Which ayanamsa is correct?

There is no settled answer; the variants differ over which star fixes the zodiac and when the two zodiacs coincided. Lahiri is the most widely used and is the Indian government standard, which is why it is the default here.

How much does switching change my chart?

Between common variants, usually under a degree — invisible in the rashi chart unless a planet sits near a sign boundary. In D30 or D60, and in dasha start dates, the same difference is clearly visible.

Ayanamsa — the sidereal offset

The ayanamsa is the angle between the tropical zodiac, which starts at the equinox, and the sidereal zodiac, which is fixed against the stars. It is the single setting that most changes a Vedic chart, and the most common reason two calculators disagree.

Why it changes

The Earth's axis precesses, completing a slow circle roughly every 25,800 years. The equinox therefore drifts backwards through the constellations at about one degree every seventy-two years. The tropical zodiac follows the equinox; the sidereal zodiac does not. The gap between them grows steadily and currently stands at about 24 degrees.

The variants

Lahiri (Chitrapaksha) is the Indian government standard and the default here, fixing the star Chitra (Spica) at 180 degrees. Raman and Krishnamurti (KP) differ from Lahiri by small fractions of a degree. Fagan-Bradley is the Western sidereal standard. The differences are under a degree between most variants — small in the rashi chart, but enough to move a planet across a boundary in a high divisional chart or to shift a dasha start by months.

What to do about it

Pick one and stay with it. The tradition you learned in almost certainly has a default — Lahiri across most of India, KP for KP practitioners — and switching between them mid-analysis produces contradictions that look like the chart being unclear when they are really the settings being inconsistent. Every chart here states which ayanamsa it used and its exact value for that date.

Questions

Which ayanamsa is correct?: There is no settled answer; the variants differ over which star fixes the zodiac and when the two zodiacs coincided. Lahiri is the most widely used and is the Indian government standard, which is why it is the default here.
How much does switching change my chart?: Between common variants, usually under a degree — invisible in the rashi chart unless a planet sits near a sign boundary. In D30 or D60, and in dasha start dates, the same difference is clearly visible.

Cast this on a real chart

Enter a birth date, time and place and every calculation described here is available on the chart page.

Open the calculator

The kundali (birth chart) — A janam kundali is a map of the sky at a birth moment. How the rashi chart is built, why birth time matters to the minute, and how sidereal positions are computed.
Divisional charts — the shodashvarga — D1 through D60: what each varga is read for, from the hora to the shashtiamsha, and why the higher divisions demand arcsecond-accurate positions.
Vimshottari dasha — How the mahadasha sequence is calculated from the Moon's nakshatra, the nine planetary periods and their lengths, and how antardasha subdivisions are derived.