How Birth Chart Compatibility Works
Birth-chart compatibility compares the positions and angular relationships of multiple chart points — the Sun, Moon, Rising sign, and every planet — between two people, not just their Sun signs. Signz computes a planet-by-planet, degree-by-degree score from the full natal chart using its own compatibility engine.
The Foundation
What Birth Chart Compatibility Measures
Your birth chart is a snapshot of the sky at the exact moment and place you were born. Compatibility compares that snapshot against someone else's — point by point, aspect by aspect.
Each chart point governs a different domain of life. The Sun shapes core identity, the Moon governs emotional needs (explored in depth in our Moon sign compatibility guide), the Rising sign colors first impressions, Mercury shapes communication, Venus shapes love language, and Mars shapes drive. When two charts are compared, every point is measured against its counterpart — your Venus against their Venus, your Mars against their Mars — and the angular distance between each pair is converted into an alignment score.
The heaviest weight goes to the Sun, Moon, and Rising — the Big Three — because they describe who you fundamentally are. Lighter weights go to the slower-moving planets that shape generations more than individuals. You can run the full comparison on the free compatibility calculator.
This full-chart comparison of two natal charts is traditionally called synastry — and the same technique applies to romance, friendship, and business connections. For a sign-by-sign breakdown, read our Sun sign compatibility guide.
Beyond the Sun
Why Sun-Sign Compatibility Is Incomplete
Sun-sign horoscopes compare one point out of twelve. That's why two people with the 'perfect' Sun-sign match can feel completely wrong together.
- The Sun is just your ego and life purpose — it says nothing about how you process emotions (Moon), communicate (Mercury), or give and receive love (Venus).
- Two people can share a Sun sign but have opposite Moon signs, completely different emotional needs, and clashing Mars drives.
- The Rising sign — which requires a birth time — changes your outward vibe entirely, so people born on the same day can feel nothing alike.
- Cross-planet bonds like your Sun against their Moon often matter more for real chemistry than matching Sun signs.
The Points
The 12 Chart Points
Signz compares 12 points between you and every match. Each one governs a different domain of life and relationship, and each carries a tier weight in the score.
Sun
IdentityGoverns core identity, ego, and life purpose
Moon
IdentityGoverns emotions, inner needs, and emotional security
Rising
IdentityGoverns first impressions, vibe, and physical presence
Mercury
ChemistryGoverns communication style and way of thinking
Venus
ChemistryGoverns love language, affection, and values
Mars
ChemistryGoverns drive, passion, and energy
Jupiter
DestinyGoverns growth, beliefs, and optimism
Saturn
DestinyGoverns commitment, discipline, and responsibility
North Node
DestinyGoverns karmic direction and soul lessons
Uranus
OuterGoverns individuality, freedom, and change
Neptune
OuterGoverns dreams, spirituality, and imagination
Pluto
OuterGoverns transformation, power, and intensity
The Geometry
How Planetary Aspects Affect Compatibility
Aspects are the angular relationships between two planets. When your Venus and their Mars sit 120° apart, that's a trine — effortless chemistry. At 90°, it's a square — friction.
Signz detects the five major aspects within an 8° orb, then adjusts the score when an especially tight aspect (within 3° of exact) forms between personal placements — Sun, Moon, Venus, Mars, or Rising.
Conjunction
Two planets blend their energies onto one wavelength — intense unity.
Tight-aspect adjustment: +5 points
Sextile
Gentle, supportive chemistry that opens doors without forcing them.
Tight-aspect adjustment: +2 points
Square
Friction that creates attraction but requires conscious work to resolve.
Tight-aspect adjustment: −5 points
Trine
Effortless, natural flow — things just click between these two energies.
Tight-aspect adjustment: +4 points
Opposition
A push-pull polarity of attraction and tension between opposites.
Tight-aspect adjustment: ±0 points
The Engine
How Signz Weights Identity, Chemistry, Destiny & Outer Placements
Every compatibility percentage comes from a four-tier weighted engine. Here are the exact weights used in the real scoring system.
Identity
Sun · Moon · Rising
Half the score. Who you are at your core — the placements that define your fundamental self.
50%
of score
Chemistry
Mercury · Venus · Mars
40% of the score. How you communicate, love, and act — the daily dynamic between you.
40%
of score
Destiny
Jupiter · Saturn · North Node
5% of the score. Growth, commitment, and karmic direction — the long-term arc.
5%
of score
Outer
Uranus · Neptune · Pluto
5% of the score. Generational currents of change, dreams, and transformation.
5%
of score
The Highlight Reel
Not all connections matter equally. The engine identifies your top 3 strongest Tier 1 and Tier 2 pairs and applies a 1.5× weight boost to them — so your most powerful bonds count more toward your final score than your weakest links.
The Chemistry
Cross-Planet Bonds
Compatibility isn't only matching the same planet in two charts. Signz also measures cross-connections — your Sun against their Moon, your Venus against their Mars — because these bonds define real chemistry.
The Soulmate Bond
Your core identity meets their emotional needs — whether you fundamentally "get" each other on a soul level.
The Passion Spark
Your love language meets their drive and desire — the magnetic pull of romantic and physical chemistry.
The Deep Comfort Connection
Your emotional needs meet their outward presence — whether you feel safe and at home around each other from the first meeting.
The Aesthetic Attraction
Your values and affection style meet their vibe and appearance — the instant "I like the look of you" pull.
Symmetric by Design
Every cross-bond is measured in both directions — your Sun against their Moon, and their Sun against your Moon. Both are real synastry aspects that shape the same bond, so the displayed score is their average. Your compatibility percentage is always identical whether you're viewing their profile or they're viewing yours.
Missing Data
What Happens When Birth Time Is Unknown
Your Rising sign requires an exact birth time to calculate. If you or your match don't know it, the engine adapts without penalizing you.
Instead of assigning a fake neutral score that drags the percentage down, the engine removes Rising-dependent pairs — the Rising mirror plus the Moon-Rising and Venus-Rising cross-bonds — from the calculation entirely and normalizes the remaining weight to 100%. Your score reflects only the planets the engine can actually measure, at their true weights — no artificial ceiling, no unfair penalty.
When this happens, a ⚠ Partial chart badge appears on the compatibility page. Add a birth time later and all four cross-bonds — including the two Rising pairs — snap back to full weight automatically.
Reading the Score
How to Read a Compatibility Score
Every score is color-coded on a gradient from red to green. Here's how to interpret what you see — and what it doesn't promise.
Challenging
Fundamental friction. Needs significant conscious effort and compromise.
Growing Edge
Real differences with some common ground. Workable with mutual awareness.
Solid
Balanced alignment with natural chemistry in several key areas.
Cosmically Aligned
Deep, multi-layered harmony across your most important placements.
Astrology compatibility is a tool for self-reflection and exploring connections, not a scientific predictor of relationship success. A high score highlights natural harmony to build on; a lower one points to areas that need conscious communication. Use it as one perspective among many.
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NETWORK
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Overall Compatibility
89%
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Cancer
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Real-World Use
Dating, Friendship & Business
The same engine that scores romantic chemistry also reveals how two charts interact in any relationship.
Dating & Romance
See beyond Sun-sign hype. Compare how you and a match communicate (Mercury), express affection (Venus), and pursue desire (Mars) before you invest time in a date.
Friendship
Understand why a friend lights you up or drains you. Moon compatibility reveals emotional safety; Mercury shows whether your conversation styles click.
Business & Collaboration
Pair co-founders and teammates by complementary strengths. Saturn reveals commitment and discipline; Mercury shows how you hash out ideas together.
FAQ
Frequently Asked Questions
Is birth chart compatibility scientifically proven?
No. Astrology compatibility is a tool for self-reflection and exploring connections, not a scientific predictor of relationship success. Use it as one perspective among many.
Do I need to know my birth time?
No. If you don't know the birth time, the Signz engine omits the Rising sign and rebalances the score against the remaining chart points at their true weights — no artificial ceiling, no penalty.
What's the difference between Sun-sign and birth chart compatibility?
Sun-sign compatibility compares one point — the Sun. Birth chart compatibility compares all 12 chart points plus four cross-planet bonds, so it captures communication, love language, drive, and emotional needs, not just ego.
How is the Signz compatibility score calculated?
Signz compares the exact degree distance between each chart point, gives more weight to the most personal connections, and adjusts the result for especially close major aspects.
Is the score the same from both sides?
Yes. Cross-planet bonds are scored in both directions and averaged, so your compatibility percentage is identical whether you view their profile or they view yours.
Can I check compatibility without a Signz account?
Yes. The free birth chart compatibility calculator compares any two birth charts using the same engine — no account required.
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