What a compatibility score can and cannot be
Any tool that returns a percentage has made a series of choices about what counts and by how much. Most of them hide those choices, which is why two sites can score the same couple at forty and eighty and neither will tell you why.
The score here is the summary of a list, and the list is printed underneath it. Three things feed the number: how exact each contact is, how much weight the bodies involved normally carry, and whether the angle is an easy one or a difficult one. Nothing else. There is no personality model behind it and no database of couples.
Read the number as a headline and the list as the article. If the two disagree, the list is right.
Why exactness matters more than the aspect
A contact within one degree of exact is doing real work, and both people will usually recognise it once it is named. A contact six degrees off is present in the arithmetic and mostly absent from the experience. That difference is larger than the difference between a trine and a square.
This is the single most common error in amateur synastry: treating a long list of loose aspects as evidence of a strong connection. Two charts always produce a hundred or more contacts if the orbs are wide enough. What distinguishes one comparison from another is how many of them are tight.
| Orb | Weight in the score | How it tends to show |
|---|---|---|
| Under 1 degree | Near full | Unmistakable to both people |
| 1 to 3 degrees | Substantial | Recognisable once pointed out |
| 3 to 6 degrees | Small | Background colour |
| Beyond the orb | None | Not listed at all |
Which contacts the score weighs heavily
Contacts involving the Sun, the Moon, Venus, Mars and the ascendant carry more weight than the rest, because those are the placements that consistently describe how a relationship feels from inside. A tight Jupiter to Neptune contact between two charts is real and is not what anyone means when they ask about compatibility.
Major aspects count fully; minor ones count at less than half. Conjunctions are treated as neutral rather than positive, because a conjunction takes the character of the bodies involved. Venus conjunct Venus and Mars conjunct Saturn are not the same event.
Why a difficult comparison is not a bad one
Squares and oppositions lower the score, and that is a modelling decision rather than a judgement. In practice, comparisons made entirely of trines and sextiles are frequently reported as pleasant and inert, while long marriages are full of squares.
A square describes friction that produces motion: the same argument recurring because it is about something real. An opposition describes complementary opposites, which is the most common signature in durable partnerships and also in durable feuds. What genuinely predicts trouble is not the presence of hard contacts but the absence of anything supportive alongside them.
If your score is low and the tight contacts are Venus and Moon aspects, the score is understating the relationship. Read the list.
Sign compatibility, and why it is not used here
Most compatibility content online works from sun signs: a table of which sign gets on with which. That method uses two of the roughly forty placements in a comparison and discards the rest, and it produces the same answer for every Aries and every Libra on Earth.
This calculator does not use it. Two Aries and Libra charts can produce completely different comparisons depending on where the Moons, Venus and Mars actually sit, and the calculator reads the actual positions. If the sun signs happen to be compatible in the traditional scheme, that will show up naturally as a Sun to Sun aspect in the list.
What you need to run it
Two birth dates are the minimum, and they give you every planet-to-planet contact. Adding birth times adds the ascendant and Midheaven contacts and both house overlays, which is a substantial improvement rather than a refinement.
The overlays are what tell you where the relationship happens. The same Venus contact reads as domestic in one person's fourth house and professional in their tenth, and a comparison without them is a description of chemistry with no setting. If only one of you has a birth time, one direction of overlay is still available.
What to do with the result
Look at the headline contacts first. Those are the Sun, Moon, Venus, Mars and ascendant links, pulled out separately at the top, and they usually contain everything you came for.
Then read the tightest three contacts in the full list, whatever they involve. Those are the parts of the comparison that both people are actually living with, and they are frequently more informative than the score.
Finally, look at what is missing. A comparison with no Moon contacts at all describes two people who may get on well and do not particularly settle each other, and that absence is as much a finding as any aspect in the list.
Common questions
How is the score calculated?
Three inputs: how exact each contact is, how heavily the bodies involved normally count, and whether the angle is easy or difficult. The full contact list is printed underneath so you can check the number against what produced it.
Why is my score different on another site?
Because every tool weights differently and most hide the weighting. There is no standard formula for compatibility, which is the main reason to read the list rather than the number.
Does a low score mean we are incompatible?
No. A low score with three exact Venus contacts describes a livelier relationship than a high score with none. The score summarises; it does not judge.
Do we need birth times?
Not strictly. Planet-to-planet contacts work from the dates alone. Times add the ascendant and Midheaven contacts and both house overlays, which is a substantial addition rather than a refinement.
Why do you not use sun sign compatibility?
Because it uses two of roughly forty placements and gives the same answer for every Aries and every Libra alive. If your sun signs happen to aspect each other, that shows up in the list naturally.
Are hard aspects bad for compatibility?
They lower the score and they do not predict trouble on their own. Long marriages are full of squares. What predicts difficulty is hard contacts with nothing supportive alongside them.