How to Run a Synastry Chart (and What to Look At)
What you need to compare two charts, which contacts to read first, and how to get something more useful than a compatibility percentage.
Comparing two charts is mechanically simple and interpretively easy to get lost in. A synastry comparison generates well over a hundred contacts between two charts, and if you read them in the order a report lists them you will drown.
This is the practical version: what to supply, what to look at, and in what order.
What you need
Two complete sets of birth data — date, time and place for both people.
Birth times matter more here than in a single chart, because house overlays are usually the most revealing part of a comparison and houses depend entirely on the time. Without both times you can still read planet-to-planet aspects, which is worthwhile, but you lose the overlay layer completely.
If you only have one person's time, say so in the reading rather than working around it silently.
The four views
A proper comparison has four charts in it, and confusing them is the most common source of muddle.
The two natal charts come first. You cannot read a relationship between two people you have not read individually. Skipping this is like reviewing a conversation without knowing who is talking.
The synastry biwheel overlays the two charts to show the contacts between them — whose planets touch whose, and where each person's planets land in the other's houses.
The composite is a chart built from the midpoints of both, read as the relationship's own chart rather than as two people compared. It describes the entity they form.
Different tools, different questions. Synastry says how these two people affect each other. Composite says what the relationship itself is like.
What to read first
Order matters, because reading everything at once produces nothing.
Sun and Moon contacts. Sun to Sun, Moon to Moon, and Sun to Moon between the charts. These are the bedrock — whether two people fundamentally get each other. Sun-Moon contacts between charts are among the most reliable indicators of long-term ease.
Venus and Mars contacts. Venus is what someone is attracted to, Mars is how they pursue it. Contacts between one person's Venus and the other's Mars are the most consistent indicator of physical chemistry in the whole comparison. If they are absent, the pairing may work in every other way and lack a spark, and people notice.
House overlays. Whose planets fall in whose houses, both directions. A stack of someone's planets in your seventh describes a very different relationship from a stack in your twelfth, and the overlay often says more than the aspects.
Saturn contacts. Saturn between charts brings weight, duration and restriction. Saturn contacts are common in long marriages and also in relationships that feel heavier than they should. Read them carefully rather than as good or bad.
Read those four and stop. The remaining hundred contacts are detail.
Why compatibility percentages are useless
A single score collapses every contact into one number, using a weighting somebody picked. Two different tools will give the same couple different scores, which tells you the number describes the tool rather than the relationship.
It also answers a question nobody actually has. People do not want to know whether a relationship is 73% good. They want to know why the same argument keeps happening, or why someone they have nothing in common with is impossible to stop thinking about. Those live in specific contacts, and a score deletes exactly that information.
Reading it honestly
Difficult contacts are not disqualifying. Squares between charts generate friction, and friction generates interest — relationships with nothing but trines are frequently pleasant and inert. Charts do not decide relationships. Two people with an excellent comparison who will not do the work will fail, and a difficult comparison between two people who are paying attention often will not.
Synastry describes the terrain. It does not tell you whether to make the journey, and any reading that presents itself as a verdict is overstepping what the technique supports.
Running one
Astrologer's synastry tool is free and needs no account: biwheel, full cross-aspect grid with orbs, and a house-overlay heatmap that makes the concentrations obvious at a glance. astro.com also produces free synastry and composite charts and is the right place to cross-check anything.
For the interpretation side, there is a longer guide to reading a comparison, and a comparison of the software that says where other tools do this better.
Frequently asked questions
What do I need to run a synastry chart?
Date, time and place of birth for both people. Birth times matter more here than for a single chart, because house overlays are often the most revealing layer and houses depend entirely on the time.
What should I look at first in a synastry chart?
Sun and Moon contacts between the charts, then Venus and Mars contacts, then house overlays, then Saturn. Those four layers carry most of the meaning; the remaining hundred contacts are detail.
What is the difference between synastry and a composite chart?
Synastry overlays two charts to show how two people affect each other. A composite is built from the midpoints of both and read as the relationship's own chart, with its own angles and houses.
Are compatibility percentages meaningful?
No. A single score collapses dozens of separate contacts using an arbitrary weighting, and two tools will give the same couple different numbers — which tells you the score describes the tool rather than the relationship.