Relationships

Synastry Chart Free: How to Read Two Birth Charts Side-by-Side (and Why Calculators Can Disagree)

By ZodiScope. Updated October 3, 2026.

A synastry chart is the technical name for laying two birth charts side by side and reading the connections between them. It's one of a handful of techniques built from two complete birth records; composite and Davison charts are the other common ones. Most of this site is about one person's placements. Your moon sign is yours. Your Saturn return is yours. Your stellium is yours. Synastry is where astrology turns from you to a relationship.

Most people looking for a free synastry chart want a tool that will tell them something about a specific other person. It's common to run the same two birthdays through a couple of sites and get aspect lists that don't match. That's usually settings rather than mystery: calculators can use different orbs, house systems and zodiacs, and each choice changes what ends up in the table.

Below: what synastry is (and how it differs from a single-chart reading, a composite chart or a Davison chart), why calculators can disagree, the four aspects this guide reads first (conjunction, opposition, trine, square), five planetary contacts to check first (Moon-Moon, Sun-Moon, Venus-Mars, Mercury-Mercury, Saturn contacts), house overlays, how transits such as Saturn in Aries differ from the natal comparison, and what synastry can't tell you about a relationship.

The short answer: a free synastry chart overlays two complete birth charts and lists the aspects between them. Start with the Moon, Sun, Venus, Mars, Mercury and Saturn contacts, then check which of your houses your partner's planets fall in. Use one calculator with known settings, and read it as description, not prediction.

What a synastry chart actually is (and what it isn't)

A synastry chart starts with two complete natal charts: every planet, the Ascendant, the Midheaven and the houses for each person, calculated from full birth data (date, exact time, place). They're shown on a single wheel, one chart inside and the other outside, and the reading looks at the relationships between the two. When a planet in one chart forms an aspect to a planet or point in the other, within the orb the reader allows, that's a synastric contact. Which planets, which aspect and which house are what the reading is built on.

Two other two-chart techniques often get mixed up with it. A composite chart is a single new chart made from the zodiacal midpoints of corresponding positions: the midpoint of the two Suns, the midpoint of the two Moons, and so on for each planet and point. A Davison relationship chart works differently. It finds the midpoint in time between the two births and a midpoint location between the two birthplaces, then casts an ordinary chart for that moment and place. Synastry keeps both natal charts intact and reads them against each other. A common way to put the difference: synastry describes how two people interact, while composite and Davison charts are read as a picture of the relationship itself. Most people searching for a free synastry chart want the first, meaning how a partner's placements land on theirs.

What a full synastry chart needs beyond a sun-sign comparison: birth time for both people. The Moon changes signs roughly every two to two-and-a-half days, so a birth time can resolve which Moon sign to use. Without birth times you can still compare many planetary signs, including the Sun, Mercury, Venus and Mars, while checking for sign changes around each date. Exact aspect angles are less certain, particularly near an orb cutoff. The Moon's sign can be uncertain on days when it changes sign, and its exact degree is uncertain on any day. The birth chart guide covers what each placement means; for synastry the key addition is the rising sign, which sets the 1st and 7th house axis that many astrologers read for partnership. (Before you build the full overlay, run the sun, moon & rising calculator for both of you. It needs a birth time before it will return a rising sign at all, which makes it a fast test of whether you actually have the data synastry needs.) The rising sign guide covers why the Ascendant matters specifically in relational reading.

Why free synastry calculators can give different answers

If you've compared a few calculators and got a few different answers, you haven't done anything wrong. The tools really can produce different results from the same birth data. Three settings account for most of it.

Reason one: orb settings. The orb is how far the angle between two planets can sit from the exact aspect angle and still count as that aspect. A trine is exact at 120°, so with a 7° orb it covers roughly 113° to 127°, and with a 3° orb only 117° to 123°. Software differs in the orbs it allows, and some tools vary them by planet or by aspect, so a Venus-Mars angle of 114° can show up as a trine on one site and vanish on another. Looser orbs also mean longer aspect lists. That's why a raw count of aspects tells you more about the settings than about the couple.

Reason two: house systems. There are many ways to divide a chart into twelve houses. Placidus, Whole Sign, Equal, Koch and Porphyry are among the familiar ones, and the Swiss Ephemeris programming reference documents many more. Each method can give different cusps from the same birth data, so a partner's planet in your 7th house under Placidus might sit in your 6th or 8th under Whole Sign. That changes the house overlay, which is one of the two main synastry layers in this guide. It doesn't change the angle between two planets, so the aspect list stays the same.

Reason three: tropical vs sidereal. Most Western astrology uses the tropical zodiac, which puts 0° Aries at the vernal equinox. Vedic astrology and some Western astrologers use a sidereal zodiac, which starts from a point fixed against the stars. The gap between the two zero points is the ayanamsha, and there are several competing values for it (the Swiss Ephemeris reference lists them). Both zodiacs still use twelve equal 30° signs; sidereal signs don't follow the uneven outlines of the actual constellations. The offset is now about 24°, so a planet in roughly the first 24 degrees of a tropical sign usually falls in the previous sign in sidereal. That moves sign placements a lot. It barely moves synastry aspects, because both charts shift by almost the same amount and the angle between your Venus and their Mars stays nearly identical. What's left over comes from the ayanamsha creeping forward between the two birth dates, and it's small.

The practical answer: pick one calculator and stay on it, and find out what its orb, house and zodiac settings are. Aspects and houses only compare cleanly within the same settings. The birth chart sites comparison looks at how popular calculators differ, which helps if the synastry you ran two years ago doesn't match the one you ran last week.

The four aspects this guide reads first

An aspect is the angular distance between two planets, measured around the zodiac. The five major aspects are the conjunction (0°), sextile (60°), square (90°), trine (120°) and opposition (180°), and there are minor ones such as the quincunx and semisextile. Many calculators put them all in one table. This guide reads four first: conjunction, opposition, trine and square. Sextiles count too, just more quietly in most readings, and minor aspects are usually treated as supporting detail. That's our emphasis; other astrologers weigh them differently.

• Conjunction (0°): the merger. Two planets at or near the same zodiac degree, within whatever orb the reader allows. The traditional reading is fusion. Your Venus on their Mars puts the planet of affection and the planet of pursuit in the same spot, which astrologers read as strong attraction or tangled dynamics depending on the planets involved. Sun-Moon, Moon-Venus and Venus-Mars conjunctions are among the contacts synastry writers discuss most.
• Opposition (180°): the mirror. Planets across the zodiac from each other. Your Sun in Aries with their Sun in Libra is the textbook example. The symbolism is polarity, each person holding what the other lacks. Oppositions aren't read as bad; they're read as a pull between two sides that goes better when both people can see it.
• Trine (120°): the easy flow. Planets about four signs apart, often in the same element. The element is a shortcut, though. A trine is defined by degrees, so two planets in same-element signs can be out of orb, and a trine near a sign boundary can link signs of different elements. Trines are read as ease and comfort. The caution in this guide: pleasant contacts don't do the work of a relationship for you.
• Square (90°): the friction. Planets about three signs apart, often in signs of the same modality (cardinal, fixed or mutable), though again the degrees decide. Squares are read as tension and recurring arguments, and also as the contacts that push people to adjust. The useful part is naming it: "this is the thing we always fight about" is easier to work with than a brand new fight every time.

An intuition worth dropping: the "best" synastry isn't simply the one with the most trines and fewest squares. In this guide's reading, the contacts that deserve your attention involve the personal planets (Sun, Moon, Mercury, Venus and Mars) and Saturn, and what matters about the friction contacts is whether the two of you can talk about them.

The five placement contacts to check first (in order)

With loose orbs, two charts can produce a long list of aspects. This guide narrows it to five contacts, read in the order below. It's a reading order we chose for clarity, not a standard every astrologer follows, and the rest of the table still adds detail.

1. Moon-to-Moon: emotional needs. In astrological symbolism the Moon describes how you comfort yourself, what you need when you're upset, what a bad day asks for. Harmonious Moon-Moon aspects (trine, sextile) are read as similar emotional styles. Squares and oppositions are read as different ones, say one person wanting to talk it through while the other wants to be left alone for a while. It's a description to check against how the two of you actually are, and it doesn't predict whether you can live together. The full framework is in the moon sign compatibility piece, which reads the two Moons by element and traditional dignity.

2. Sun-to-Moon: the "feels like home" contact. One person's Sun (identity, vitality, the public self) in aspect to the other's Moon (the private emotional side). Conjunctions and trines are the classic "feels like home" reading, where the Moon person finds the Sun person familiar and easy to be around. Astrology writing often calls it a "marriage aspect." That's a traditional label; we aren't aware of good evidence that it turns up in married couples more often than chance would predict. Squares and oppositions are read as contrast rather than ease, which isn't the same as bad.

3. Venus-to-Mars: chemistry and pursuit. Venus is read as what you love; Mars as how you go after what you want. A Venus-Mars contact puts the planet of attraction next to the planet of desire, which is why it's the best-known "chemistry" aspect in pop synastry. The caution: attraction isn't durability, and no single contact decides whether a relationship lasts. The Venus sign meaning piece covers what each Venus sign is said to want, and the Venus-Mars question comes down to whether your partner's Mars style matches what your Venus is asking for. Missing someone's Venus sign? Find their Venus placement with their birth date and birthplace.

4. Mercury-to-Mercury: communication tempo. Easy to overlook. Mercury is how you think and talk: pace, how much you process out loud, whether you reason your way to a conclusion or go on hunches. Close harmonious Mercury aspects are read as conversations that flow; tense ones as conversations that need a bit of translation, which is fine in good moods and tiring in bad ones. Not glamorous, but it's the day-to-day layer.

5. Saturn contacts: commitment and pressure. Saturn stands for structure, duty and time in astrology, so Saturn-Sun, Saturn-Moon, Saturn-Venus and Saturn-Ascendant contacts are read as the serious, committed side of a connection. They can also be read as heavy, with one person feeling judged or held back by the other. That's symbolism about the tone of a bond, not a forecast of how long it lasts. The piece on the Saturn return covers what Saturn is said to do in one chart; in synastry the same themes play out between two people.

Beyond these five: Ascendant/Descendant contacts (attraction and the partnership axis), Jupiter contacts (growth and shared optimism), and outer-planet contacts (Uranus, Neptune, Pluto), which tend to be generational and are usually read only when they form tight aspects to personal planets across the two charts. The stellium piece covers another pattern worth checking: a stellium in your chart that your partner's planets aspect.

House overlays: the second layer

If aspects are the first synastry layer, house overlays are the second. A house overlay is which of your twelve houses your partner's planets land in, so it depends on your birth time and on the house system you use. It's read as the life area their planets touch. The twelve houses explainer covers what each house means in a single chart; in synastry, the relevant question is not "what does my 7th house mean" but "what happens when my partner's Moon lands in my 7th house specifically."

Six overlays worth knowing, and how they're traditionally read:

• Their planets in your 7th house (partnership). The 7th house is the partnership house. Partner planets here are read as touching your idea of partnership directly, which is why this overlay gets so much attention.
• Their planets in your 1st house (identity). Partner planets in your 1st house are read as touching your sense of self. A partner's Moon here is often described as the "I feel more like me around you" overlay.
• Their planets in your 4th house (home / private self). The home and family house. Partner planets here are read as someone who feels like family, and whether that feels good may depend on what family means to you.
• Their planets in your 5th house (romance, creativity, play). The 5th is the house of romance and play. Partner planets here, especially Venus or Mars, are read as the "fun" overlay.
• Their planets in your 8th house (intimacy, shared resources). The 8th is the house of deep intimacy and shared money. Partner planets here are read as intensity, emotionally, sexually or financially.
• Their planets in your 12th house (private / unconscious). Probably the most mythologized overlay. It's read as hidden, hard to put into words and sometimes fated-feeling, and it can also describe a connection that stays out of view. The pattern is more interesting than the verdict.

Why house overlays matter alongside aspects: in the symbolism, a Venus-Mars trine falling in the 11th house (friendship) reads differently from the same trine in the 5th (romance) or the 7th (partnership). Same aspect, different emphasis. If a calculator shows only aspects, look for a houses or overlay view, and remember that overlays need an accurate birth time for the person whose houses you're using.

Synastry vs transits: the fixed map and the moving sky

Synastry compares two natal charts, and those don't change. Transits are where the planets are now, read against each chart. Astrologers use them for timing questions, like why something feels heavier this season or why an old topic is back. That's a separate layer from compatibility, and a transit doesn't turn a good match into a bad one or the other way round.

• Saturn transits. Saturn is read as structure and commitment. When it crosses a sensitive point in one partner's chart, the symbolism is a sober review of what that person is carrying, and relationships often come up in that review. For the current passage and its dates, see Saturn in Aries and the Saturn-Neptune conjunction guide. The first Saturn return is the personal version of the same theme.
• Outer-planet transits. Uranus, Neptune and Pluto move slowly, so their transits are read as long background themes, such as a change in how someone communicates or what they want from a partner. They say more about the individual chart they touch than about the pairing.
• Venus and Mercury retrogrades. A common astrological convention treats Venus retrograde as a time to revisit a relationship rather than start one, and Mercury retrograde as a time for crossed wires and second drafts. They're conventions rather than rules, and the dates and signs change every cycle, so check a current calendar such as our Mercury retrograde guide.

A synastry chart answers "what does this pairing look like in astrological terms?" A transit answers "what's being emphasized right now?" If your question is why something changed this month, that's a transit question. The natal charts didn't move.

What synastry can't tell you

The limit worth stating plainly: synastry shows the terrain, not the destination. The chart describes, in symbolic terms, where ease and tension might show up and which contacts to talk about. It can't tell you whether the two people involved will do anything with that.

A study by Helgertz and Scott (2020) found no consistent support for the Sun-sign compatibility rules it tested against Swedish marriage and divorce records. It did not test full synastry. More chart detail does not, by itself, demonstrate predictive accuracy.

Specifically, what synastry does not predict:

• Whether the relationship will last. No aspect pattern guarantees a long relationship or rules one out. What happens depends on the two people, their circumstances and what they choose to build.
• Whether you should leave or stay. Synastry offers themes to consider, but it cannot establish what is happening between two people. Your experience of the relationship, including how you treat each other, matters more than any aspect label.
• Whether someone is your "soulmate." This is the question synastry searches are usually secretly asking, and the honest answer is that the soulmate framing is not in the chart. The chart offers a detailed symbolic description; the "soulmate" label is a story people place on top of the chart afterwards.
• What to do about a contact you don't like. A Moon-Mars square in synastry is read as one person's feelings and the other person's temper rubbing against each other. It does not tell you whether to therapy through it, name it out loud, or treat it as a deal-breaker. The chart names the pattern; the people decide what to do with it.

If you find the symbolism useful, turn it into a specific question. A Moon-Mars square might prompt you to ask what happens when one person needs comfort and the other wants action. Then talk about an actual disagreement: what each of you needed, what was said, and what you would like to do differently. The chart is a starting point for that conversation.

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