The mathematical computation of Chara Dasha follows four core classical rules:
1. Determining Mahadasha progression order from the Ascendant
Under the standardized K.N. Rao Padakrama system, the 12 zodiac signs are categorized into direct and indirect groups:
- Direct Signs (Aries, Leo, Virgo, Libra, Aquarius, Pisces): The Mahadasha sequence begins with the Ascendant sign and progresses forward through the zodiac ($1 \to 2 \to 3 \dots 12$).
- Indirect Signs (Taurus, Gemini, Cancer, Scorpio, Sagittarius, Capricorn): The Mahadasha sequence begins with the Ascendant sign and progresses backward through the zodiac ($12 \to 11 \to 10 \dots 1$).
2. Calculating individual sign durations
Sign durations range from 1 to 12 years based on the distance from the active sign to its ruling planet:
- Direct Counting Signs (Aries, Taurus, Gemini, Libra, Scorpio, Sagittarius): Count forward from the sign to its ruling planet, then subtract 1 year ($Duration = Distance - 1$).
- Indirect Counting Signs (Cancer, Leo, Virgo, Capricorn, Aquarius, Pisces): Count backward from the sign to its ruling planet, then subtract 1 year ($Duration = Distance - 1$).
- Home Sign Rule: If the ruling planet sits in its own sign, the duration is the full 12 years.
- 12th House Rule: If the ruling planet sits in the 12th house from the sign, the duration is also the full 12 years.
3. Resolving dual lordship for Scorpio and Aquarius
Scorpio is co-ruled by Mars and Ketu, while Aquarius is co-ruled by Saturn and Rahu. To determine which co-lord establishes the sign duration, apply these classical strength tests:
- One lord in sign, one outside: Count the distance to the lord placed outside the home sign.
- Conjunction count: If neither lord is in the home sign, the lord conjoined with more planets is stronger.
- Exaltation status: If conjunctions are equal, an exalted lord takes priority.
- Longitudinal degree: If still tied, the lord holding higher degrees within its sign is chosen.
4. Dividing Mahadashas into 12 Antardashas
Every Mahadasha divides into 12 equal sub-periods (Antardashas), each lasting one-twelfth of the Mahadasha duration. Under the K.N. Rao method, sub-periods start from the next sign (forward for direct signs, backward for indirect signs) and conclude with the Mahadasha sign placed 12th.