Do both people need an exact birth time?

For a full synastry reading, yes. The ascendant and midheaven are among the most-read contacts in the technique, and both depend entirely on the birth time and place. Without a time you can still compare the planets, which is what our love compatibility calculator does - it accepts a missing time and says so in the result.

Which synastry aspects matter most?

Traditionally, Sun-Moon and Venus-Mars contacts carry the most weight, followed by anything touching the ascendant, then Moon-Venus and Moon-Saturn. Beyond the pairing itself, exactness decides: a contact within one degree is felt considerably more than the same aspect at six, which is why this tool prints the orb of every one it finds.

Are square and opposition aspects bad in synastry?

No, and reading them that way is the most common mistake with the technique. Squares and oppositions are where the friction is, and friction is what gives a relationship charge and impetus. A comparison made entirely of trines and sextiles is usually described as comfortable and inert. The useful question is not whether a hard aspect exists but which two bodies it joins.

Why does this calculator ignore Uranus, Neptune and Pluto?

Because they barely move. Uranus spends about seven years in a sign, Neptune fourteen, Pluto up to thirty - so two people of similar age have them in nearly identical positions and will always show contacts between them. Those contacts describe a generation, not a pair, and including them pads the grid with rows that would appear for any two people born in the same decade.

What orbs does the synastry chart calculator use?

Conjunctions and oppositions count within 8°, trines and squares within 7° and sextiles within 5°, with 2° more whenever the Sun or Moon is involved. With Include minor aspects ticked, the quincunx counts within 3° and the semi-sextile, semi-square and sesquiquadrate within 2°, with no luminary allowance. The orbs are fixed and cannot be changed on the page, so a contact just outside them is left off the list.

Is there a twin flame synastry chart calculator mode?

No. Twin flame is a popular way of describing an intense connection, not a separate astrological calculation, so there is no twin flame setting and no score. People who read synastry that way usually look at the tightest contacts at the top of the list, especially Sun-Moon, Venus-Mars and anything touching the ascendant or the North Node. The same list works for that reading as for any other.

What is a double whammy in synastry?

A double whammy is when the same pairing aspects in both directions - your Venus to their Mars and their Venus to your Mars, for instance. Astrologers treat it as significantly stronger than either contact alone, on the reasoning that the exchange is mutual rather than one-sided. Because this tool lists both directions, they are easy to spot: look for the same two bodies appearing twice with the names swapped.

Can synastry predict whether a relationship will last?

No. Synastry describes tendencies in how two people interact, in the same way any personality framework describes tendencies. It cannot account for what either person chooses to do, and the literature is full of long marriages with difficult charts and short relationships with easy ones. Treat it as a vocabulary for talking about a dynamic, not a forecast.

Is this synastry chart calculator free, and is our birth information stored?

It is free, no signup, and nothing is stored. Both charts are computed in your browser and neither set of birth details is transmitted, saved or logged. The page continues to work with the network disconnected.

Synastry Chart Calculator

Compare two birth charts and see every aspect between them, in both directions, with the orb each was found at

First person

Name (optional)

Birth date

Birth time

Birth place

Sets the coordinates and the time zone. Both are needed for this calculation. City data from GeoNames, CC BY 4.0.

Second person

Name (optional)

Birth date

Birth time

Birth place

Sets the coordinates and the time zone. Both are needed for this calculation. City data from GeoNames, CC BY 4.0.

Include minor aspects

What is a synastry chart?

Synastry is the comparison of two birth charts. Rather than reading either person on their own, it looks at the angles between one person's planets and the other's: your Venus against their Mars, their Moon against your ascendant. Those inter-chart aspects are what the technique is about, and they are read as the specific texture of the relationship - where the two of you fit easily, where you rub, and which parts of each person the other reliably activates.

Direction matters here in a way it does not within a single chart. Your Venus on their Mars and their Venus on your Mars are two different statements: the first says something about how you find them attractive, the second about how they find you. A synastry grid that lists each pairing once has thrown away half the information. This calculator crosses both charts in full, and every row names which chart each body belongs to.

Here is what the free synastry chart calculator returns for two sample charts: 15 June 1990 at 14:30 in London and 3 March 1992 at 08:15 in New York. With both forms filled in and Compare the charts pressed, the header reads "32 within orb". The first row is First person's Mars sextile Second person's Mars at an orb of 0.1°, and the second is First person's Ascendant trine Second person's Mars at 0.2°. Tick Include minor aspects and the count rises to 44. Below the list, each person's full birth chart table sits side by side.

It also leaves the outer planets out, and that is deliberate. Uranus, Neptune and Pluto spend seven to thirty years in a sign, so two people born within a few years of each other share them almost exactly. An aspect between two Neptunes describes a generation rather than a couple, and including it fills the grid with contacts that carry no information about the pair. What remains are the personal planets, Jupiter and Saturn, the ascendant and midheaven, and the North Node - the placements that genuinely differ between two people and that the tradition reads. Both charts need a birth time, since the ascendant and midheaven are among the most-read contacts in synastry.

How to read a synastry chart?

Enter both birth details
Date, time and place for each person. Both times are needed - the ascendant and midheaven are among the contacts synastry weighs most heavily.
Start with the tightest aspects
The list is sorted by how close each contact is to exact, measured against the orb that aspect allows, and a partile contact is felt far more than one at the edge of its orb. The top few describe the relationship more than the rest combined.
Read the direction, not just the pairing
Each row names whose body is whose. Your Moon on their Saturn is a different experience from their Moon on your Saturn, and the tradition treats them separately.
Weigh friction as well as ease
Squares and oppositions are not failures. They are where the energy is, and a comparison with none at all is usually a flatter relationship than one with a few.

Key Features

Both Directions Compared
A full cross-product rather than the upper triangle, because A's Venus on B's Mars is a different statement from the reverse
Every Aspect With Its Orb
Each contact prints how far from exact it is, so you can judge how much weight it deserves instead of taking a yes-or-no
Generational Planets Excluded
Uranus, Neptune and Pluto are left out - two people of similar age share them, so their contacts describe a cohort, not a couple
Both Charts in Full
Complete positions for each person alongside the comparison, so a contact can be traced back to where each body sits

Frequently Asked Questions

Synastry is the comparison of two birth charts to describe a relationship. It measures the angles between one person's planets and the other's, and reads those inter-chart aspects as the particular dynamic between the two people - where they support each other, where they clash, and which parts of each the other brings out.

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Read the full Synastry Chart Calculator guide

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Synastry Chart Calculator

First person

Second person

What is a synastry chart?

How to read a synastry chart?

Enter both birth details

Start with the tightest aspects

Read the direction, not just the pairing

Weigh friction as well as ease

Key Features

Both Directions Compared

Every Aspect With Its Orb

Generational Planets Excluded

Both Charts in Full

Frequently Asked Questions

Related Tools

Birth Chart Calculator

Dating & Personal Growth Calendar

Love Compatibility Calculator

Venus Sign Calculator

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First person

Second person