Synastry chart: what's between your birth chart and theirs
What is it between the two of you that neither of you can explain?
Synastry puts your birth chart next to theirs and shows where you connect, where you're drawn to each other, and where you clash. We calculate it with NASA ephemeris data, the same calculation our readings use.
First up: you
When were you born?
The order doesn't change the result.
Just to personalize the result on screen. We don't store it.
What time were you born?
Your birth time sets your rising sign (also called your Ascendant) and your houses, the areas of your life where the other person's planets land. With the exact time, we can see which of your houses each one falls in. You'll find it on your birth certificate.
We can still run it using noon. The exact time adds that person's rising sign and houses, the areas of their life where the other person's planets land. Without it, their Moon could be off by as much as 7°.
Where were you born?
Start typing your city and pick it from the list. That's how we get the exact coordinates of where you were born.
We set the time zone for each date and place automatically, including historical changes.
Your half is done. Now for theirs
When were they born?
The order doesn't change the result.
Just to personalize the result on screen. We don't store it.
What time were they born?
Their birth time sets their rising sign and their houses. With their exact time, we can see which of their houses your planets fall in. It's on their birth certificate.
We can still run it using noon. The exact time adds that person's rising sign and houses, the areas of their life where the other person's planets land. Without it, their Moon could be off by as much as 7°.
Where were they born?
Start typing their city and pick it from the list. That's how we get the exact coordinates of where they were born.
We set the time zone for each date and place automatically, including historical changes.
Still calculating. It's taking a little longer than usual…
The two of you now have a compatibility score.
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Your synastry chart
Houses are calculated with the Placidus house system, the same one the synastry report uses.
overall compatibility
One birth time is missing, so we used noon. That person's rising sign and houses aren't calculated, and their Moon could be off by as much as 7°. Enter the exact time and we'll add them.
What the two charts already show
This first look is a sample of how we read a synastry chart.
If you want to go deeper, our full reading walks through what's between the two of you, area by area, and what to do about it.
Your full synastry reading
14+ pages interpreting the synastry between the two of you.
- A portrait of your relationship: what kind of couple you make
- What sparks the chemistry between you
- How each of you feels understood, and where you clash
- How you talk to each other and where you misread each other
- What holds the relationship together long term, and what strains it
- Your 3 priorities as a couple, with concrete steps for both of you
What is a synastry chart?
A synastry chart compares two birth charts. One chart goes on top of the other, and we measure the angle each planet in one makes with every planet in the other. Those angles, which astrology calls aspects, describe the chemistry, the friction, and what holds the two of you together.
It doesn't measure whether a relationship will last. It measures the kind of energy that flows between two specific charts: where you recognize each other without trying, where understanding each other takes work, and the themes you're likely to keep repeating until you actually look at them.
What you need to calculate a synastry chart
Six details: the birth date, time, and place for each of you. The birth time sets each person's Ascendant, or rising sign, which shows how you come across and the first impression you leave. It also sets the twelve houses, one for each area of life. Without the houses, you lose one of the most revealing layers: which area of the other person's life each of your planets falls into.
If one of you doesn't know their birth time, check the box and the calculation still runs. In that case we don't show that person's Ascendant, because an Ascendant calculated from a made-up noon isn't an approximation. It's simply wrong.
What this calculator shows you
- The Sun, Moon, and Ascendant for each of you, down to the degree.
- Five scores for the relationship: attraction, emotional connection, communication, stability, and chemistry, plus an overall score.
- A first look at what's between the two of you: two key themes in your synastry, each one explained and tied to the aspect or signs behind it.
- A preview of what the synastry reading covers: more aspects between your two charts, and which of the other person's houses one of your planets falls in.
Each score compares the two of you as a pair against a reference distribution and runs from 30 to almost 100. So it's not a percentage, and it's not a test grade. Everything is calculated from NASA ephemeris data (the tables NASA's Jet Propulsion Laboratory publishes to track where each planet is at any moment) and the Placidus house system. No card needed, no account to create.
How to read the aspects between two charts
Each aspect names two planets, one from each person's chart, and the angle between them. The orb is how many degrees that angle is from exact, and the fewer there are, the more it shows in everyday life.
- Sun-Moon contacts between two charts are among the most cited markers of a lasting relationship: one of you brings direction, the other brings support.
- Venus-Mars contacts speak to attraction and desire, the pulse that quickens between the two of you.
- Moon-Moon contacts show whether the two of you feel safe together without having to explain yourselves.
- Mercury-Mercury contacts show whether you get each other when you talk or whether you have to translate.
- Saturn brings structure and weight. It bears down, and it's also what makes things last.
Harmonious aspects flow and tense ones create friction, but a relationship that's all harmony tends to stand still. Tension is what gets things moving, which is why the report names it instead of hiding it.
What's in the paid report that isn't here
The numbers are the same. The calculator runs on the same engine as the report, so the score you see here is exactly the one that would appear on the report's cover. What changes is how much of it gets unpacked, and whether it gets interpreted at all.
The synastry reading is a 14+ page PDF report that adds the written interpretation, area by area. It points to the specific aspects behind each dynamic and ends every section with steps you can act on. It also includes the calculations this page doesn't show:
- The composite chart: one chart for the relationship itself, with its own Sun, Moon, and aspects.
- The Davison chart, cast for the halfway point in time and space between your two births, with its own date.
- Every house overlay: which of the other person's houses each of your planets falls in.
- Juno, Vesta, Black Moon Lilith, Chiron, the lunar nodes, and the Vertex.
- The declination parallels and contraparallels.
- The combined element profile for the two of you, plus where the relationship stands in its development, read from applying and separating aspects.
Other free calculators
If you'd like to start with your own chart before comparing it with anyone else's, try the birth chart calculator. If you're more curious about places than relationships, the astrocartography calculator draws your planetary lines on a world map. And if your birthday is coming up, the solar return calculator gives you the exact minute your next year starts, birthday to birthday, and the tone it sets.
Frequently asked questions
What exactly is synastry?
Synastry compares two full birth charts, point by point. It lines up one person’s planets against the other’s and measures the angle between each pair. That angle, and its orb (how many degrees it is from exact), is what describes the dynamic between the two of you. A synastry chart isn’t one chart for the couple. It’s two birth charts laid on top of each other.
Do I need both birth times?
Ideally, yes. Without a birth time, that person has no Ascendant (how they come across and the first impression they leave) and no houses (the chart is divided into twelve houses, one for each area of life). So you lose every aspect, or angle, to their Ascendant, and you can’t see which of their houses the other person’s planets land in. That’s half the good stuff in a synastry chart. If either of you doesn’t know your birth time, check the box and the calculation still runs: the scores are worked out the same way, and we don’t make up an Ascendant to fill the gap.
Is synastry the same as a composite chart?
No. Synastry compares your two birth charts and shows how the two of you affect each other. A composite chart is built from the midpoints between your two charts: one chart for the relationship itself. They answer two different questions: synastry shows what happens between the two of you, and the composite shows what the relationship itself is like. The full composite chart comes with the paid synastry reading.
Does a low score mean the relationship won’t work?
No. The scores measure which kind of energy is strongest between you, not whether the relationship will last. A relationship with a lot of tension can be the one that changes you most, and a very harmonious one can end up feeling boring. Astrology describes the dynamic. It doesn’t write the ending, and what you do with that is up to the two of you.
Are the free scores the same as the ones in the paid report?
Yes, number for number. The calculator runs on the same engine as the paid report. It trims what it shows, not what it calculates. There’s no scaled-down version of the math: the 78 you see here is the same 78 that would appear on the cover of the report.
Do you store both people’s birth details?
No. We use both birth dates, times, and places for the calculation and never write them to a database. That’s worth saying, because with a synastry chart you’re entering the details of someone who isn’t there to say yes to it.
Does it work for relationships that aren’t romantic?
The calculation works for any relationship, because the aspects (the angles the planets in two charts form with each other) are the same no matter who the two people are. What’s built for couples is the reading: the five scores and the paid report are written with romance in mind. For a friendship or a family bond, the aspects still hold, but the framing may feel a little narrow.