Synastry Chart Calculator

What Is A Synastry Chart?

A synastry chart is a comparison of two natal charts. It measures the exact angles, called aspects, between one person's planets and the other person's. Those angles show where two people's energies meet easily and where they rub against each other.

Synastry is the traditional technique behind what people often call birth chart compatibility. If your Venus sits within a few degrees of your partner's Mars, that is a contact, and astrologers read it as a specific flavour of attraction. The same applies to Sun and Moon contacts, Saturn contacts and everything else the comparison finds.

The technique is old and simple in principle. Each person's chart stays whole and separate. Nothing is averaged or merged. Astrologers set the two charts side by side and ask which points touch, which points ignore each other, and whether the contacts cluster around the soft angles or the tense ones. A reading then weighs the pattern as a whole rather than counting hits.

How Does This Synastry Chart Calculator Work?

The Synastry Chart Calculator takes two birth dates, times and places, calculates both natal charts, and then measures the angle between every planet in one chart and every planet in the other. It compares Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn, plus each person's Ascendant when both birth times are known. Every aspect it finds is listed in your result with its orb, tightest first, and the table below gives the orbs: 6 degrees for conjunction and opposition, 5 for trine and square, and 4 for sextile.

Alongside the aspect list you see both people's Sun, Moon and rising signs and how their element balance compares, so you can tell at a glance whether the pair is fire heavy or earth heavy, for example.

There is deliberately no compatibility score. A single number would hide which contacts are easy and which are tense, and that distinction is the whole point of synastry. Positions come from Astronomy Engine, an open-source MIT-licensed library, and everything is computed in your browser. No account is needed and birth details are not stored on our servers.

Which Synastry Aspects Matter Most?

Which synastry aspects matter most is a question astrologers answer with a short list. Sun-Moon contacts come first: they are read as the core of emotional recognition, the feeling that the other person simply gets you. Venus-Mars contacts come a close second, associated with attraction and chemistry. Moon-Venus contacts are considered central to emotional ease, the comfort of being cared for in a way that lands.

Saturn contacts deserve special weight. Astrologers read them as commitment and longevity on one hand and friction or heaviness on the other. A tight Saturn aspect to a partner's Sun or Moon is often described as glue, sometimes as a weight, frequently as both at once.

After the personal planets come the slower ones. Jupiter contacts are read as generosity and shared optimism, while hard angles between outer planets tend to colour a relationship less vividly because those placements are shared by whole generations. Tight orbs matter more than the aspect count: a 1-degree square says more than a 5-degree trine.

What Does Each Planet Show In Synastry?

What each planet shows in synastry is the same thing it shows in a single chart, now seen through the other person's planets. The Sun reflects identity and what feels vital, the Moon reflects emotional needs and habits, Venus what a person finds pleasant and lovable, Mars how they pursue and assert. When your Moon lands on your partner's Venus, the caring instinct and the receiving instinct meet directly, and your result lists exactly which of these meetings occur in your two charts.

Mercury shows how each person explains and listens, so Mercury contacts describe whether conversation flows or misfires. Jupiter shows where one person expands the other's confidence. Saturn shows where one person structures, limits or tests the other, which astrologers read as both the commitment and the friction in a bond.

The Ascendant matters too, when both birth times are known. It represents the face each person presents first, so contacts to it describe first impressions and the immediate pull between two people.

Synastry Vs Composite Chart: What Is The Difference?

Synastry and a composite chart differ in what they compare. Synastry keeps both natal charts intact and measures the angles between one person's planets and the other person's. It answers questions about how person A experiences person B. A composite chart is a different technique entirely: it averages the two charts into one, taking the midpoint of each pair of planets, and treats the result as a chart of the relationship itself.

astrologers often say synastry shows the wiring between two people while the composite shows the entity they create together. The two techniques are complementary rather than competing.

This page calculates synastry only. It does not draw a composite chart, because averaging positions answers a different question from the one this tool asks. If you want the composite too, you can find both midpoints by hand from the two natal charts, or use a service that offers it. For most questions about attraction, ease and friction between two specific people, synastry is the technique astrologers reach for first.

Can A Synastry Chart Tell If A Relationship Will Last?

No. A synastry chart cannot tell whether a relationship will last. Astrology describes relationship dynamics symbolically, and the aspects between two charts sketch tendencies: where affection flows, where patience thins, where commitment is likely to be tested. None of that predicts outcomes as fact.

Two people with a sheet full of trines can drift apart. Two people with grinding squares can stay together for decades and be glad of it. Human choice, circumstance and effort sit outside any chart, and honest astrologers say so. The chart describes the weather of a bond, not its destination.

That is also why this calculator refuses to hand out a compatibility score. A number like 78 percent would pretend to a prediction the underlying method cannot make, and it would hide the only useful information synastry offers, which is the detail. Read the aspect list as a map of tendencies, weigh the Sun-Moon, Venus-Mars, Moon-Venus and Saturn contacts most heavily, and then make your own decisions with the people involved.

More Free Astrology Calculators

• Full natal chart with free reading: every planet, house and aspect.
• Rising Sign Calculator: Your Ascendant.
• Moon Sign Calculator: Your emotional sign.
• Big Three Calculator: Sun, Moon, Rising.
• Venus Sign Calculator: How you love.
• Mars Sign Calculator: How you act.
• Saturn Return Calculator: Your turning point.
• Transit Chart: Today vs your chart.
• North Node Calculator: Your growth path.
• Midheaven Calculator: Career and calling.
• Lilith Calculator: Your untamed side.
• How we calculate: the method and the Swiss Ephemeris accuracy test.