How the compatibility score is calculated
Every compatibility site gives a percentage and almost none of them say where it comes from. Here is ours, in full, so you can reproduce any number on this site with a pen.
Step 1: the element base
The two signs' elements set the starting value. This follows the traditional groupings: fire with air, earth with water.
| Relationship | Example | Base |
|---|---|---|
| Same element | Fire with fire | 88 |
| Complementary elements | Fire with air, earth with water | 82 |
| Neighboring group | Fire with air's own group, earth with water's | 70 |
| Contrasting groups | Fire with water, air with earth | 62 |
Step 2: the modality adjustment
Two signs of the same modality both want to operate the same way, which the tradition treats as friction: minus 4. Different modalities complement each other: plus 3.
Step 3: the traditional best-match bonus
If the second sign appears on the first sign's traditional best-match list, the score gains 6.
Step 4: the scale limits
The result is held between 55 and 96. Same-sign pairs skip steps 1 to 3 entirely and take a fixed 76, because comparing a sign with itself has no element or modality difference to measure.
A worked example: Aries and Leo
| Step | Points | Running total |
|---|---|---|
| Both signs are Fire: same element | sets the base | 88 |
| Cardinal with Fixed: different modality | +3 | 91 |
| Leo is on the traditional best-match list for Aries | +6 | 97 |
| Capped at the scale maximum | -1 | 96 |
| Final score | 96% |
Aries and Leo sit 120 degrees apart on the wheel, which is a trine. See the Aries and Leo page for the same table alongside the written notes.
The aspect angle
Every pair page also shows the angle between the two signs. Each sign occupies 30 degrees of the 360-degree wheel, so the angle is simply the shorter distance between them in signs, multiplied by 30. It is not part of the score; it is included because it is the classical way of describing a pairing, it is checkable, and almost nobody publishes it as data.
| Degrees | Name | Signs apart |
|---|---|---|
| 0 | Conjunction | 0 |
| 30 | Semi-sextile | 1 |
| 60 | Sextile | 2 |
| 90 | Square | 3 |
| 120 | Trine | 4 |
| 150 | Quincunx | 5 |
| 180 | Opposition | 6 |
What this number is not
It is not a measurement, not a prediction and not advice. It is arithmetic over two pieces of traditional symbolism, published openly so that it can be checked rather than believed. Birthdexmakes no claim that a zodiac sign affects any real relationship. The wider reasoning behind the site's data is on the methodology page.
Frequently asked questions
Why does every site give a different compatibility percentage?
Because every site uses a different formula, and almost none of them publish it. The numbers are not measurements of anything, so there is nothing for them to agree on. Ours is set out in full on this page precisely so you can see what it is and is not.
Does a high score mean a relationship will work?
No. The score is arithmetic over two pieces of traditional symbolism. It knows nothing about the two people involved and predicts nothing about them.
Why is the maximum 96 and not 100?
Because a scale that reaches 100 implies a perfect match exists, and this model has no basis for saying that. The range is deliberately capped at 96 at the top and 55 at the bottom.
What is an aspect angle?
It is the angle between two signs on the 360-degree wheel, where each sign takes up 30 degrees. Signs opposite each other are 180 degrees apart, four signs apart is 120 degrees, and so on. It is plain geometry from the sign order.