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Free Synastry Chart – Astrology Compatibility Calculator

Learn what synastry is in relationship astrology, discover how to interpret a chart to gain insight about your compatibility, and use our synastry chart calculator to create your own for free.

What is a Synastry Chart Calculator in Astrology?

Synastry chart calculator | Woman comparing two natal charts and drawing out a synastry chart.

We may earn a small commission from the affiliate links in this post, at no extra cost to you.

A synastry calculator easily generates a synastry compatibility chart (or chart comparison) for you.

Synastry is related to the astrology of relationships, in a more meaningful way than simply reading a relationship horoscope.

If you imagine a natal chart to be a snapshot of the exact planetary coordinates of your time of birth, then a synastry chart is essentially two individual’s birth charts, overlaid one on top of the other.

The idea behind synastry in astrology is to interpret how one person’s chart relates to another.

Synastry charts are similar to composite charts in that their purpose is to explore relationships and provide deeper understanding, however they differ in their approaches.

Rather than showing both sets of data on a single chart for comparison, a composite chart plots the mathematical midpoint of two separate charts, creating one entirely new chart which represents the relationship itself.

The idea behind synastry in astrology is to interpret how one person’s chart relates to another. (You may have heard of solar return synastry, which looks at a single year, from one birthday to the next.)

So a synastry chart looks at how two people influence one another, while a composite chart examines how the family / business / platonic / romantic relationship itself functions.

Looking for a different kind of calculator? We have a variety of free birth chart calculators available.

Where Can I Create a Synastry Chart?

You can create a free synastry chart below. Simply enter the following birth data for yourself and your partner:

  • Date of birth
  • Birth time
  • Place of birth

Note: For the most precise synastry reading, it’s important to use accurate data. If you don’t have the exact time of birth, use 12am or 12pm instead. If either of your places of birth are not available, use the nearest city.

Click ‘Create chart’, and your chart will appear. You will also find an optional synastry report to purchase, if you wish.

You may also like our free birth chart online calculator.

Generate Your Free Synastry Chart

Create your chart below, and then scroll down to learn how to interpret the information.

If you created your astrology chart above, you’ll be able to see that the outside circle displays the zodiac symbols. The next white wheel shows your birth chart placements, and then inside of that are the other person’s planets.

It provides insight into aspects of harmonious and challenging relationship potential.

In the center of the wheel various aspects are demonstrated by coloured lines (more on this later).

As you can see, the synastry chart format gives a side-by-side comparison of two separate natal charts, and how their houses match up. This is important because it provides insight into both harmonious aspects and also potentially challenging aspects of the relationship.

Below the chart you’ll find a list of each partner’s planets, followed by a list of their aspects.

You may also be interested in our free Vedic chart calculator.

Synastry Chart Interpretation

When it comes to reading a synastry chart there are several important factors to consider. The first thing to be aware of is that we’re not necessarily referring to romantic relationships.

It’s necessary to understand the terminology that makes up the basis of a synastry chart.

It could be that a you want to examine your love life, but equally you may be interested in better understanding your relationship with a business partner or family member; synastry charts work for any kind of relationship.

The next thing that’s important to understand is the terminology that makes up the basis of a synastry chart, and what it means. Let’s take a look…

Components of a Synastry Chart – the Terminology

These are the most important elements to know about in synastry astrology:

  • Planets – Represent a theme, eg. the Moon rules intuition, while Venus rules desire.
  • Signs – Represent how energy is expressed, eg. Cancer is intuitive and nurturing.
  • Houses – Represent various areas of your life, eg. the 7th House is associated with partnerships and marriage.
  • Aspects – The angle planets make with one another, eg. Venus square Mars.
  • Orbs – The margin of difference from an exact aspect, eg. Venus square Mars, with an orb of 2°.

Now let’s take a deeper dive into each of these components…


The planets in astrology differ to the astronomically recognised planets. In astrology, they refer both to celestial bodies, as well as other significant points in the sky. Depending on where you look, you may see slight variations as to which are included. 

In general, the planets include:

  • Sun
  • Moon
  • Mercury
  • Venus
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto
  • Chiron
  • North Node
  • South Node

You may note the similarity to the planets in Human Design.

Zodiac Signs

You will likely be very familiar with the 12 star signs in Western astrology. The zodiac sign in which the Sun is positioned on a map of the cosmos at the moment of birth is said to have the most influence over you.

Also known as sun signs, they comprise of:

  • Capricorn
  • Aquarius
  • Pisces
  • Aries
  • Taurus
  • Gemini
  • Cancer
  • Leo
  • Virgo
  • Libra
  • Scorpio
  • Sagittarius


Houses are a key component of an astrological chart. There are 12 houses in traditional Western astrology, and 12 or 13 in Vedic astrology, depending on the system used (when you get deeper into astrology, there are a few different house systems you can try experimenting with to see which resonates most).

The 12 houses in traditional astrology comprise of:

  • 1st House – represents identity and the self.
  • 2nd House – relates to possessions and finances.
  • 3rd House – pertains to communication and siblings.
  • 4th House – represents home, family, and security.
  • 5th House – associated with creativity, romance, and fertility.
  • 6th House – relates to work and health.
  • 7th House – represents partnerships and marriage.
  • 8th House – pertains to shared resources, commitment, and transformation.
  • 9th House – associated with education, travel, and culture.
  • 10th House – represents ambition and career.
  • 11th House – pertains to friends, community, and charity.
  • 12th House – represents intuition, spirituality, and healing.


Synastry aspects refer to the angles planetary positions make to each other. The following table shows the five major aspects for each sign.

AriesLeo, SagittariusGemini, AquariusCancer, CapricornLibra
TaurusVirgo, CapricornCancer, PiscesLeo, AquariusScorpio
GeminiLibra, AquariusAries, LeoVirgo, PiscesSagittarius
CancerScorpio, PiscesVirgo, CapricornLibra, AriesCapricorn
LeoAries, SagittariusGemini, LibraTaurus, ScorpioAquarius
VirgoTaurus, CapricornCancer, ScorpioGemini, SagittariusPisces
LibraGemini, AquariusLeo, SagittariusCancer, CapricornAries
ScorpioCancer, PiscesVirgo, CapricornLeo, AquariusTaurus
SagittariusLeo, AriesLibra, AquariusVirgo, PiscesGemini
CapricornVirgo, TaurusScorpio, PiscesLibra, AriesCancer
AquariusLibra, GeminiSagittarius, AriesScorpio, TaurusLeo
PiscesScorpio, CancerCapricorn, TaurusSagittarius, GeminiVirgo

In every natal chart there are five major aspects, as follows…


Angle: 0° – 10°

Typically, an orb of ~10° is considered to be a Conjunction.

Conjunctions can produce either harmony or tension, depending on the planets involved.

Venue Pluto is one of the hottest aspects a synastry chart can have.

Highly favourable Conjunctions may involve the Sun, Venus, or Jupiter. Highly unfavourable Conjunctions may involve the Moon, Mars, or Saturn.

Venue Pluto is one of the hottest aspects a synastry chart can have. Pluto conjunct Venus represents sex, passion, intimacy and obsession, while Venus represents love, connection, beauty, and affection.


Angle: 60° – 1⁄6 of the 360° ecliptic or 1⁄2 a trine (120°)

An orb of 3 – 4° is allowed, depending on the planets involved.

This is the weakest planetary aspect. It resembles a Trine in terms of influence, but is less intense.

A Sextile indicates birth chart compatibility and an easy relationship.


Angle: 90° – 14 of the 360° ecliptic or 12 an opposition (180°)

An orb of 5 – 10° is allowed, depending on the planets involved.

A Square traditionally involves the least compatible signs.

Typically involves houses in different quadrants, and is so-called for the square formed if lines are drawn from the center to the two signs.

Creating negative qualities such as tension and friction, a Square traditionally involves the least compatible signs.


Angle: 120° – 13 of the 360° ecliptic

An orb of 5 – 10° is allowed, depending on the planets involved.

A Trine is very beneficial, bringing shared temperament and goals to a relationship, resulting in natural harmony.


Angle: 180° – 12 of the 360° ecliptic

An orb of 5 – 10° is allowed, depending on the planets involved.

Oppositions may indicate discord or balance.

Said to be the second most powerful aspect.

An Opposition is similar to a Conjunction, without the uniting element of a Conjunction. All significant axes in astrology are essentially Oppositions, meaning they may indicate discord or balance.

Any aspect which is not major is said to be a minor aspect, of secondary importance.

How to Read a Synastry Chart

Now that we’ve covered synastry chart terminology, let’s take a look at how it all comes together…

Planets and Aspects

The first step is to look at the most important planet positions and their aspects to each other. For a synastry reading, these will be as follows…


The Moon represents our inner self, and is therefore one of the most important planets to consider in synastry. It provides insight about the way we need to be loved and nurtured.

Harmony between the moons and other planets will give you an overview of how well your relationship may work in the longer term.

Venus and Mars

Venus represents what we’re attracted to and Mars represents strong attraction and physical intimacy.

In the most passionate and magnetic unions, a woman’s (feminine) Venus will be in contact with the man’s (masculine) Mars.

Saturn and Pluto

A lack of such connections may indicate weaknesses in the relationship.

Saturn contacts suggest longevity and resilience, even in the face of relationship difficulties, especially so if they exist between an individual’s Saturn and the partner’s Moon or Venus.

A lack of such connections indicates weaknesses in the relationship which may prevent endurance.


Next, look at which houses a partner’s planets overlay. If there are many planets in harmonious angles overlaying a specific house, it illustrates balance and compatibility in that specific area of life.

The most important houses to consider are…

Fifth House

The 5th House is the house of romance and fertility. 5th House overlays indicate thrilling adventures with your partner.

Seventh House

The 7th House is the House of partnerships, including marriage. 7th House overlays can denote commitment to one another and the relationship.

Eighth House

The 8th House is the House of commitment and shared resources. 8th House overlays can indicate dedication and the long-term potential of a successful marriage.

Get a Personalised Guide to Your Synastry Chart

Get over 20 pages of detailed interpretations and unique diagrams!

The above information is intended to help you interpret a synastry chart by yourself. 

If you prefer a guided report to walk you through the insights offered by a synastry chart, you have the option to purchase a personalised report, based on your unique birth details.

The front cover of a synastry chart report.

The guide is offered in collaboration with Astro Charts, and for just $33 provides the following benefits:

  • Discover your relationship’s unique voice. In this guide we analyse and interpret your relationship’s shared planetary aspects and uncover its strengths and growth areas.
  • Over 20 pages of detailed interpretations and unique diagrams exploring the planetary aspects you share with your partner.
  • Interpretations are guided by an evolutionary perspective of astrology, seeking to see the value and highest expression of each aspect of your chart.
  • Highlights the individual qualities you each bring into each aspect.
  • An easy to read PDF delivered directly to your email.

Simply click ‘GET MY REPORT’ at the bottom of the synastry chart you’ve generated above! Use this jump link to take you back to your chart.

You may also be interested in our Human Design chart calculator.

What Does the Science Say?

As we’ve explored in other astrology posts, there’s no scientific evidence to support the modality.

Studies suggest the popularity of relationship horoscopes is likely due to a combination of the Barnum Effect and ‘self-attribution bias’.

Read more in our birth chart calculator post.

Signature reading Kate, with a heart.

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