Zodiac Decan Calculator
Every Sun sign is split into three 10-day decans, each with its own sub-ruling planet and flavour. Enter your birth date to find your Sun sign, your decan, and the planet that fine-tunes who you are. Free, no signup.
What is a zodiac decan?
A decan (sometimes spelled “decanate”) is a ten-degree slice of a zodiac sign. Each sign covers exactly 30 degrees of the zodiac wheel, and astrologers split that span into three equal parts of 10 degrees each. Because the Sun moves about one degree a day, each decan works out to roughly a ten-day window inside your sign — so two people who are both, say, Leos can still be quite different depending on which third of Leo their birthday falls in.
The idea is ancient: the Egyptians used 36 decans to track the night sky and mark time, and the system was later folded into Hellenistic and modern Western astrology. Today decans are used to add nuance — a second planetary “sub-ruler” layered over your sign’s main ruler, tinting its core traits with an extra flavour.
The three decans of every sign
Each sign’s three decans run in order through your birthday:
- First decan (days 1–10). The “purest” expression of the sign, ruled by the sign’s own planet — a double dose of its core energy. An early Aries is the most headfirst Aries; an early Cancer the most tender Cancer.
- Second decan (days 11–20). The sign coloured by the next planet of its element. The base traits stay, but a new sub-ruler shifts the tone — more charm, more discipline, more drive, depending on the planet.
- Third decan (days 21–30). The sign blended with the third planet of its element, often the most distinctive version — your sign stretched toward a different mood entirely.
How decan rulers work
The sub-ruling planets come from the triplicity (or Chaldean) system, the most common one in popular astrology. Within each element — Fire, Earth, Air, Water — the three signs share their planets across one another’s decans, walking in zodiac order.
Take the Fire signs as an example. Aries’ decans are ruled by Mars, then the Sun (Leo’s ruler), then Jupiter (Sagittarius’ ruler). Leo’s decans run Sun, Jupiter, Mars; Sagittarius’ run Jupiter, Mars, Sun. So a third-decan Aries carries a streak of Sagittarian adventure, while a third-decan Leo gets an extra shot of Martian boldness. The same pattern repeats inside each element, which is why your decan ruler is always a planet that “belongs” to your element — it deepens your sign rather than contradicting it.
Your first decan is always ruled by your sign’s own planet, which is why first-decan natives tend to feel like the most textbook version of their sign. The second and third decans are where the interesting blends show up.
Are decans accurate?
Decans are a real, traditional layer of astrology — but they’re still only a refinement of your Sun sign, which itself is just one placement in your chart. Treat your decan as a useful shading on your sun-sign personality: a reason two people of the same sign can read so differently. It is not a substitute for your full birth chart, where your Moon, Rising, and every planet’s exact placement tell the complete story.
The calendar dates we use are the conventional ten-day thirds of each sign. Because Sun-sign boundaries drift by about a day across the leap-year cycle, a birthday right at the edge of a decan is best confirmed against an exact chart — but for almost everyone, the third you fall in is clear.
Questions about zodiac decans
What is my decan?
Your decan is which third of your Sun sign your birthday falls in. Every sign spans 30 degrees, split into three 10-degree (roughly ten-day) decans. The first decan covers the first ten days of your sign, the second the middle ten, and the third the final stretch. Enter your birth date above to see yours.
How are decans calculated?
Each zodiac sign is divided into three equal 10-degree parts. Since the Sun moves about one degree per day, each decan works out to about ten calendar days inside your sign. This calculator finds your Sun sign from your birth date, then checks which ten-day third your birthday lands in.
What is a decan's ruling planet?
Each decan has a sub-ruling planet from the triplicity (Chaldean) system. Your first decan is ruled by your sign's own planet; the second and third are ruled by the planets of the other two signs in your element, in zodiac order. That sub-ruler adds a second flavour on top of your sign's main traits.
Does my decan change my zodiac sign?
No. Your Sun sign stays the same — the decan only refines it. Think of it as fine-tuning: two people of the same sign born in different decans share the sign's core but differ in tone, because each decan carries a different sub-ruling planet.
Are zodiac decans real or accurate?
Decans are a genuine, ancient layer of astrology, dating back to the Egyptians' 36 decans. They're a meaningful refinement of your Sun sign, but they describe only one placement. For the full picture you need your complete birth chart, which includes your Moon, Rising, and every planet's exact position.
