Type two birthdays into most love compatibility sites and a number comes back: 82%, 47%, "soulmates", "challenging". What almost none of them show is how that number was produced. Was it two Sun signs looked up in a table? Were the Moon and Venus included? Did a square subtract five points or fifty? Without that, a percentage is a claim you cannot check.
This guide opens the arithmetic up. It explains what astrological compatibility compares, how aspects between two charts are found, how one transparent scoring scheme turns them into a percentage, and why the number deserves less weight than the contacts behind it. The worked examples use the Toolz love compatibility calculator, and the formulas quoted are the ones it runs.
A note on framing before starting. Astrological compatibility is a traditional practice with a long literature, not a tested predictor of how relationships go. The planetary positions in this guide are real astronomy; the meanings attached to them are cultural tradition.
TL;DR: Astrological love compatibility, also called synastry, compares the planets in two birth charts and reads the angles between them. The Toolz calculator compares the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn in each chart, plus the ascendant when birth times and places are given. Each contact scores by aspect type (trine +1, sextile +0.7, conjunction +0.6, square -0.8, opposition -0.5), times a weight for the pair of planets (Sun-Moon and Venus-Mars highest at 3), times exactness. Friction counts at half, and the total is scaled to 0-100%. The percentage is a scheme, not a measurement, so the tool lists every contact and what it contributed. It is client-side, free, no signup.
What is astrological love compatibility?
Astrological compatibility is the comparison of two people's birth charts to describe how they interact. The technique is called synastry. Each chart records where the Sun, Moon and planets stood in the zodiac at the moment of birth, and synastry looks at the angles between one person's placements and the other's.
Sun sign compatibility, the "Leo and Aquarius are opposites" style, is the simplest version: it compares one placement from each chart and ignores everything else. A full synastry comparison uses many placements. The difference is large. Two people whose Sun signs are said to clash often have easy contacts between their Moons or Venus placements, and the reverse is common too.
A compatibility score goes one step further than synastry by summarizing all those contacts in a single number. That step has no basis in historical astrology; it is a modern convenience. Done honestly, it means stating the scoring rules so anyone can see what the number is made of.
What does the calculator compare?
The calculator computes positions for eight placements in each chart, using the same astronomy engine as the rest of the Toolz astrology set, built on Jean Meeus's Astronomical Algorithms.
| Placement | Traditionally read as | Needs |
|---|---|---|
| Sun | Identity, direction | Birth date |
| Moon | Emotional nature, instinct | Birth date; time helps |
| Mercury | Communication | Birth date |
| Venus | Affection, what one values and enjoys | Birth date |
| Mars | Desire, drive, how one fights | Birth date |
| Jupiter | Generosity, growth | Birth date |
| Saturn | Commitment, duty, restraint | Birth date |
| Ascendant (rising sign) | How one comes across | Birth time and place |
Without a birth time, every position is calculated for noon on the birth date and the ascendant is left out. The result then says that one or both charts has no ascendant and the score is based on fewer contacts.
How are aspects between two charts found?
An aspect is an angle between two placements that is close to one of a few key divisions of the circle. The calculator checks every placement in the first chart against every placement in the second, so with ascendants that is 8 x 8 = 64 pairs, and keeps the closest matching aspect for each pair.
"Close to" is set by the orb, the tolerance in degrees. Orbs are not standardized in astrology, which is the main reason two calculators disagree about whether an aspect exists. These are the ones this calculator uses:
| Aspect | Exact angle | Base orb | Orb with Sun or Moon involved | Read as |
|---|---|---|---|---|
| Conjunction | 0 | 8 | 10 | Fused, acting as one |
| Sextile | 60 | 5 | 7 | Opportunity, support to be taken up |
| Square | 90 | 7 | 9 | Friction, drive and frustration |
| Trine | 120 | 7 | 9 | Ease, support without effort |
| Opposition | 180 | 8 | 10 | Tension that needs balancing |
The two extra degrees whenever the Sun or Moon is involved follow the common practice of giving the luminaries wider orbs. Only these five major aspects are used for the score.
Each aspect also gets a strength between 0 and 1 that measures exactness:
strength = 1 - (distance from exact angle / allowed orb)
A Sun-Moon trine at 121 degrees is 1 degree from exact with a 9-degree orb, so its strength is 1 - 1/9 = 0.889. The same trine at 126 degrees has a strength of 1 - 6/9 = 0.333. Tighter aspects count for more, which is the one part of any scoring scheme nearly every astrological tradition agrees on.
How does the compatibility score get calculated?
Each contact's score is three numbers multiplied together:
contact score = aspect value x pair weight x strength
Aspect value says whether the contact helps or creates friction: trine +1, sextile +0.7, conjunction +0.6, square -0.8, opposition -0.5.
Pair weight says how much that pairing of planets matters in romantic synastry. The weights follow the emphasis in the tradition, but the exact values are a design choice:
| Pair | Weight | Traditional reading |
|---|---|---|
| Sun - Moon | 3 | Identity meeting instinct, the classic marriage contact |
| Venus - Mars | 3 | Attraction and desire |
| Moon - Venus | 2 | Affection and emotional ease |
| Sun - Venus | 2 | Warmth and liking |
| Moon - Moon | 2 | Two emotional natures reading each other |
| Sun or Moon - Ascendant | 2 | Recognizing and feeling at home with how the other arrives |
| Venus - Ascendant, Sun - Sun, Moon - Mars, Venus - Saturn | 1.5 | Attraction on sight, shared direction, passion or abrasion, commitment |
| Moon - Saturn, Sun - Mars | 1.2 | Security or coldness; cooperation or competition |
| Venus - Jupiter, Mars - Mars, Venus - Venus, Mercury - Mercury, Sun - Saturn | 1 | Enjoyment, fighting style, shared values, easy conversation, restraint |
| Moon - Mercury, Sun - Jupiter | 0.8 | Putting feelings into words, encouragement |
| Any other pair | 0.3 | Counts, faintly |
Then the contacts are totaled with two rules:
- Friction counts at half. The sum of all negative scores is halved before it is subtracted. The reasoning is that a pairing with no tension at all is not the ideal a score should point to; much astrological writing treats some friction as what gives a relationship energy.
- The total is scaled against what this pair could have scored. The "possible" figure is the sum of the pair weights of every contact found, which is what the total would be if every contact were an exact trine.
raw = sum of supportive scores - 0.5 x sum of |frictional scores|
possible = sum of pair weights of all contacts found
percent = round( clamp( 50 + (raw / possible) x 60, 0, 100 ) )
If no contacts are found at all, the score is 50%.
Worked example with three contacts
Take a simple, made-up pair of charts with just three contacts, to see each step. (A real comparison usually finds ten or more.)
| Contact | Aspect value | Pair weight | Orb used / allowed | Strength | Score |
|---|---|---|---|---|---|
| A's Sun trine B's Moon | +1 | 3 | 1 / 9 | 0.889 | +2.667 |
| A's Venus square B's Mars | -0.8 | 3 | 2 / 7 | 0.714 | -1.714 |
| A's Mercury sextile B's Jupiter | +0.7 | 0.3 | 1 / 5 | 0.800 | +0.168 |
Now the totals:
supportive = 2.667 + 0.168 = 2.835
friction = 0.5 x 1.714 = 0.857
raw = 2.835 - 0.857 = 1.978
possible = 3 + 3 + 0.3 = 6.3
raw/possible = 1.978 / 6.3 = 0.314
percent = 50 + 0.314 x 60 = 68.8 -> 69%
The result is 69%. If friction were subtracted in full instead of at half, raw would be 1.121, raw/possible 0.178, and the score 61%. That eight-point gap is the entire effect of one design decision, which is why it is stated on the page.
A real example from two dates
Enter 14 February 1991 for the first person and 30 August 1989 for the second, with no birth times. The calculator finds five supportive and five frictional contacts and scores the pair at 47%. The contact that moved the number most is the first person's Sun opposite the second person's Moon: a heavily weighted pair, in a frictional aspect. A Sun sign table would have compared Aquarius with Virgo and stopped there.
What do the score bands mean?
The percentage is never shown on its own. A one-line description sits under it:
| Score | Description shown |
|---|---|
| 80 to 100 | A great many supportive contacts and little friction |
| 65 to 79 | More support than friction, with enough tension to keep it interesting |
| 50 to 64 | A mixed picture, the ordinary case |
| 35 to 49 | More friction than support; not a verdict, but it will take work |
| 0 to 34 | A demanding combination on paper; charts describe tendencies, not outcomes |
Is a compatibility percentage meaningful?
Only as a summary of the rules that produced it. Three facts are worth keeping in mind.
Different schemes give different numbers. Change the orbs, the weights or the friction rule and the same couple scores differently. There is no standard, so comparing a percentage from one site with a percentage from another tells you nothing.
Few contacts can produce extreme scores. Because the total is scaled against the contacts actually found, a pair of charts with a single exact Sun-Moon trine and nothing else would compute to 50 + 1 x 60 = 110, capped at 100%. Real charts almost always form many contacts, so this rarely happens, but it shows why the list of contacts matters more than the headline.
Astrology is not a tested predictor. When astrological claims have been put to controlled tests, such as the double-blind experiment Shawn Carlson published in Nature in 1985, astrologers did not match charts to personality profiles better than chance. Treat the score as a way into a traditional reading, not as evidence about a relationship. The calculator's result carries the same caveat.
That is why the tool lists every supportive and frictional contact with its orb, then the six contacts that moved the score most with the exact amount each added or removed. If you disagree with a weight, you can see precisely what it did.
Why is Sun sign compatibility not enough?
Sun sign compatibility reduces two charts to one placement each. The traditional element rule, that signs of the same element get along, comes from the fact that same-element signs sit 120 degrees apart, a trine.
| Element | Signs | Traditionally easy with |
|---|---|---|
| Fire | Aries, Leo, Sagittarius | Other Fire signs, often Air |
| Earth | Taurus, Virgo, Capricorn | Other Earth signs, often Water |
| Air | Gemini, Libra, Aquarius | Other Air signs, often Fire |
| Water | Cancer, Scorpio, Pisces | Other Water signs, often Earth |
That is one contact out of dozens a full comparison considers, and sign-based matching also ignores exactness: two Suns at 0 and 29 degrees of their signs are treated the same as two Suns at matching degrees. The calculator asks for two dates rather than two signs for exactly this reason. Your Sun sign still counts, because the date fixes it, but so do the other six planets.
Do you need birth times for a compatibility reading?
Not strictly, but they help. The planets from Mercury outward move at most a degree or two a day, so a noon position is close to the real one. The Moon is different: it moves about 13 degrees a day, so a noon position can be six or seven degrees away from the true position at birth. That is enough to move a Moon contact in or out of orb, and occasionally to change the Moon's sign.
For 30 August 1989, for example, the noon Moon is at 28 degrees 43 minutes Leo, and it moved into Virgo later that day. Someone born that evening has a Virgo Moon, and the calculator, without a time, would not flag it. Adding times and places fixes the Moon and adds both ascendants, which carry some of the higher pair weights.
How to check compatibility, step by step
- Open the astrology compatibility calculator.
- Enter the first person's birth date, and a birth time and city if known. Names are optional.
- Enter the second person's details the same way.
- Click "Compare compatibility".
- Read the percentage with its band description, and the note if an ascendant is missing.
- Read "What helps" and "Where the work is": the supportive and frictional contacts, each with its orb.
- Check "What moved the number most" to see the six strongest contacts and their exact contributions.
For the full grid of every aspect between the two charts without any scoring, use the synastry chart calculator. To look at one placement in detail, the Venus sign calculator covers the planet most associated with love and attraction.
Is our birth information stored?
No. Both charts are computed client-side, in JavaScript in your browser, and neither set of birth details is transmitted, logged or saved. Once the page has loaded it works offline.
Common mistakes when reading compatibility results
| Mistake | Why it misleads | Better approach |
|---|---|---|
| Comparing percentages across sites | Each uses different orbs, weights and friction rules | Compare the contacts, not the number |
| Reading a low score as a verdict | Scores describe tendencies on paper, not outcomes | Read the frictional contacts as themes |
| Ignoring the missing-ascendant note | The score used fewer contacts | Add birth times and places |
| Trusting a noon Moon near a sign change | The Moon moves about 13 degrees a day | Add the birth time |
| Treating 100% as ideal | High scores can come from few, tight contacts, and the scheme discounts friction for a reason | Look at how many contacts produced it |
| Using only Sun signs | One placement out of eight per chart | Compare full charts |
The rest of the set, including Sun, Moon and rising sign calculators and a dating and personal growth calendar, is on the astrology tools hub. When you want to see the scheme applied to two real charts, compare two birth charts and read the contacts before the number.
