Synastry Chart Calculator

A synastry chart is two birth charts drawn as one wheel, with each person's planets laid over the other's so you can read the aspects between them. The two-ring drawing is called a biwheel. Where one person's Moon meets the other's Venus, where Saturn lands on a Sun, those contacts are where the relationship actually lives.

That is the whole idea: keep both charts intact and look at the conversation between them. A natal chart tells you about one person. A synastry chart tells you what happens when their planets sit in the same room as someone else's. Some contacts feel like recognition. Some feel like a lesson. Most relationships run a mix of both, and the wheel shows you which planets are doing the talking.

What it will not hand you is a verdict. There is no pass or fail here, and a chart full of squares is not a doomed relationship any more than a chart full of trines is an easy one. If you want the single weighted number, the compatibility calculator runs it from the same two charts. This page is for reading the picture yourself.

Start at the rings. The first person you enter goes on the outer ring; the second person sits on the inner ring. The legend in the corner names both by color, so you never have to guess whose Mars you are looking at. The whole wheel is turned to the first person's Ascendant, which means their rising sign anchors the left of the chart and the signs run counter-clockwise from there.

The lines across the center are the aspects. Each one connects a planet in one chart to a planet in the other, and the color tells you the tone: ease against friction. Hover any line and you get the two bodies, the aspect, and the exact orb. A tight line is a loud contact. A wide one is a whisper. If the middle looks busy, that is normal for two full charts; read the tightest few lines first and let the rest sit.

Around the outside, small ticks mark each person's Ascendant and Midheaven when the birth time is exact. Those angles carry real weight, because a partner's planet sitting on your Ascendant or Descendant shows up in how the meeting physically feels. When you want every contact spelled out as a sortable matrix instead of lines on a wheel, the synastry aspect grid is the companion to this view.

Not every line on the wheel pulls the same weight. The ones that shape the day-to-day tend to involve the personal planets: the Sun, Moon, Mercury, Venus, and Mars. A Sun to Moon contact between two charts is one of the classic signatures of feeling at home with someone. Venus to Mars is the chemistry axis. Mercury to Mercury tells you whether the two of you actually think in the same language.

Then there is Saturn, which deserves its own paragraph. Saturn contacts show where the relationship asks for structure and staying power, and where it can feel like weight. A Saturn on a Moon can read as steadying or as heavy depending on everything else in the chart, which is exactly why the wheel gives you the contact and lets you weigh it rather than scoring it for you. Node contacts, where one person's North Node touches the other's personal planet, often carry that sense of meeting for a reason.

Tightness is the tiebreaker. A contact inside one degree of orb is doing real work; one near the edge of orb is faint. Sort the contacts by orb and read from the top, whichever bodies they connect. For a weighted read of the chemistry signatures specifically, the soulmate synastry calculator scores Venus, Mars, Juno, and the Lot of Eros contacts.

Four tools get confused for each other, so here is the short version. A synastry chart keeps both birth charts and reads the aspects between them, which is what this page draws. A composite chart merges the two into a single chart built from the midpoint of each pair of planets, reading the relationship as one entity. A Davison chart also makes one chart, but from the midpoint in time and space between the two births, so it is a real chart for a real moment that you can transit and progress.

The compatibility calculator takes the same contacts this wheel shows and returns a weighted percentage, for when you want a fast summary rather than the full picture. And if the question is whose houses light up whose planets, the house overlays calculator lays that out in both directions, since the biwheel here anchors the signs but leaves the house-by-house overlay to that tool. Most readings use two or three of these together: the wheel for the live contact, then a composite or a house overlay for depth.