Synastry Compatibility Calculator

Synastry compares two birth charts to explore relationship themes — how one person's planets interact with another's through aspects and house overlays. Professional readings weigh moon signs for emotional rhythm, Venus and Mars for attraction and conflict style, Saturn for longevity tests, and dozens of other combinations. This calculator uses a simplified model only: it derives each person's sun sign from birth date and scores element harmony between them.

The result is a tier — Harmonious, Balanced, or Challenging — plus a short traditional summary written in the language of pop astrology. That tier is a starting point for conversation, not a relationship verdict. Real partnerships depend on communication, shared values, life stage, and context far beyond sun signs alone.

Worked example: Person A born July 15, 1990 (Cancer sun) and Person B born November 8, 1992 (Scorpio sun). Both are water signs in the tropical zodiac, so many element-based tables describe the pair as emotionally intuitive and mutually understanding in symbolic terms. The calculator shows each derived sun sign, a harmony score, and the tier label.

Another worked example: a fire-sign sun paired with an earth-sign sun may score as Challenging in element folklore — meaning different pacing and priorities in symbolic language, not incompatibility. Fire tends toward action and visibility in horoscope write-ups; earth toward stability and routine. Couples with that mix often report both attraction and friction regardless of score.

This page differs from the zodiac compatibility calculator, which lets you pick signs from dropdowns without entering dates. Here, birth dates drive the sun signs automatically — useful when you know birthdays but not which sign names correspond to them.

Synastry cannot predict breakups, marriage, or chemistry. Astrological comparison is symbolic and for entertainment. Do not use results for major life decisions such as ending a relationship, choosing a roommate, or hiring someone.

For deeper comparison, both people would need full birth times and places so moon, rising, and inter-aspects can be calculated. Sun-sign synastry is the shallowest layer in the tradition — accessible, but incomplete relative to a professional chart reading.

The default example uses July 15, 1990 and November 8, 1992 — Cancer and Scorpio suns in the tropical zodiac. Run those dates first to see how birth-date inputs map to derived signs and harmony score, then substitute real birthdays without retyping sign names manually.

Harmony scores are integers derived from element tables — they are not percentages and do not sum to one hundred across all possible pairs. Compare tiers and summary text rather than treating the score as a precise compatibility percentage.