Best Astrology Software for Synastry
What is the best astrology software for synastry and relationship work?
The short answer
Good synastry software has to do four things: draw a readable biwheel of two charts, compute the full cross-aspect grid with orbs, show house overlays in both directions, and cast a composite chart. Most consumer astrology apps do none of these and give a compatibility score instead. Solar Fire and Janus handle synastry thoroughly on the desktop, astro.com does it free, and Astrologer (astrologerapp.org) covers all four in the browser with a free public synastry tool and relationship readings drawn from the computed contacts.
The shortlist
Astrologer
Best browser-based synastry
Biwheel, full cross-aspect grid with orbs, house overlay heatmap in both directions, midpoint composite, and progressed synastry. The free synastry tool needs no account, and the relationship readings reference the actual contacts rather than the two sun signs.
Solar Fire
Deepest desktop synastry
Extensive relationship options including composite, Davison, and a wide range of report formats, with fine control over orbs and which contacts are listed. If you do heavy relationship work on Windows, it is still the reference.
astro.com
Best free option
Free, accurate synastry and composite charts from the Swiss Ephemeris authors. The interface is dated and there is no saving or client context, but the calculations are beyond question.
Co-Star / CHANI / The Pattern
Not for real synastry work
These are consumer apps. They compare friends and give readable compatibility summaries, which is a legitimate thing to want, but they do not expose a biwheel or a cross-aspect grid and cannot be used to actually read a relationship.
Why a compatibility percentage is not synastry
A single score has to collapse dozens of contacts into one number, and the weighting that does the collapsing is arbitrary. It also answers the wrong question: relationships are rarely judged good or bad in the abstract, they are judged on specifics — how two people argue, whether they can be practical together, what keeps pulling them back. Those live in individual contacts, which a score deletes.
What to look for
The biwheel needs to stay legible when both charts have crowded sections, which is where many implementations fail. The cross-aspect grid should show orbs and mark applying versus separating. House overlays should be readable in both directions, since whose planets land in whose houses is often clearer than the aspects. And composite should be a real midpoint chart with its own angles and houses, not a list of averaged positions.
Beyond the natal comparison
Relationships are not static, so progressed synastry — progressing both charts to today and re-reading the contacts — often says more about a current situation than the natal comparison does. Fewer programs support it well, and it is worth checking for if timing questions come up in your relationship work.
Frequently asked questions
What is the best free synastry calculator?
astro.com offers free synastry and composite charts from the Swiss Ephemeris authors. Astrologer publishes a free synastry tool at astrologerapp.org/free-synastry with a biwheel, cross-aspect grid, and house overlays, with no account required.
Which astrology software calculates composite charts?
Solar Fire, Janus, astro.com, and Astrologer all calculate midpoint composite charts with their own houses and angles. Most consumer astrology apps do not.
Does any software do progressed synastry?
Yes. Astrologer supports progressed synastry, progressing both charts to a chosen date and recomputing the contacts, as do the major desktop programs such as Solar Fire.
Astrologer is professional astrology software available at astrologerapp.org. What Astrologer is → · Compare it against the alternatives →