About SynastryCalc

What SynastryCalc is

SynastryCalc is a free online toolkit for relationship astrology. Enter two birth dates, times, and places, and the site calculates a fullsynastry chart— a double wheel that shows each person's planets, the aspects between them, and where one person's planets fall in the other's houses. You can also build acomposite chart, which blends two birth charts into one midpoint chart for the relationship itself.

Alongside the calculators we publish plain-language guides to synastry aspects, house overlays, and reading technique, so you can understand the “why” behind every line and number on the chart instead of receiving a single vague score.

Why we built it

Most synastry tools online fall into two camps: paywalled reports that hide the chart itself, or widgets that reduce a whole relationship to a sun-sign verdict. We think the interesting part of synastry is the chart — the actual geometry between two people's planets — and that this part should be free, instant, and without a login.

So SynastryCalc does the opposite: it shows you the full bi-wheel, the complete aspect grid with orbs, and house-by-house overlays first, then offers interpretation on top. You get the raw technique and the reading in one place, at no cost.

What you can do here

• Synastry chart calculator — the full double-wheel comparison of two birth charts.
• Free synastry report — a meaning-first walkthrough of your strongest connections.
• Composite chart calculator — the midpoint chart of the relationship as its own entity.
• Synastry aspects guide — what each planet pair and aspect means between two charts.
• Synastry houses guide — how house overlays describe where someone lands in your life.
• How to read a synastry chart — a step-by-step method with an annotated example.
• Synastry vs composite chart — when to compare two charts versus blend them into one.

How the calculations work

Chart positions are computed from high-precision professional ephemeris data — the same data trusted by leading astrology software — so planetary positions are accurate to the arc minute. When you type a birth place, the site looks up its exact coordinates and automatically applies the correct time zone and daylight-saving rules for that date in history, which is where most free calculators quietly go wrong.

Aspects are calculated with standard orbs, house overlays use the widely used Placidus house system, and compatibility themes are summarized with a transparent scoring method built from the aspects you can see on the chart — never a hidden number.

Our editorial principles

• Interpretations are grounded in the traditional meanings astrologers actually use — signs, houses, aspects, and overlays — not invented personality quizzes.
• Interpretations are written in advance with astrologers and matched to your chart — not generated on the fly by AI — so the same chart always produces the same reading.
• No fear-based language: difficult aspects are described as dynamics to work with, not curses.
• Everything on the site is free to use, with no account, no paywall, and no email required.

What we are not

Astrology is a symbolic language, and SynastryCalc is offered for entertainment and educational purposes. Our tools do not predict the future, diagnose anything, or replace professional advice of any kind. A synastry chart describes tendencies and dynamics between two charts — what you do with that information is entirely yours. You can read more in our terms of service and privacy policy.

Get in touch

Found a bug, spotted a calculation that looks off, or have an idea for the next tool? We read every message — reach us any time at support@synastrycalc.com or through the contact page.