What synastry is
Synastry is relationship astrology done properly: it takes two complete natal charts and looks at how they talk to each other. Each person's planets are cast from their own birth data, and then we measure the aspects between one person's planet and the other's.
Your partner's Moon conjunct your Venus, say, is a classic 'easy affection' thread; their Saturn squaring your Sun reads as a more effortful, structuring bond. The chart also shows which of your houses their planets occupy — the areas of your life the relationship switches on.
Because it works from two real charts, synastry is far more specific than a sun-sign compatibility article that sorts everyone into twelve buckets. It needs both people's birth dates, times and places to be at its best.
How we build your synastry
First, each person's chart is computed independently and identically to our natal calculator — planets from sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine) (VSOP87/ELP), houses with the Porphyry system, angles from local sidereal time and latitude. Nothing about one chart depends on the other.
Then we generate cross-aspects: every pairing where one person's planet forms a major aspect (conjunction, sextile, square, trine or opposition) to the other's, within a sensible orb, listed tightest first. Separately, we drop each of the second person's planets into the first person's houses to produce the overlay table.
The compatibility score is a transparent heuristic, not a black box. It starts at 50 and adds for harmonious aspects and subtracts for hard ones, weighting the personal planets — Sun, Moon, Venus and Mars — heaviest and the outer planets least, and giving tighter orbs more influence. It is capped to a 5–95 band so no pairing reads as perfect or hopeless. We always show the aspects behind it, because the number means nothing without them.
What you get
Once both charts are entered:
- A compatibility score with a plain-language band and the reasoning behind it.
- The cross-aspect list — whose planet aspects whose, the aspect, and the orb.
- A short reading of each cross-aspect using both planets' traditional themes.
- House overlays showing where your partner's Sun, Moon, Venus and Mars land in your chart.
How to read a synastry chart honestly
No single aspect makes or breaks a relationship, and a chart with many 'hard' aspects is not doomed while a flowing one is not guaranteed. Synastry describes dynamics, not outcomes: squares and oppositions create friction, but friction is also energy and attraction; too many trines can read as comfortable but static. The skilled way to use it is to look at the whole weave — which planets are involved, whether the personal planets (especially the Moon and Venus) are supported, and where the load-bearing tension sits.
House overlays add the 'where'. Your partner's planets landing in your 7th (partnership), 4th (home) or 8th (intimacy) houses are traditionally read as the relationship finding natural rooms in your life. Treat every line as a question to reflect on with the actual person, not a verdict about them.
What an output looks like
Imagine one person's Sun is at 20° Aries and the other's at 22° Leo. That is a trine — same element, 120° apart, a 2° orb — and it would top the list as an easy, mutually supportive core-identity link. If, meanwhile, one person's Venus squares the other's Saturn, that appears as a tighter, more effortful thread around affection and pacing. The report shows both, each labelled with who aspects whom and how exact the aspect is.
Your own results are drawn entirely from the two real charts you enter, so they will differ in every placement from this illustration.
What this tool does not claim
Synastry is a symbolic framework, not a relationship forecast. There is no evidence that aspects between two charts predict whether a partnership lasts, and controlled research has not supported astrology's claims generally. The score especially is offered as one reflective summary, not a compatibility verdict — a low band does not mean 'don't', and a high one does not mean 'effortless'. Real relationships run on communication and choices, not on the sky.
Practically: the reading is only as good as both birth data. If either time is unknown, that person's Rising and houses are approximate, so the overlays for them are indicative only. We cast and compare the charts in your browser and never transmit either person's data.
Synastry vs. composite & Davison charts
Synastry keeps two separate people and maps the aspects between them. The composite chart instead merges you into a single 'relationship chart' by taking the midpoint of each matched planet, and the Davison chart casts one chart at the couple's midpoint in time and space. Those are different lenses on the same pair — synastry asks 'how do we interact?', the composite asks 'what is the relationship itself?'. You can build either with the composite & Davison tool once both natal charts exist.
Our method in one line
Two independent natal charts (sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine), Porphyry quadrant houses (whole-sign available on every chart) houses) overlaid by cross-aspects and house placements; score is a documented weighted heuristic. Reviewed by Dr. Amara Osei, Astronomy Reviewer.
Step by step
- 1
Enter your birth details
Your date, time and place — used to cast your natal chart exactly as the standalone calculator would.
- 2
Enter your partner's birth details
Their date, time and place in the second panel. Ask them, or use data they're happy to share.
- 3
Compare the charts
The score, cross-aspects and house overlays build instantly, entirely in your browser.
- 4
Read the aspects, not just the number
Use the score as a headline and the aspect list as the actual substance.
Tip — Both birth times matter most for the house overlays.
Frequently asked questions
- What is a 'good' synastry score?
- There isn't a threshold that makes a relationship work. The score is a weighted summary of how many core planets aspect each other harmoniously versus challengingly. Treat it as a reflection of the aspect list below it, and read the specific aspects rather than chasing a high number.
- Do we both need our birth times?
- For the deepest reading, yes. Cross-aspects between planets only need birth dates, but the house overlays — where one person's planets fall in the other's chart — depend on each Ascendant, which needs an accurate time. Without a time we estimate and flag the houses as approximate.
- Is a hard-aspect-heavy synastry a bad sign?
- Not necessarily. Squares and oppositions describe friction, and friction is also charge and attraction — many long-lasting charts have plenty. It's the balance, the planets involved and the overall weave that matter, not a raw count of 'difficult' aspects.
- Is our birth data private?
- Calculations run entirely in your browser. Your birth data is never uploaded, never stored on a server, and never shared. Both charts are computed locally in your browser; neither person's data is uploaded, stored or logged.
- Can I compare friends and family, not just partners?
- Yes. Synastry works for any two people — you can read the aspects between a parent, child, friend or colleague's chart and your own. The technique is the same regardless of the relationship type.
Written by
Elena Marchetti
Founder & Staff Astrologer
Methodology reviewed by
Dr. Amara Osei
Astronomy Reviewer
Published 2026-10-01 · Updated 2026-10-01 · Calculated with sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine)