The calculation begins by dividing the 360-degree sidereal zodiac into 27 equal nakshatras. Each nakshatra spans 13°20′, or 13⅓ degrees. Every nakshatra is further divided into four padas of 3°20′ each. The Moon's exact longitude determines both its nakshatra and its progress through that nakshatra.
The planetary ruler repeats in this order: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, and Mercury. Because there are 27 nakshatras and nine rulers, each ruler governs three nakshatras. Ashwini begins with Ketu, Bharani with Venus, Krittika with Sun, and so forth until Revati, ruled by Mercury.
The first formula calculates the fraction of the birth nakshatra still remaining. If the Moon has completed 35% of a nakshatra, the remaining fraction is 65%, or 0.65. Multiply this fraction by the Mahadasha ruler's complete duration to determine the balance remaining at birth.
Mahadasha Balance = (1 − Nakshatra Progress Fraction) × Planetary Duration. For a Moon-ruled nakshatra with 35% completed, the remaining balance is 0.65 × 10 = 6.5 years. The next planetary period begins after this remaining interval has elapsed.
The Vimshottari Dasha Calculator converts these durations into approximate calendar dates using 365.2425 days per year. This is a Gregorian mean-year convention. Some Jyotish programs use a 360-day savana year instead, producing different calendar dates even when the starting nakshatra and planetary order agree.
The Antardasha formula is similarly proportional: Antardasha Duration = Mahadasha Duration × Antardasha Ruler Duration ÷ 120. Within a twenty-year Venus Mahadasha, for example, the Venus Antardasha lasts 20 × 20 ÷ 120 = 3⅓ years, while the Sun Antardasha lasts 20 × 6 ÷ 120 = one year.
The first Antardasha of a Mahadasha is ruled by the Mahadasha lord itself. The remaining subperiods follow the established nine-planet sequence. When the Moon's exact birth position is known, the calculator can also locate the portion of the birth Mahadasha's Antardasha sequence active at the birth instant.
For calendar accuracy, the birth date, local birth time, and UTC offset must be entered consistently. The UTC offset does not alter a Moon longitude already calculated elsewhere. It simply establishes the instant from which this calculator's dated periods are measured.