⚭ Free Synastry Calculator: Compare Two Birth Charts

Umbriel's free synastry calculator compares two birth charts. Every aspect between them with its exact orb, and where each person's planets fall in the other's houses. No account, no email. It draws the two charts as a bi-wheel, and gives the pair a score out of 100, built from the same contacts.

• What you'll needBoth people's birth dates, times and places
• What you getEvery aspect between two charts with exact orbs, each person's planets in the other's houses, and the overlay drawn as a bi-wheel
• CostFree, no account required

Synastry is chart comparison: laying one birth chart over another and reading what touches what. It is the oldest and most direct of the relationship techniques, and unlike a composite it keeps both people intact. You can always see who is doing what to whom.

Two things are being looked at. The inter-aspects are the contacts between one person's planets and the other's, and the tight ones do most of the work. A Moon on a Saturn at one degree describes something a ten-degree contact does not. The house overlays are where each person's planets land in the other's houses, which is often the more recognisable half. It describes the area of life each person shows up in for the other.

A word about the score. It is out of 100, and it is the average of four parts: romance, communication, harmony and passion. Each part is built from the aspects between the two charts, and a tight aspect counts for more than a loose one. Every contact it was built from is listed with its orb, so you can see where each point came from and weigh them yourself. The house overlays also need both birth times, with only one, the aspects still stand but half the picture is missing, and Umbriel will say which half.

Synastry compares two birth charts by finding the aspects between every body on one chart and every body on the other, then adds a score from a fixed formula. Both charts are computed by the same engine. The cross aspects cover all points on the two charts except asteroids and Ceres, Pallas, Juno and Vesta, which are held out so that what you switch on cannot change the score. The aspects are conjunction and opposition at 8 degrees, trine and square at 7, sextile at 5, and quincunx and semisextile at 3. The five major aspects get 2 degrees more when either body is the Sun or the Moon, and the whole set can be scaled tight (0.6 times) or wide (1.4 times) in settings. The score starts four sub-scores (romance, communication, harmony, passion) at 50 each and adds or subtracts for a short list of pairs, such as Venus with Mars, Sun with Moon, Mercury with Mercury and Moon with Venus, weighting each contact by how close it is out of 8 degrees. Each sub-score is held between 5 and 98 and the total is their average. The pair's name is taken from the closest contact within 8 degrees among the Sun, Moon, Mercury, Venus and Mars, plus the Rising sign only when both people have a birth time. The score is a count of a few contacts, not a measurement of a relationship. The positions rest on the accuracy page figures (Sun under 1 arcsecond, Moon 0.27 arcminutes, Mercury, Venus and Mars under 0.2 degrees), and the Moon, the Rising sign and any house contact move with the birth time.