Enter your birth date to find your Sun sign decan, its degree range and its ruling planet under both the Chaldean and triplicity systems.
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Example: someone born on 14 April has the Sun at about 24 days into Aries, which makes it the third decan of Aries, ruled by Venus (Chaldean) and Jupiter (triplicity).
A decan is a 10-degree slice of a zodiac sign. Every sign covers 30°, so each one splits into three decans (0°–9°59′, 10°–19°59′ and 20°–29°59′), which gives 36 decans around the zodiac. Each decan has its own ruling planet that adds a second shade to the sign, so two people with the same Sun sign can read quite differently.
The word comes from the Greek deka, meaning ten. Decans are also called decanates or faces. They began as Egyptian star groups whose rising helped mark the hours of the night, and Hellenistic astrologers later linked them to planets. Your Sun sign, for example, may sit in the first, second or third decan, and the same applies to your Moon, Ascendant and every other placement.
The calculator does this instantly, but the method is simple enough to check yourself:
Worked example: a 14 April birthday is 24 days after 21 March, so it falls in the third decan of Aries. Chaldean ruler: Venus. Triplicity ruler: Jupiter (the third decan belongs to Sagittarius, the last fire sign).
Astrologers use two ways of naming decan rulers, and they often disagree. This calculator shows both so you can compare them.
| System | How rulers are assigned | Aries decans | Mostly used in |
|---|---|---|---|
| Triplicity | The sign, then the next two signs of the same element | Mars, Sun, Jupiter | Modern sun-sign astrology |
| Chaldean | Seven planets repeat in Chaldean order from Mars at 0° Aries | Mars, Sun, Venus | Traditional and Hellenistic astrology |
The Chaldean order is Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, from the slowest to the fastest planet as seen from Earth. In traditional astrology, a planet in its own decan (face) earns a small dignity point. If you follow modern horoscopes, read the triplicity result. If you study traditional charts, read the Chaldean one, and stay with one system within a single reading.
Dates are the standard Sun sign dates and can shift by a day from year to year. Use the calculator above or a birth chart if you were born close to a boundary.
Mar 21–Apr 19. Fire sign ruled by Mars.
Apr 20–May 20. Earth sign ruled by Venus.
May 21–Jun 20. Air sign ruled by Mercury.
Jun 21–Jul 22. Water sign ruled by Moon.
Jul 23–Aug 22. Fire sign ruled by Sun.
Aug 23–Sep 22. Earth sign ruled by Mercury.
Sep 23–Oct 22. Air sign ruled by Venus.
Oct 23–Nov 21. Water sign ruled by Mars.
Nov 22–Dec 21. Fire sign ruled by Jupiter.
Dec 22–Jan 19. Earth sign ruled by Saturn.
Jan 20–Feb 18. Air sign ruled by Saturn.
Feb 19–Mar 20. Water sign ruled by Jupiter.
The ruling planet colours how the sign shows up. Treat these as themes to reflect on rather than predictions. Under the Chaldean cycle, Mars rules six decans and every other planet rules five.
| Planet | Themes it adds | Chaldean decans ruled |
|---|---|---|
| Mars | drive, courage, assertion | 6 |
| Sun | vitality, pride, leadership | 5 |
| Venus | charm, harmony, desire | 5 |
| Mercury | wit, curiosity, speech | 5 |
| Moon | feeling, intuition, memory | 5 |
| Saturn | discipline, structure, endurance | 5 |
| Jupiter | optimism, growth, meaning | 5 |
A useful trick from traditional practice: treat the decan ruler as a helper planet. If your Sun sits in a Mars-ruled decan, look at Mars in your chart, because its sign and house add detail to how your Sun expresses itself.
The Sun moves about 1° a day, so a birth date is usually enough for the Sun decan. People born within a day of a boundary should be careful, because the exact moment the Sun crosses 0°, 10° or 20° changes slightly each year.
Other placements move faster. The Moon changes decan in under a day, and the Ascendant (rising sign) moves through a decan in roughly 40 minutes, so both need an exact birth time. To find the decan of any planet, get its degree from the Birth Chart Calculator or the Planetary Positions Hub, then enter it in the zodiac degree tab above.
| Term | What it means |
|---|---|
| Decan | One of three equal 10° parts of a sign. Also called a decanate. |
| Face | In traditional astrology, another name for the decan. Some esoteric writers use it for the older Egyptian star-decans. |
| Term (bound) | One of five unequal parts of a sign, each ruled by a planet. It carries more weight than a face in the traditional dignity system. |
| Cusp | The border between two signs or houses, not a division inside a sign. |
A decan is a 10-degree division of a zodiac sign. Each sign spans 30°, so it holds three decans, giving 36 in total. Each decan has its own ruling planet that adds a second layer of meaning to the sign. Decans are also called decanates or faces.
Enter your birth date in the calculator above. It finds your Sun sign, counts the days since that sign began and divides by ten: days 0 to 9 are the first decan, 10 to 19 the second and 20 onward the third. For other planets or the Ascendant, use the zodiac degree tab.
They depend on the sign. For Aries, the first decan runs about 21 to 30 March, the second 31 March to 9 April and the third 10 to 19 April. Every sign follows the same 10-day pattern from its start date, and the full list is in the 36-decan guide above.
The triplicity system gives each sign's three decans to the sign itself and the other two signs of its element, so Aries is ruled by Mars, Sun and Jupiter. The Chaldean system repeats the seven classical planets in a fixed order from Mars at 0° Aries, so Aries gets Mars, Sun and Venus.
Neither is wrong, they are two traditions. Modern sun-sign astrology mostly uses triplicity, while traditional and Hellenistic astrologers use Chaldean. This calculator shows both so you can compare, and it is best to stay with one system when interpreting a chart.
Not for the Sun decan unless you were born within a day of a decan or sign boundary. The Sun moves about 1° a day, so a date is usually enough. The Moon changes decan in under a day and the Ascendant roughly every 40 minutes, so those need an exact birth time.
No. Your Sun sign stays the same. The decan adds a secondary influence: in the triplicity system, a second-decan Leo is said to carry a Sagittarius flavour, while in the Chaldean system the same degrees are ruled by Jupiter.
No. A cusp is the border between two signs or houses, while a decan is one third of a single sign. A person born on a cusp sits at the edge of a sign, and a first or third decan is simply the first or last 10° of it.
The Sun enters each sign at a slightly different hour every year, so start dates shift by about a day. This tool uses standard dates, so if you were born near a sign or decan boundary, confirm the result with a full birth chart.
No. Astrology is a symbolic tradition, and studies have not shown that star signs or decans predict personality or events. Treat the decan as a lens for reflection and storytelling rather than a fact about you.
Astrology is a symbolic tradition and is not scientifically validated. This tool is for reflection and entertainment.