Synastry Chart Calculator: Two Birth Charts, Every Aspect Both Ways

What a synastry chart shows

Synastry lays two natal charts over each other. On the bi-wheel the base person sits inside with their houses, the partner's planets sit on the outer ring, and lines join the points that form an aspect across the two charts. Cafe Astrology's overview describes the usual approach: start with Venus, then work through every inter-aspect, with Venus, Mars, Mercury, Jupiter and Saturn carrying most of the story.

What you need for both people

• Date of birth on the local calendar.
• Time of birth, ideally from a birth certificate. The sky turns one degree every four minutes, so a guess that is half an hour out can move the Ascendant by five to eight degrees.
• Place of birth, for coordinates and the time zone in force that day.

If a time is unknown, tick the unknown-time box rather than typing noon.

Getting a clean result from the synastry chart calculator

1. Type each birth city. Coordinates and the time zone fill in, and the offset is worked out for that exact local moment, labelled standard or summer time.
2. Check the offset first. The form warns when a clock time did not exist (spring change), when it happened twice (autumn change) and for births before 1970; compare with a record you trust and override the offset if needed.
3. Enter hours and minutes. Blank minutes count as :00; an impossible time shows an error.
4. Choose whose houses form the base of the wheel; switch it to see overlays the other way. Placidus is the default, Whole Sign an option.
5. Open the aspect list or grid, the overlays, the composite and the placements (retrograde planets are marked Rx); swap the partners, copy a summary, save a PNG or SVG or print. Nothing leaves your browser, and the last entries stay only in this browser until you press the clear button.

The order I read two charts in

I sort by orb and go through them in a fixed order, so wide contacts do not drown the exact ones.

1. Venus and Mars: affection and desire.
2. Sun and Moon: identity meeting emotional needs.
3. Every other Moon contact: feeling at home.
4. Saturn: commitment and weight, the glue and the homework at once.
5. Ascendant, Midheaven and the Node, with tighter orbs.
6. Generational contacts: Uranus, Neptune and Pluto to the same three in the partner. I skip them; you share them with your whole school year.
7. House overlays, then the composite chart last.

The example pair throughout this page: Sam, born in Chicago at 09:15 on a July morning (UTC−5, summer time), and Lena, born in Madrid at 21:40 on an April evening (UTC+2, summer time). Their key contacts:

Why both directions matter

Sam's Venus squares Lena's Sun at 0°23'. Sam's Sun squares Lena's Venus at 5°20'. Same two planets, two different contacts, and the first is about fourteen times tighter. A tool that merges them keeps whichever it meets first; the old version of this site kept the wide one. The calculator now checks every point of one chart against every point of the other, up to 13 × 13 pairs.

The aspect grid and the orbs I use

The grid puts one person's points in rows and the other's in columns; each cell shows the aspect glyph and the orb, coloured by nature, generational cells in grey. Orbs are inclusive; angles and the Node get tighter ones because they are only as good as the birth time.

One tight square between personal planets tells me more than ten wide sextiles.

In the Aspects tab every line names whose planet is whose, shows its c value (the contribution defined below) and gives a one-line meaning. You can filter by point or by nature (harmonious, challenging, intense, generational) and hide the generational contacts.

How the harmony score is built, and why it is not a verdict

Every scored aspect gets a contribution:

Pair weight is 2 when both points are personal (Sun, Moon, Venus, Mars, Ascendant), 1.5 when one is, 1 when neither is, and 0 for generational contacts. For Sam's Venus □ Lena's Sun: t = 1 − 0.39 ÷ 7 = 0.94, so c = −1 × 2 × 0.94 = −1.89, the largest single contribution in the pair. Positive contributions add up to H = 15.40, negative ones to N = 11.89, and the score is 100 × H ÷ (H + N) = 100 × 15.40 ÷ 27.29 = 56.

The same formula on subsets gives the category bars: Emotional (Moon contacts) 62, Mind (Mercury) 100, Attraction (Venus or Mars) 52, Long-term (Saturn) 30. Mind at 100 only means no hard Mercury contacts; long-term at 30 is mostly the exact Saturn–Moon square, effort to talk about, not a forecast. More aspects do not raise the number: it is a balance, not a count. Under "Why this number" the calculator lists the five biggest contributions with nature × weight × tightness written out.

One forgotten hour of summer time

Enter Lena's birth as UTC+1 instead of UTC+2 and her Ascendant jumps from Scorpio 0°23' to Scorpio 12°11', her Midheaven from Leo 5°59' to Leo 21°01'. Sam's Sun, Moon, Mercury, Jupiter and Saturn each slip back one house in her chart, and contacts appear that are not real, such as Sam's Sun △ Lena's Ascendant at 0°38'. Check both offsets before reading overlays.

When one birth time is unknown

With Lena's time unknown, the chart is cast for local noon and her Ascendant, Midheaven and houses are dropped. The Moon is the problem: over that day it ran from Scorpio 4°00' to Scorpio 18°37', so all seven of Sam's contacts to her Moon are flagged as depending on the birth time. Contacts possible at other hours are listed separately; the pair drops to 48 aspects and a score of 53. Signs and planet aspects hold, houses and angles do not (The Astrology Podcast).

Overlays and the composite chart

An overlay is the partner's planet in your house: where in life you feel them. Lena's Mars falls in Sam's 11th; Sam's Mars in Lena's 7th and his Pluto in her 1st.

The composite is one chart of midpoints on the shorter arc: Sam's Sun at Cancer 12°50' and Lena's at Aries 18°58' give a composite Sun at Gemini 0°54'.

What I checked before trusting the numbers

Planet positions use the astronomical formulas behind the JPL Keplerian elements (JPL Table 1, valid 1800–2050), with ΔT and nutation corrections and the mean Node. Beyond about 66.5° latitude Placidus fails, so houses fall back to Porphyry, as in the Swiss Ephemeris. Time zones come from the IANA database in your browser. Orbs are truncated: 2.995° shows as 2°59'. Results were compared with Swiss Ephemeris:

Going further

• How to read a synastry chart: the full reading order.
• Synastry house overlays: their planets in your houses.
• Composite chart vs synastry: when I use each.