Free Synastry Chart Calculator
Compare two birth charts to see how you actually interact — your cross-aspects, house overlays, compatibility score, and a personalized AI reading. No signup.
Enter both people's birth details. Birth times sharpen the house overlays and angles, but the planet-to-planet aspects work even without them.
What Is a Synastry Chart?
A synastry chart overlays two people's birth charts to reveal the aspects between them — how your planets touch each other's. It shows the chemistry, friction, and dynamics of how two people experience one another.
Synastry is the astrology of relationships. Instead of one chart, you read two charts together, looking at how each person's planets connect to the other's — your Venus to their Mars, your Moon to their Sun, and so on.
These cross-aspects describe the living dynamic between you: where there is ease and attraction, where there is tension and growth, and where two lives naturally fit or rub against each other.
How to Read a Synastry Chart
Four steps from chemistry to the bigger picture.
- 1
Read the inter-chart aspects
Look at how one person's planets aspect the other's — your planet to their planet. These cross-aspects are the heart of synastry.
- 2
Check the house overlays
See which of your houses the other person's planets fall into. This shows what areas of life they light up for you.
- 3
Find the strongest contacts
Sun-Moon, Venus-Mars, Moon-Moon, and Saturn contacts are the headline aspects — they reveal emotional fit, attraction, and commitment.
- 4
Weigh the overall balance
Step back and read the whole picture — the mix of easy and challenging aspects, and the compatibility score, tells you the relationship's overall tone.
The Strongest Aspects in Synastry
These contacts carry the most weight in a relationship.
Sun – Moon
One of the most important bonds in synastry. It signals deep emotional fit and a sense of belonging — the heart of many lasting relationships.
Venus – Mars
The classic chemistry aspect. Venus (love and attraction) meeting Mars (desire and drive) creates magnetism, passion, and physical spark.
Moon – Moon
When two Moons connect, you feel emotionally at home with each other. Your instincts, moods, and needs naturally understand one another.
Saturn contacts
Saturn brings commitment, structure, and longevity — but also responsibility and lessons. Strong Saturn ties often appear in serious, lasting bonds.
Synastry House Overlays
House overlays show where a partner's planets fall in your chart — and therefore which areas of your life they activate. If their Sun lands in your 7th house, they feel like a natural partner; in your 5th, the connection is playful and romantic.
Overlays into the 1st, 5th, 7th, and 8th houses are especially powerful for romance, identity, and intimacy. They tell you not just how you relate, but where the relationship lives in your life.
House overlays need both birth times to be accurate — without them, the houses shift and the overlay reading becomes approximate.
Synastry vs Composite Chart
Two different ways to read a relationship.
| Synastry | Composite | |
|---|---|---|
| What it is | Two charts overlaid | One merged midpoint chart |
| Question it answers | How do we interact? | What is the relationship itself? |
| Method | Cross-aspects between the two charts | Midpoint of each planet pair |
| Read as | Two people in dialogue | A single chart, like a couple's birth chart |
| Best for | Chemistry, friction, dynamics | The relationship's identity and purpose |
Do You Need Birth Times for a Synastry Chart?
Not for everything. The planet-to-planet aspects — the core of synastry — can be calculated from birth dates alone. You only need exact birth times for the house overlays and the angle contacts (Ascendant and Midheaven).
So even if one or both of you do not know your exact birth time, you can still get a meaningful synastry reading of your chemistry and dynamics. Add the times when you have them for the full house-overlay picture.
Ready to compare your charts?
Generate your synastry chart now, or see the full compatibility picture with composite and more.