Compare two charts, not just two signs
Synastry Calculator
This free synastry calculator compares two birth charts and lists the aspects between them, such as Sun to Moon, Venus to Mars and Moon to Moon. Enter both birth details and the synastry calculator shows each person’s placements, the closest connections and what traditional astrology says about each one, without calling a relationship good or bad.
Compare two birth charts
Enter both birth details. A birth time and place give the fullest result; without a time the tool leaves out the Ascendant and marks the Moon as approximate. The calculation runs in your browser; nothing you enter is sent to the site.
Your synastry result
Key connections
All aspects between the two charts
| First chart | Aspect | Second chart | Orb | Nature |
|---|
No aspects fall within the orbs used. That is unusual and says nothing bad about a relationship.
Placements used
Remember: synastry is a symbolic tradition for reflection and conversation. Supportive aspects do not guarantee ease and challenging ones do not doom a relationship.
How this was calculated
How to use the Synastry Calculator
- Enter the date of birth, birth time and birthplace for both people. Names are optional labels.
- Choose a nearby city or enter latitude and longitude, then check the time zone that applied at birth.
- If a birth time is unknown, tick the box for that person. Expect a less complete result.
- Select Compare the two charts, then read the key connections first and the full aspect table after that.
What is a synastry chart?
A synastry chart is a comparison of two birth charts. Instead of asking what each person’s planets say about them alone, it asks how one person’s planets sit against the other’s. When a planet in one chart makes a close angle to a planet in the other, astrologers read that as a link between the two themes: for example, one person’s Venus touching the other’s Mars is traditionally linked with attraction.
Searches for relationship compatibility by birth chart often lead to scores and percentages. Synastry does not really work that way. It produces a list of connections, each with its own tone, and leaves the weighing to the reader. That is how this tool presents it too.
Method: positions, aspects and orbs
The tool calculates each chart from first principles in the browser and then compares them. The steps are listed so that you can check them.
Step 1: Convert each local birth time to UTC using the selected time zone, including daylight saving in force on that date. If no time is given, 12:00 local time is used.
Step 2: Calculate the ecliptic longitude of the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Planets use orbital elements with perturbation terms for Jupiter and Saturn; the Moon uses a 60-term lunar series. With a birth time, the Ascendant is added from sidereal time, obliquity and latitude.
Step 3: For every body in the first chart and every body in the second, measure the angular separation and check it against the five major aspects and their orbs.
Step 4: Sort the aspects by orb (the closer to exact, the stronger) and highlight the classic relationship contacts.
| Aspect | Angle | Orb used | Traditional reading |
|---|---|---|---|
| ☌ Conjunction | 0° | 8° | Blending: the two themes are fused and strongly felt |
| ⚹ Sextile | 60° | 4° | Supportive when you use it |
| □ Square | 90° | 6° | Friction that can motivate growth |
| △ Trine | 120° | 6° | Flowing, easy support |
| ☍ Opposition | 180° | 7° | Attraction and push-and-pull |
Aspects are measured between tropical longitudes. In the sidereal option both charts shift by the same ayanamsa, so the angles are identical and only the sign names change.
Worked example: two charts, one comparison
Take Person A, born 24 August 1990 at 14:30 in Karachi (UTC+05:00, so 1990-08-24 09:30 UTC), and Person B, born 3 May 1993 at 21:40 in London (UTC+01:00 in summer, so 1993-05-03 20:40 UTC). The tool finds these placements for A: Sun 0°57′ Virgo; Moon 17°27′ Libra; Mercury 23°29′ Virgo; Venus 12°49′ Leo; Mars 26°19′ Taurus; Jupiter 1°15′ Leo; Saturn 19°25′ Capricorn; Ascendant 21°15′ Sagittarius. For B: Sun 13°24′ Taurus; Moon 11°55′ Libra; Mercury 29°54′ Aries; Venus 6°02′ Aries; Mars 2°50′ Leo; Jupiter 5°56′ Libra; Saturn 29°11′ Aquarius; Ascendant 27°21′ Scorpio.
Comparing the two charts gives 13 aspects within orb: 5 supportive, 6 challenging and 2 blending. The tightest are A’s Venus square B’s Sun (orb 0.6°), A’s Venus sextile B’s Moon (orb 0.9°) and A’s Mars opposition B’s Ascendant (orb 1.0°). Among the classic contacts, A’s Venus sits in a close square to B’s Sun and a sextile to B’s Moon. Read traditionally, that mixes warmth and appreciation toward B’s emotional side with a bit of friction toward B’s core identity. A real reading would weigh these against the rest of the list rather than picking one.
Reading synastry aspects sensibly
- Start with the tightest orbs. An aspect within 2° or 3° is usually discussed first; one at the edge of its orb is weaker.
- Look for patterns. Several aspects pointing the same way tell you more than one on its own.
- Remember both people. An aspect belongs to both charts, but the two people may experience it differently.
- Do not read “hard” as “bad”. Squares and oppositions are frequently found in long, committed relationships.
- Keep it conversational. The most useful use of a synastry reading is to ask each other whether it rings true.
The classic synastry connections
These are the pairs the tool highlights as key connections, with the theme each is traditionally linked to.
- Sun and Sun: core identity and life direction.
- Sun and Moon: ego and emotional needs.
- Moon and Moon: emotional rhythms and what feels like home.
- Venus and Mars: attraction and desire.
- Sun, Moon and Venus: affection, appreciation and tenderness.
- Mercury and Mercury: how you talk and think together.
- Mars and Mars: drive and the way each handles conflict.
- Saturn to Sun, Moon or Venus: commitment, responsibility and sometimes restriction.
- Ascendant to Sun or Moon: first impressions and recognition.
Why birth time and place matter
Planet signs depend mostly on the date. The Ascendant depends on the exact time and place, and the Moon changes enough over a day that an unknown time blurs it. If you do not know a birth time, you can still use the tool; it will leave out the Ascendant and tell you that Moon contacts are approximate. Check hospital or birth-certificate records where you can, and remember that the time zone in force at the time of birth is what counts, not today’s.
What this tool does not do
It does not calculate houses, house overlays, Uranus, Neptune, Pluto, Chiron, the lunar nodes, asteroids or a composite chart, and it does not give an overall score. Those are deliberate limits: we would rather show a smaller set of correct calculations than a long report we cannot verify. For a simpler comparison of Sun signs only, use the Zodiac Compatibility Calculator.
Common mistakes and limitations
- Wrong time zone or daylight saving: a one-hour mistake can shift the Ascendant by a whole sign for some births.
- West longitudes entered as positive: places in the Americas have negative longitudes.
- Counting every aspect equally: a 0.5° orb is very different from a 7° one.
- Treating the result as a verdict: synastry is for reflection, not for deciding whether to stay together or part.
- Comparing across zodiac systems: keep both charts in the same system, as the tool does.
Frequently asked questions
What is synastry in astrology?
Synastry is the comparison of two birth charts. Astrologers place one person’s planets against the other’s and look at the angles, called aspects, between them. Each aspect is read as a theme in the relationship, such as attraction, communication or commitment.
Do I need my birth time for a synastry chart?
It helps a lot. The Ascendant needs an accurate time and place, and the Moon moves about 13° a day, so without a time it could be up to roughly 6° away from the position we use at noon. Without a time the tool leaves out the Ascendant and flags the Moon as approximate.
Which synastry aspects matter most?
Astrologers differ, but the contacts most often discussed are Sun–Moon, Moon–Moon, Venus–Mars, Sun–Venus and Moon–Venus, along with Saturn contacts to the Sun, Moon or Venus. The tool highlights these as key connections and lists every other aspect below them.
Are squares, oppositions and Saturn aspects bad?
Not in the way the word suggests. Tradition reads them as friction, tension or responsibility, which can feel hard and can also drive growth and commitment. Easy aspects can lead to complacency. Neither type decides whether a relationship works.
Why does the tool only use the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn and Ascendant?
Those are the bodies we can calculate reliably in the browser and they carry most traditional synastry readings. Uranus, Neptune, Pluto, the lunar nodes, houses, house overlays and composite charts are not included, and we say so rather than imply a full report.
How accurate are the planet positions?
We checked them against an independent astronomy library over 300 dates between 1920 and 2030. The Sun, Moon, Mercury and Venus agreed within about 0.02°, and Mars, Jupiter and Saturn within about 0.05°. That is accurate enough for signs and aspects at normal orbs, though a result very close to an orb limit can still tip either way.
What is the difference between synastry and a composite chart?
Synastry compares two separate charts and keeps both people’s planets. A composite chart blends the two charts into a single chart for the relationship itself, using midpoints. This tool does synastry only.
Is my birth data saved?
The calculation runs in your browser and the details you enter are not sent to the server by this tool. Ordinary hosting logs and other website technologies may work separately; see the Privacy Policy.
Related tools and resources
Keep exploring, or read how SOULTRIANGLE documents its methods.
Our commitment to transparency
We explain the method, show the working and aim to correct problems when they are reported. If you spot an error, please use our Contact page and include the tool name and a non-sensitive example.
Last updated: October 18, 2026 · Method: planet longitudes from orbital elements, Moon from a 60-term series, Ascendant from sidereal time; five major aspects with stated orbs.