How Our Moon Phase Calculator Works
By Bilal Yaseen · Last updated 4 November 2026
The moon phase soulmate test turns two birth dates into two natal moon phases, then reads how those phases relate. The first part is astronomy you can check. The second part is astrological tradition. This page explains both, step by step.
Step 1: Find the moon’s age on each birth date
For each date, the calculator takes 12:00 noon UTC on that day. It counts the days since a known new moon, 6 January 2000 at 18:14 UTC. It then divides that count by the mean synodic month of 29.530588853 days, the average time from one new moon to the next.
The remainder is the moon’s age: how many days into its cycle the moon was on that date. An age of 0 is a new moon. An age of about 14.8 days is a full moon.
Step 2: Name the phase
The cycle is split into eight equal slices of about 3.7 days. Each slice is centred on one of the eight named phases used by NASA and the U.S. Naval Observatory.
| Natal moon phase | Moon age |
|---|---|
| New Moon | 27.7 to 29.5 and 0 to 1.8 days |
| Waxing Crescent | 1.8 to 5.5 days |
| First Quarter | 5.5 to 9.2 days |
| Waxing Gibbous | 9.2 to 12.9 days |
| Full Moon | 12.9 to 16.6 days |
| Waning Gibbous | 16.6 to 20.3 days |
| Third Quarter | 20.3 to 24.0 days |
| Waning Crescent | 24.0 to 27.7 days |
Step 3: Work out how much of the moon was lit
The lit share of the moon’s disc is calculated from its age with the standard formula (1 − cos(2π × age ÷ 29.53)) ÷ 2. This drives the moon drawings in your result. It also drives the “lit together” figure, which shows how much of one full disc your two moons light when you put them side by side. That is the idea behind the TikTok question “do our moons make a full moon?”
Step 4: Turn the two phases into a compatibility score
Each of the 64 possible phase pairs (8 × 8) has a rating from 2 to 5 in a traditional pairing table. The rating sets the score band:
| Rating | Label | Score band |
|---|---|---|
| 5 | Highly harmonious | 92–97% |
| 4 | Strong compatibility | 80–89% |
| 3 | Moderate and workable | 68–78% |
| 2 | Challenging but possible | 56–64% |
The exact number inside a band is fixed for each pair. The same two dates always give the same score. Nothing is random.
The full pairing table is below. Read across from your phase and down from your partner’s phase.
| New | Wax. Cres. | First Q. | Wax. Gib. | Full | Wan. Gib. | Third Q. | Wan. Cres. | |
|---|---|---|---|---|---|---|---|---|
| New Moon | 4 | 4 | 3 | 3 | 5 | 3 | 3 | 5 |
| Waxing Crescent | 4 | 4 | 4 | 3 | 3 | 3 | 2 | 3 |
| First Quarter | 3 | 4 | 3 | 4 | 3 | 3 | 3 | 2 |
| Waxing Gibbous | 3 | 3 | 4 | 4 | 4 | 5 | 3 | 3 |
| Full Moon | 5 | 3 | 3 | 4 | 4 | 4 | 3 | 3 |
| Waning Gibbous | 3 | 3 | 3 | 5 | 4 | 4 | 4 | 3 |
| Third Quarter | 3 | 2 | 3 | 3 | 3 | 4 | 4 | 4 |
| Waning Crescent | 5 | 3 | 2 | 3 | 3 | 3 | 4 | 4 |
The three pairs rated 5 are New Moon + Full Moon, New Moon + Waning Crescent, and Waxing Gibbous + Waning Gibbous.
What the score is, and what it is not
The percentage is a symbolic score drawn from astrological tradition. It is not a measured probability, and no study has shown that birth moon phases predict how a relationship goes. Read it as a prompt for conversation, not a verdict. See our disclaimer.
Known limits of the calculation
- Mean cycle, not the exact moon. The real moon speeds up and slows down along its orbit, so an actual new moon can come several hours earlier or later than the average cycle predicts.
- No birth time. The calculator uses noon UTC on each date. If you were born within about a day of a phase boundary, your true phase may be the neighbouring one. You can check exact phase times on the U.S. Naval Observatory phase tables.
- Dates from 1900 onward. The tool accepts birth years from 1900 to the current year.
- Nothing is stored. The calculation runs in your browser. The dates and names you enter are not saved.