Astrology Compatibility Calculator (Synastry Chart)

Sun sign matching or synastry: what does astrology compatibility compare?

Most people check astrology compatibility by putting two Sun signs side by side. Think of Aries with Leo or Taurus with Virgo. That comparison looks at only one part of a birthday. Synastry, by contrast, overlays two complete birth charts. It then measures the angles between seven bodies one by one. Our calculator offers both on the same page, but they answer different questions.

The Quick zodiac match mode pairs the twelve signs by element and modality. For example, two signs of the same element form a trine and score 80. Two signs in square, however, sit at 26. These numbers depend on the Sun sign alone. As a result, millions of people with the same sign get the same answer.

Birth date only and Full synastry work differently. Here our tool finds the real positions of the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn for that day. So two people with the same Sun sign can still get very different results. In short, the quick table answers general curiosity. Synastry, in contrast, applies the traditional rules to two specific charts. Either way, the result summarises astrological tradition; it is not a forecast.

Why do the Moon, Venus, Mars and the Ascendant matter in synastry?

In the tradition, each body stands for an area of life. Synastry looks at how those areas meet between two people. Astrologers usually check one person's Sun with the other's Moon first. Tradition reads this contact as identity meeting emotional needs. That is why our tool gives the Sun and Moon pair the top weight for emotional bond: 1.0.

Tradition also links Venus and Mars contacts with attraction. So the Venus and Mars pair carries a weight of 1.0 in the attraction category. Mercury with Mercury describes communication style. Saturn contacts, meanwhile, describe commitment and seriousness. For instance, the Sun with Saturn and Venus with Saturn each carry 0.8 for stability. Likewise, tradition reads Moon and Venus contacts as warmth and closeness.

The Ascendant is the sign rising on the eastern horizon at the moment of birth. It moves about 15 degrees per hour on average. Therefore the tool uses the Ascendant only when you know both birth times. Ascendant contacts then carry a weight of 0.6. Uranus, Neptune and Pluto, on the other hand, move very slowly. People of a similar age have them in almost the same place. To keep the result readable, we leave those three planets out.

What happens to astrology compatibility when the birth time is unknown?

The Moon is the fastest moving body in the sky. We measured 4,000 samples between 1950 and 1986 with our astronomy library. The Moon covered between 11.77 and 15.35 degrees a day. A noon chart can therefore put the Moon roughly 6 to 8 degrees off when the time is unknown. Moreover, that error is as large as the orbs we use in synastry. A single noon score would suggest a precision that does not exist.

Instead, our tool splits a day with an unknown time into 49 moments, 30 minutes apart. It builds a chart for each moment. With both times unknown, that makes 49 × 49, or 2,401 pairings. The tool calculates the index for every pairing. You then see the median together with the 10th and 90th percentiles. Some Moon aspects only hold for part of the day. In that case the table says so, for example "valid for 60 percent of the day".

If the Moon changes sign on that date, the tool flags it. Without a time, the Moon sign stays uncertain. The tool also never shows the Ascendant or Descendant without a birth time. Mercury, Venus and Mars usually move less than 2 degrees within a day. Their aspects therefore stay largely the same either way. Finally, a time from a birth certificate or family record narrows the range in Full synastry.

How do time zones and daylight saving time change a chart?

A birth certificate shows local clock time. Our tool, however, places the planets by Universal Time (UTC). The gap between the two depends on the country and the year. New York, for instance, was on daylight saving time in January 1974. London used UTC+1 in January 1970 under British Standard Time. In other words, the same written time can mean a different moment in another year.

Germany has followed EU summer time since 1980. Berlin records show UTC+1 in summer 1979 and UTC+2 in summer 1980. In summer 1945, Berlin even ran on UTC+3. Turkey is another good example. From 1978 to 1984 its standard time was UTC+3. After 1984 it used UTC+2 with summer time. Since 7 September 2016, it has stayed on UTC+3 all year.

Our calculator reads this history from your browser's IANA time zone data. Once you pick a place of birth, it finds the offset for that date by itself. Some times never existed because clocks went forward; the tool warns you about those. Other times happened twice because clocks went back, so it asks which one you mean. For dates before 1970, and before 1985 in Turkey, it shows the offset it used. It also offers Change UTC offset for a manual fix. If your town is missing, use Enter location manually with decimal degrees. Our coordinate converter turns degrees, minutes and seconds into decimals.

Synastry or composite chart: what is the difference?

Synastry keeps two separate charts and measures the angles between them. A typical question is how far A's Venus sits from B's Mars. A composite chart, on the other hand, creates a third chart out of the two. Robert Hand systematised the midpoint method in 1975. In it, the composite Sun is the point exactly halfway between the two Suns. The same rule applies to the other planets. The Davison method (1977) instead builds a chart for a real moment halfway between the two births and places.

Both methods also address different questions. In the tradition, synastry describes how two people affect each other. A composite, meanwhile, describes the relationship as a unit of its own. Our tool calculates synastry only and does not produce a composite chart. The reason is transparency. In synastry, every aspect rests on two real planetary positions, so we can show each contribution. A midpoint chart, by contrast, needs extra choices such as the shorter or the longer arc. Without explaining those choices, a number would make the index harder to trace.

How can you read harmonious and challenging aspects without fatalism?

Traditional classification calls trines and sextiles easy aspects. Squares and oppositions count as hard ones. Modern astrology puts less weight on that fatalistic split. It describes hard aspects as areas to work on instead. Our tool follows the same approach. A square carries a direction of −0.8, but an opposition only −0.3. That is because tradition reads opposition as both attraction and tension.

We also never deduct tension from the index. Instead, it appears separately as the Friction level. Values below 35 count as low, 35 to 65 as medium and anything above as high. That way a pair with many harmonious and many challenging aspects sees both sides. The verbal bands stay plain too. A value of 60 and above means harmonious themes outweigh challenging ones. From 40 to 59 means balance, and below 40 means challenging themes stand out.

None of these words is a verdict. The index is a number from 0 to 100, but it is not a percentage. A value of 50 marks the middle of random birth pairs. A low value carries no negative outlook for a relationship. Likewise, a high value carries no positive one. So use the result as a conversation starter: which themes feel familiar, and which do not? UK advertising guidance also warns against exploiting the credulity of susceptible people. Our result texts therefore avoid fear based labels.

What does science say about astrology compatibility?

The short answer: large studies have not shown that astrology compatibility predicts marriage or divorce. The box on this page summarises the findings. Here we look at what the studies actually tested.

In Carlson's double blind test, published in Nature in 1985, physicists and astrologers agreed on the protocol in advance. Then 28 astrologers tried to match more than 100 birth charts with personality profiles. They did no better than chance. Voas (2007) looked at the birthdays of married couples in the 2001 census of England and Wales. His first table showed about 22,100 more same sign couples than expected. However, most of that excess came from errors in filling in the forms. Specifically, couples with the same sign but different birth months were no more common than chance.

After that, Helgertz and Scott (2020) used Swedish register data from 1968 to 2001. They found no consistent evidence that astrologically compatible couples marry more often or divorce less. These studies have limits: Voas tested Sun signs only, without the Moon or Ascendant. Still, Carlson's astrologers worked with full birth charts and did no better than chance either. That is why our tool calculates planetary positions astronomically. It never presents the compatibility reading as a scientific measurement.

Where do your birth details go, and how long are they kept?

Nowhere. Only your browser handles the date, time, place and nickname. To run the calculation, it downloads only the calculation files and a city list from our server. That request names the country you chose and nothing else. Your browser also runs the city search itself. No query goes to an external location service.

The tool keeps birth details out of the page address as well. On this tool, the Copy link button only copies the page address. Copied results contain no birth details either. No data from this tool goes to analytics events, error logs or AI services. Form fields also stay masked for session recording tools. Our guide to Microsoft Clarity explains how session recording works.

We store your details in local storage only if you press Save on this device. That copy stays on your device, and Delete saved data removes it at once. We never ask for names or gender, because the calculation does not use them. If you enter someone else's details, please make sure they are comfortable with that. For the full picture, see our privacy policy. Our guide to building a GDPR compliant website covers this approach of keeping data off the server. We also build similar browser based calculators in our custom software development projects.