Birthday compatibility calculator
A birthday compatibility calculator turns two dates of birth into a score out of 100.
This one shows its working: the formula is published below, every step that fed the number appears next to the result, and the same two dates always produce the same score. Nothing is stored and nothing is sent anywhere — the calculation runs entirely in your browser.
How to use it
Enter the first birth date
Type or pick the first person's date of birth. The tool accepts any valid date; the order you enter them doesn't change the score.
Enter the second birth date
Add the other person's date of birth. Both fields are treated the same way, so swapping them later gives the identical number.
Press Calculate
The compatibility score appears immediately, along with the life path numbers and the group match behind it.
Read the breakdown
Every factor that produced the score is shown next to the result: base group score, modifiers, and final clamp.
Check the formula
Scroll down to see the exact arithmetic the tool uses. You can reproduce the score on paper with the two dates.
How birthday compatibility is calculated
The score is built from three steps: life path numbers, a group matrix, and three modifiers. First, convert each birth date to its life path number. Write the date as YYYY-MM-DD, remove the dashes, add every digit together, and if the total is more than 9, add its digits again until you land on a single digit from 1 to 9.
That gives two life path numbers. Then place each number into one of three groups: independent (1, 5, 7), grounded (2, 4, 8), or expressive (3, 6, 9). Look up the pair in the group matrix: same group = 88; independent + expressive = 72; grounded + expressive = 64; independent + grounded = 48.
Start with that base score. Then apply three modifiers: subtract 6 if the two life path numbers are identical; add 10 if their sum is a multiple of 9; subtract 10 if the difference between the two numbers is 4 or 5. Finally, clamp the total to the range 0–100: anything below 0 becomes 0, anything above 100 becomes 100.
The main score is exactly this arithmetic, nothing else, and the result screen shows each part so you can verify it. The zodiac signs shown below the score are a separate calculation.
Questions people ask
How is birthday compatibility calculated?
Each birth date is reduced to a single digit from 1 to 9 by adding all digits of YYYY-MM-DD together and repeating until one digit remains. Those two life path numbers are placed into three groups — independent, grounded, expressive — and a base score comes from a fixed matrix. Three modifiers then adjust the score, and the total is clamped between 0 and 100. The full formula is published above.
Is birthday compatibility the same as star sign compatibility?
No. Star sign compatibility compares zodiac signs; the main score here uses the full birth date as digits. It does not consult astrology, positions of planets, or any birth chart. The score comes only from arithmetic on the two dates you enter. Below it you also see both zodiac signs and how well they match: that is a separate calculation, and it does not change the score.
What if we have the same birthday?
Then both life path numbers are identical. The base score starts at 88 because you are in the same group, and the 'same number' modifier subtracts 6. If the sum of the two identical numbers happens to be a multiple of 9, an extra 10 is added before the final clamp.
Why does the score never change for the same two dates?
Because the calculation is fixed arithmetic on the two dates, with no randomness and no hidden state. Enter the same pair next week or next year and the result will be identical. That is deliberate: a score that drifted between visits would be entertainment pretending to be measurement, and you could never check it.
Are our birth dates saved anywhere?
No. The two dates never leave your browser. There is no server, no account, no email, and no storage on your device. The calculation happens locally on this page, and as soon as you close the tab, nothing you typed remains.
What does a low score actually mean?
Only that the arithmetic produced a number near the bottom of the range. It does not mean a relationship is bad, doomed, or dangerous. The tool measures nothing about two people; it just applies a fixed formula to two dates. Treat a low score the same as a high one: as a game, not a judgement.
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Stop wondering — run the test
Two names, ten seconds, and the whole formula in the open.