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Birth Chart Compatibility Calculator
Birth chart compatibility compares two full natal charts: how each person's Sun, Moon, Venus, Mars, Mercury and rising signs relate, and which planets form close aspects between the charts. Enter birth date, time and place for both people. Unknown times are fine: we leave out the rising sign.
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What is birth chart compatibility?
Comparing Sun signs only tells you how two people's core temperaments relate. A birth chart holds much more: the Moon for emotional needs, Venus for affection and taste, Mars for desire and conflict, Mercury for communication and the rising sign for everyday style. Birth chart compatibility compares all of these between two people.
This calculator gives you a compatibility report in two parts. First, a table that sets your placements next to theirs and says how each pair of signs relates. Second, the closest aspects between the two charts, the exact angles that astrologers weigh most when they look at a relationship.
Is birth chart compatibility the same as a synastry chart?
They come from the same comparison. A synastry chart lists every aspect between two charts, with orbs and weights, and is the tool astrologers use for detailed work. This page turns the same calculation into a shorter report organized by placement, which is easier to read if you are new to chart comparison.
Both use the same chart engine, the same orbs and the same weighting, so the closest aspects you see here are the same ones that top the synastry list.
Which placements matter most for compatibility?
Astrologers usually start with the personal placements. They change quickly and differ even between people born a few days apart, which makes them more individual than the outer planets.
| Placement | What the comparison shows |
|---|---|
| Sun | Core identity and what each of you is building in life |
| Moon | Emotional needs, comfort and how you settle down at home |
| Mercury | How you talk, listen and solve problems together |
| Venus | What each of you finds attractive and how you show affection |
| Mars | Desire, energy and how each of you handles conflict |
| Rising sign | First impressions and everyday style |
Contacts between one person's Venus and the other's Mars, or between the two Moons, are classic signs of attraction and ease in relationship astrology. Contacts with Saturn are read as commitment and also as pressure.
How is the compatibility calculated?
Both charts are calculated from birth date, time and place with the open-source astronomy-engine library, in the tropical zodiac, with historical time zones. For each placement we compare the two signs by element and by the angle between them, from same sign to opposition.
For the aspects, we compare every planet in one chart with every planet in the other, plus both rising signs when both birth times are known. A conjunction or opposition counts within 8 degrees, a trine within 7, a square within 6 and a sextile within 4, with one extra degree when the Sun or Moon is involved. Aspects between personal points and tight aspects count most. The full method is on the how we calculate page.
How the birth chart compatibility report works: rules and sources
Where astrologers disagree, we pick one rule and say which. These are the rules behind your result:
| Rule | Value we use | Where it comes from |
|---|---|---|
| Aspects | Conjunction, opposition, trine, square, sextile | The five major aspects of classical astrology |
| Orbs | Conjunction and opposition 8, trine 7, square 6, sextile 4 degrees, plus 1 with Sun or Moon | Common orbs in modern practice; our choice |
| Weighting | Personal points (Sun, Moon, Venus, Mars, Ascendant) highest, outer planets lowest, tighter aspects higher | Our own rule, stated so you can check it |
| Scores | No percentage | No traditional method produces one |
| Unknown birth times | Ascendants left out | Our convention |
Do we need exact birth times?
No, but they help. Without a birth time we use noon, leave out the rising sign and houses, and note that the Moon can be several degrees off. Sun, Mercury, Venus and Mars are rarely affected, because they move slowly enough that the time of day changes little.
If one birth time is unknown, the rising sign is left out for both people so the comparison stays fair.
Can I check soulmate compatibility by date of birth?
Many people search for soulmate or marriage compatibility by date of birth. No chart can tell you whether someone is your soulmate or whether a marriage will last; those depend on two people's choices. What a chart comparison can do is show where you are likely to understand each other easily and where you will need patience.
If you only have birth dates, the numerology compatibility calculator compares your life path numbers, and the zodiac compatibility calculator compares your Sun signs.
What the report cannot do
Compatibility astrology is a tradition of interpretation, offered here for entertainment and reflection. It does not predict the future of a relationship and is not relationship, health, financial or legal advice. A challenging aspect is not a warning, and an easy one is not a guarantee.
Questions
Frequently asked questions
Is this birth chart compatibility calculator free?
Yes. The placement table, the closest aspects and the placement-by-placement report are free. We ask for your first name and email before the full report.
What is the difference between birth chart compatibility and Sun sign compatibility?
Sun sign compatibility compares one placement. Birth chart compatibility compares six placements and the exact aspects between both charts, so it is far more individual.
Do I need both birth times?
No. Without birth times we use noon, leave out the rising sign and note that the Moon may be uncertain. The other placements stay reliable.
Which aspects matter most between two charts?
Aspects between the Sun, Moon, Venus, Mars and rising signs, and the tighter the better. Venus and Mars contacts are read as attraction, Moon contacts as emotional ease.
Can a birth chart show if we are soulmates?
No. A chart comparison shows patterns of ease and friction. Whether a relationship works depends on the people in it.
What is the difference between synastry and a composite chart?
Synastry compares two charts side by side, which is what this calculator does. A composite chart blends two charts into one by midpoints and is read as the chart of the relationship itself.
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