Astrological prescriptions for gemstones are almost always given in ratti — "wear a five-ratti yellow sapphire". Laboratories report weight in carats and grams. Sellers use whichever unit flatters the stone or confuses the buyer. Understanding the conversion is a small piece of arithmetic that closes a surprisingly common gap between what was prescribed, what was sold and what was delivered.
The units
Carat (ct). The international unit for gem weight: exactly 0.2 gram. One gram is five carats. Standardised in 1907 and used by every laboratory.
Gram (g). The SI unit. Used for Rudraksha, beads and larger articles; gemstones are reported in carats with grams sometimes alongside.
Ratti. A traditional Indian unit derived from the weight of the seed of the Abrus precatorius (gunja, or rosary pea). Because it was originally a seed weight, it varied by region and period. Two values are in modern use:
- Standardised ("pakki") ratti — conventionally 0.1215 g, i.e. about 0.6075 ct. This is the figure most jewellers and laboratories mean today and the one used in most conversion tables.
- Traditional ("kachi") ratti — often quoted at about 0.1094 g (some sources 0.115 g), i.e. about 0.55 ct, historically the older Indian standard, and still used by some practitioners.
A ten-ratti stone is therefore either about 6.1 ct (standardised) or about 5.5 ct (traditional) — a difference of more than half a carat, which on a fine sapphire is a great deal of money.
Quick conversions (standardised ratti = 0.1215 g)
| Ratti | Carats | Grams |
|---|---|---|
| 1 | 0.61 | 0.12 |
| 2 | 1.22 | 0.24 |
| 3 | 1.82 | 0.36 |
| 4 | 2.43 | 0.49 |
| 5 | 3.04 | 0.61 |
| 6 | 3.65 | 0.73 |
| 7 | 4.25 | 0.85 |
| 8 | 4.86 | 0.97 |
| 10 | 6.08 | 1.22 |
| 12 | 7.29 | 1.46 |
To convert carats to ratti: divide by 0.6075. To convert grams to ratti: divide by 0.1215.
Where the confusion is exploited
Selling by the smaller ratti, prescribing by the larger. A buyer told to wear "five ratti" and sold a stone described as "five ratti" by the traditional unit receives about 2.75 ct rather than 3.04 ct — 10% less stone.
Quoting ratti with no weight. A stone described only as "5 ratti" with no carat or gram figure cannot be checked. Ask for the weight in carats.
Rounding up. A 2.85 ct stone described as "five ratti" (which would be 3.04 ct).
Confusing weight with size. Ratti, carat and gram are all weight. Two stones of the same weight can differ in size because species differ in density — a 3 ct sapphire is smaller than a 3 ct emerald. Prescriptions are by weight; appearance is by size. Specific gravity explains why.
What the laboratory reports
Weight in carats to two decimal places (three on a precision balance), and in grams where useful. Dimensions in millimetres. The report does not usually state ratti, because the unit is ambiguous; the buyer converts using whichever ratti their adviser intends, and the carat figure is the anchor.
Frequently asked questions
Which ratti should I use?
Ask the person who prescribed the weight which ratti they mean. If they do not know, the standardised 0.1215 g is the safer assumption because it gives the larger stone.
Does the weight include the setting?
No. Gem weight is for the loose stone. A set stone's weight is estimated from its dimensions unless it is removed, and the report should say which.
Is a heavier stone always better?
Beyond the prescribed minimum, weight is a preference and a cost. Quality — colour, clarity, freedom from treatment — matters more than a fraction of a ratti.
How do I weigh a stone at home?
A jeweller's balance reading to 0.01 g gives grams; multiply by 5 for carats. Kitchen scales are not precise enough for gemstones.
Do Rudraksha use ratti?
No. Rudraksha are described by size in millimetres and weight in grams.