What real chart-to-chart compatibility involves, and how it differs from comparing two Sun signs.
Real relationship astrology compares two entire birth charts to each other — not just two Sun signs.
Synastry is the branch of astrology dedicated to comparing two people's birth charts against each other, planet by planet, to see where they naturally align and where they create friction. A full synastry reading looks at dozens of relationships between the two charts: where one person's Venus (affection, values) sits relative to the other's Mars (drive, desire), how each person's Moon (emotional needs) speaks to the other's Sun (core identity), and many more combinations besides.
To be direct about scope: the Compatibility Calculator on this site is not synastry. It compares two Sun signs by element — Fire, Earth, Air, or Water — which is a real, long-standing piece of astrological theory, but it's one layer of a much bigger picture. Two people with wildly different Sun signs can still have deeply compatible Moons, or two people with a "difficult" elemental pairing can have a Venus-Mars connection that makes the friction feel more like chemistry than conflict. Sun-sign compatibility is a reasonable starting point for reflection; it was never meant to be the final word.
Real synastry needs a full birth chart for both people — birth date, time, and location for each — and a comparison of at least the major planets (Sun, Moon, Mercury, Venus, Mars) between the two charts, not just their Sun signs against each other. This site's Birth Chart Calculator currently computes the Sun, Moon, and Rising sign for one person at a time — a genuine step closer to synastry than a Sun-sign-only comparison, since you can compare two people's Moons to each other manually, but still short of a full automated synastry report.
Until a dedicated two-chart synastry tool exists here, the most honest way to get a fuller picture with what's already available: run the Birth Chart Calculator for both people separately, then compare all three placements — Sun, Moon, and Rising — between the two charts by hand, rather than relying on the Sun-sign-only Compatibility Calculator alone. Two Moons in compatible elements often say more about day-to-day emotional ease than two Suns do.
Start with the Compatibility Calculator for a quick elemental read, then run the Birth Chart Calculator for both people to compare Moon and Rising signs for a fuller picture.