Synastry chart calculator
Two birth charts in one wheel: every aspect between your planets with a short reading, where each person's planets fall in the other's houses, and both sets of astrocartography lines on one map, with the places that work for both of you. Free, nothing stored.
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Enter both birth details
Enter both birth details: the page draws both charts in one wheel, lists every aspect between your planets with a short reading, shows where each person's planets fall in the other's houses, and puts both sets of lines on one world map with the places that work for both of you.
Planets on the map
What a synastry chart shows
Synastry is the oldest way astrologers compare two people: lay one birth chart over the other and look at how the planets of one person meet the planets of the other. The word joins the Greek for "together" and "star".
The calculator above draws the first person's chart in the inner ring with its houses, and the second person's planets in gold around the outside. Lines across the center connect the planets that form an aspect: blue for the easier angles, red for the tense ones.
It works for any two people: partners, a crush, a parent and a child, two friends or two business partners. Astrology reads the contacts as a description of how two people tend to meet each other, not as a verdict on whether a relationship will last.
How to read the aspects between you
An aspect forms when a planet of one person sits at a set angle to a planet of the other. The list starts with the closest ones, because astrologers give the most weight to contacts within a degree or two.
- Conjunction (same degree): the two themes merge, and both people usually notice it.
- Trine and sextile: read as ease and support, the parts of the bond that need little work.
- Square and opposition: read as friction and attraction at once, the places where both people adjust.
- Quincunx: an awkward angle that asks for small, repeated adjustments.
Astrologers usually look first at a few classic contacts: the Sun of one person with the Moon of the other (daily life together), Venus with Mars (attraction), the Moon with Venus (tenderness), and Saturn with any personal planet (commitment, and sometimes weight). The "At a glance" box pulls these out for you.
We count the aspects but give no compatibility score. A single close Sun and Moon contact can say more than a dozen wide trines, and many lasting couples have plenty of squares. A percentage would hide that.
House overlays
With both birth times, the page also places each person's planets in the other person's houses. The house is read as the part of life where you feel the other person's planet. If your partner's Moon falls in your 4th house, tradition reads their emotional presence as tied to home and family for you; their Mercury in your 10th house would be read as their ideas touching your work and direction.
Overlays run both ways and are rarely symmetrical, which is part of what makes them interesting: the same pair of people can mean home to one and adventure to the other.
Synastry on the world map
This is the part no ordinary synastry tool shows. Below the chart, both people's astrocartography lines are drawn on one world map: the first person's in solid and dashed lines, the second person's as thin dotted lines in the same planet colors. Each line keeps its meaning for the person it belongs to, so a city on your Venus line and your partner's Jupiter line is read as warm for you and expansive for them.
The list "Places that work for both of you" finds the large cities that sit near a line of the theme you choose (love and partnership, home and family, career, a fresh start, or luck and travel) on both maps at once. Tap any place on the map to see both people's nearest lines. To explore the overlay on the 3D globe, open it on the couples map.
A map is one input among many. Cost of living, visas, family and work count for more than a line; couples mostly use the shared places to choose a trip or to compare cities they are already considering.
Synastry, composite or Davison?
| Chart | What it compares | Best for |
|---|---|---|
| Synastry (this page) | Two separate charts, planet against planet | How each person experiences the other |
| Composite chart | One chart built from the midpoints of both charts | The relationship as a unit, and where it does best on the map |
| Davison chart | A real chart for the moment and place halfway between the two births | The relationship as a unit, as a real sky |
The three pages share the same form and the same link format, so after drawing one you can open the other two for the same two people with one tap.
How it is calculated
Positions come from the same planetary engine as the rest of the site, tested against NASA JPL Horizons (see method and accuracy). Aspects use the orbs of a birth chart: 8 degrees for conjunctions and oppositions, 7 for squares and trines, 5 for sextiles and 3 for quincunxes, with 2 more when the Sun or Moon is involved. The North Node, Chiron, and with a birth time the Ascendant and Midheaven are included.
Without a birth time a chart is drawn for noon. Its houses and angles are left out, and because the Moon moves about 13 degrees in a day, its contacts are kept only within 6 degrees. The map needs both birth times: the Earth turns 1 degree every 4 minutes, so the lines move with the time. Everything runs in your browser.
Can a synastry chart tell if we are compatible?
It describes how two charts meet, in the language of a symbolic tradition. Astrologers read it as a map of the easy and the demanding parts of a bond, not as a prediction of how a relationship will go. That is why this page counts the aspects but gives no score.
What are the most important aspects in synastry?
Close contacts between the Sun, Moon, Venus and Mars of the two people, and contacts to the Ascendant when both birth times are known. Many astrologers also watch Saturn contacts, read as commitment and staying power. The tighter the orb, the more weight an aspect gets.
Do we both need an exact birth time?
Not for the planet to planet aspects. Houses, house overlays, the Ascendant and Midheaven, and the world map all need the birth time. If one time is unknown, the page draws everything that does not depend on it and says what is left out.
What is a house overlay in synastry?
It is where one person's planets fall in the other person's houses. The house is read as the area of life where you feel that planet in the other person: their Venus in your 7th house, for example, is read as affection landing in your sense of partnership.
Can I use synastry for friends or family?
Yes. The math is the same for any two people. Read the Venus and Mars contacts as affection and energy rather than romance when the bond is a friendship or a family tie.