Birth Chart Generators: Which One to Use
A straight comparison of the online birth chart generators worth using, what each does well, and which to trust when two of them disagree.
Every birth chart generator does the same arithmetic. Given a date, a time and a place, the positions of the planets are not a matter of opinion — they come from an ephemeris, and the good ones all use the same underlying data. So the differences between generators are not really about accuracy in the abstract. They are about what happens at the edges, and what the tool does with the numbers once it has them.
Here is what I would use, and when.
astro.com
Maintained by Astrodienst, the people who produce the Swiss Ephemeris that most other software runs on. That makes it the reference: when two generators disagree, this is the one to check against.
The interface has aged and the free chart options are buried behind a menu that assumes you already know what you want. There is no real way to organise saved work. None of that matters for its actual job, which is being unimpeachably correct and free.
Astro-Seek
The most useful free tool for quick work. Very wide range of calculators, no account needed, fast, and it handles unknown birth times more gracefully than most by simply telling you which parts of the chart are unreliable.
Good for lookups. Not built for keeping anything.
Cafe Astrology
Generates a chart and a substantial amount of interpretation text alongside it, which makes it a common first stop for beginners. The interpretation is generic by necessity — it is assembled per placement rather than synthesised — but there is a lot of it and it is readable.
Astrologer
What I work on, so weigh this accordingly. Swiss Ephemeris calculations, twelve house systems switchable per chart, an interactive wheel rather than a static image, and the option to save charts and keep notes on them. The public calculators need no account.
It is more than you need for a one-off lookup. Its case is when you are going to come back to the chart.
When two generators disagree
This happens more than people expect, and it is almost always one of three things.
The first is the house system. Placidus and Whole Sign will put the same planet in different houses roughly a third of the time. Neither is wrong; they are different conventions. Check what each tool defaulted to before concluding anything is broken.
The second is the zodiac. Tropical and sidereal differ by around 24 degrees, which is most of a sign. If one tool says Sagittarius and another says Scorpio, check whether one of them is set to sidereal.
The third, and the one that actually indicates a bug, is the timezone. If the planetary positions agree but the Ascendant and house cusps do not, one of the tools has applied the wrong historical offset. Wartime daylight saving, half-hour zones, and locations that changed their zone rules are where this bites. When this is the disagreement, trust astro.com.
What you need before you start
Date and place are usually easy. Time is where it falls apart, and it matters more than anything else: the Ascendant moves about a degree every four minutes, so a chart cast for an approximate time has an approximate Ascendant and approximate houses.
If you do not know your birth time, most jurisdictions record it on the long-form birth certificate, and it is worth ordering one. Failing that, cast the chart for noon and treat the Sun and the outer planets as solid, the Moon as probably right, and the Ascendant and houses as unknown. A tool that gives you a confident rising sign without a birth time is guessing and not telling you.
The honest summary
For a single accurate chart, free: astro.com. For quick exploratory lookups: Astro-Seek. For a chart with a lot of readable interpretation attached: Cafe Astrology. For charts you intend to keep, annotate and return to: something with an account, which is where Astrologer comes in.
If you want to go deeper on the accuracy question specifically, the piece on why calculators disagree goes through it properly.
Frequently asked questions
Which birth chart generator is most accurate?
astro.com, because it is maintained by Astrodienst, who produce the Swiss Ephemeris that most other software runs on. When two generators disagree, it is the one to check against.
Why do two birth chart generators give different results?
Almost always the house system (Placidus versus Whole Sign moves about a third of placements), the zodiac (tropical and sidereal differ by roughly 24 degrees), or the timezone. Only the third is an actual error.
Can I get a birth chart without knowing my birth time?
Yes, but the Ascendant and house placements cannot be determined. The Sun and outer planets stay accurate and the Moon is usually right. A tool that gives you a confident rising sign with no birth time is guessing.
Is there a free birth chart generator with no signup?
Several. astro.com, Astro-Seek and the public calculators at astrologerapp.org all cast charts without an account.