Total Carat Weight Calculator

What total carat weight means

Total carat weight, written TW, TCW or "ct tw", is the combined weight of every stone in a piece of jewelry. It is useful for comparing two pieces with many small stones, and it is the number many listings lead with. It is also easy to misread, because a big total can come from many tiny stones.

This calculator lets you build up a piece one group of stones at a time. Enter a group by weight if you know it, or by size if you only have measurements. The tool adds them up and shows grams and the number of stones too.

Worked example: a halo ring

The calculator opens with a common design: a 1.00 ct center stone and a halo of eighteen round diamonds of 1.3 mm. Each 1.3 mm melee comes out at about 0.008 ct, so the halo adds about 0.15 ct. The total is about 1.15 ct TW. Because the halo stones are estimated from size, the total is shown as 1.14 to 1.16 ct.

A three-stone ring with a 1.00 ct center and two 0.30 ct sides is simpler: all three are known weights, so the total is exactly 1.60 ct.

An eternity band with twenty-five 2 mm round diamonds totals about 0.74 ct (likely 0.71 to 0.81 ct). That is a useful check when a listing for a similar band claims a much bigger number.

How to enter stones

1. One group per size and shape. If a ring has 1.3 mm stones in the halo and 1.1 mm stones down the shank, those are two groups.
2. Count carefully. Photograph the piece and count on the photo. Halos are easy to miscount by two or three.
3. Use weight when you have it. Lab reports and manufacturer specifications often list the center stone weight. Enter that exactly.
4. Use size for everything else. Measure the stone, or read the size from the manufacturer's spec sheet, and pick the material and shape.

Reading listings with total weight

• Ask for the center stone weight separately. It drives most of the price and most of the look.
• Check whether the total includes colored stones or only diamonds.
• Watch for rounding up. "1/2 ct TW" can cover a range of real totals depending on the trade rules in your country. Ask for the exact figure.
• Watch for DEW. Moissanite totals may be diamond equivalent, not real weight. See the DEW calculator.

Need the weight of a single stone from its measurements? Use the mm to carat calculator. Need the melee sizes for a diamond? See the diamond size chart.

Why melee is the hardest part

Small stones are cut quickly and their proportions vary much more than those of a center stone. A batch of 1.3 mm melee can easily differ by 10 to 20% in weight from stone to stone. Weight also grows with the cube of size, so a 1.4 mm stone weighs about 25% more than a 1.3 mm stone. A tenth of a millimetre matters. When melee makes up most of a total, treat the result as a good estimate, not a precise figure.