelles (stone guide).
- Hessonite or spessartite: assume about 35 to 60 carats per 16 inches of rondelles (stone guide).
How many carats are 4 to 8 mm strands by stone?
Plan bigger beads differently. Here are common non micro sizes we handle often, with the strand weights we rely on. These figures help plan matched bracelets and fuller necklaces. If you design graduated runs, total the mid bead size for most of the strand, then add a small margin for the larger center section, which keeps estimates close.
- Ethiopian opal: about 20 carats per 8 inches at 4 mm (stone guide).
- Turquoise: about 45 carats per 16 inches of 4 mm rondelles or heishi; nugget strands about 200 carats per strand (stone guide).
- Lapis lazuli: about 70 carats per 16 inches of 5 mm rondelles (stone guide).
- Hessonite or spessartite: smooth 8 mm rounds about 240 carats per 16 inches (stone guide).
- Apatite: smooth 8 mm rondelles about 210 carats per 16 inches (stone guide).
- Iolite: about 150 carats per 16 inches of 8x10 mm ovals (stone guide).
How many 6 mm beads do you need?
Plan your run. We count beads first, then convert to carats using the stone-specific strand weights above. These counts assume plain stringing without knots. Adjust for clasps and spacers.
- 7.5 inch bracelet, about 6 inches beaded: 6 mm about 25 beads; 8 mm about 19; 10 mm about 15. 4 mm about 38.
- 7 inch bracelet, about 5.5 inches beaded: 6 mm about 23 beads; 8 mm about 17; 10 mm about 14. 4 mm about 35.
- Stretch bracelets use the full length: 7 inches of 8 mm about 22 beads; 7 inches of 6 mm about 29.
- 18 inch necklace: 6 mm about 70 beads; 8 mm about 52; 10 mm about 41. 4 mm about 104.
- 16 inch choker: 6 mm about 61 beads; 8 mm about 46.
How can I get carats per bead from a strand?
Divide the strand carats by the bead count for that same run. Use the bead-per-inch benchmarks above or the IGS bead chart for your size. Then confirm against our stone notes for that material. That crosscheck saves time when you reorder or remake a design for a client who wants a different length, and it reduces surprises when mixing batches.
Example, 3 mm sapphire: take about 20 carats per 8 inches and divide by the beads in that 8 inch run. For 4 mm Ethiopian opal, use about 20 carats per 8 inches and the bead count you plan to string. Example, 4 mm turquoise: start from about 45 carats per 16 inches, then divide by the beads on that strand.
Can I use an SG formula when I do not have a strand weight?
Yes. Measure diameter and height for a rondelle, or take the main dimensions for an oval, then apply the IGS constants. For rounds, use diameter squared times depth times specific gravity times 0.0018 to get carats. For ovals, multiply length by width, then by depth, then by specific gravity, then by 0.0026. Keep units in millimeters.
Specific gravities we rely on when measuring: apatite about 3.2, hessonite about 3.65, citrine about 2.65, pyrite about 5. Sample a few beads for depth to smooth out variation. When in doubt, check our stone pages like sapphire or tourmaline, then message us for a quick check. We reply fast.
For bead runs, we confirm by counting beads on the strand, estimating carats with the formulas, and sanity-checking against our known strand weights. It keeps design math consistent across sizes.