Free Decan Calculator

A decan is a 10-degree subdivision of a zodiac sign. Each sign holds three decans (0° to 9°59′, 10° to 19°59′, 20° to 29°59′), and each decan has its own ruling planet that colors how the sign expresses. Decans refine a placement beyond the sign alone, the way bounds and faces do in traditional astrology.

That definition sounds tidy until you go to use it. Then you hit the question most decan calculators skip past: which decan ruler? Two systems exist, they disagree, and the choice between them actually matters.

Three steps:

1. Find the placement's degree. Not the date. The degree. The Sun moves a little less than 1° per day, so anyone born within three days of the start or end of a sign needs an actual chart, not a date lookup. The calculator above handles this with arc-second precision.

2. Identify the decan. 0°00′00″ through 9°59′59″ is the first decan. 10°00′00″ through 19°59′59″ is the second. 20°00′00″ through 29°59′59″ is the third. The boundary degrees count as the start of the next decan, not the end of the previous one.

3. Look up the ruler. Augurine uses the Chaldean cycle: Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, repeating across all 36 decans starting from Aries 0°. Venus at 12° Virgo lands in the second decan of Virgo, ruled by Venus, so Venus picks up an extra dose of its own keynote inside that segment.

Most pages teach you one system and act like the other does not exist. Both are real. They come from different lineages and they answer different questions.

Chaldean decans (Hellenistic, traditional). The seven visible planets cycle in the fixed Chaldean order: Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter. You start with Mars on the first decan of Aries and walk that cycle through all 36 decans in zodiacal order. This is what Vettius Valens uses, what shows up in Hephaistio, and what traditional astrologers reach for when they are working dignity stacks or timing techniques. The Chaldean ruler is the planet that contributes to a +1 dignity score in the classical five-tier dignity system.

Triplicity decans (modern). Each sign gets divided into thirds, and the ruler of each third is drawn from the same elemental triplicity in cardinal, fixed, mutable order. Aries (cardinal fire) gets Aries-Leo-Sagittarius rulers, so Mars, Sun, Jupiter. This is the system you will see in most contemporary books. It feels coherent because the rulers stay inside the element. It also has almost no ancient pedigree: it is a synthesis that came together in the 19th and 20th centuries.

Augurine defaults to Chaldean because the calculator is part of a traditional dignity workflow that feeds the bounds, almuten, hyleg, and alcocoden tools. If you are doing modern psychological astrology and you want a reading rooted in element theory, the triplicity system is internally consistent. Pick one and stay consistent within a chart.

Date-only decan calculators fall down here. The Sun travels roughly 0°59′ per day. If you are born within three days of a decan boundary, a date lookup will guess wrong about a third of the time. Every other placement is faster or slower than the Sun, but the same logic holds: the Moon crosses a decan in about a day, the Ascendant in roughly forty minutes.

People born close to a boundary often describe themselves as feeling like both decans. Sometimes that is intuition. More often it is bad data. Run the calculator with your birth time and the ambiguity disappears.

The boundary rule: 0°00′00″ inclusive belongs to the new decan. 9°59′59″ still belongs to the previous one. The same rule applies at sign cusps.

Most decan articles stop at the Sun. That is strange, because the technique applies to every chart point. Decans get interesting when you read them across multiple bodies.

Sun decan refines core identity. A Leo I (Saturn-ruled in the Chaldean system) reads very differently from a Leo III (Mars-ruled). The first plays Leo as elder; the second plays it as combatant.

Moon decan flavors the emotional life and habits. A Cancer III Moon under a Moon decan ruler doubles down on watery saturation; the same Moon at 5° Cancer (Venus-ruled in Chaldean) softens toward attraction and appetite.

Ascendant decan shapes how the persona lands in a room. The Ascendant moves about 1° per four minutes, so this decan is the most birth-time-sensitive of all.

Decan ruler condition. Treat the decan ruler as a mini-dispositor. If your Sun is in a decan ruled by Mars, look at where Mars sits in your chart. That placement colors the Sun's expression even though Mars and the Sun are not aspecting each other directly. This is one of the most underused diagnostic moves in traditional practice.

Decans are not just descriptive. They time things.

Transits to the decan ruler. A Jupiter return to the natal Mars that rules your Sun's decan can read like a Sun-Jupiter event even if Jupiter does not aspect the Sun directly. Decan rulers transmit.

Profections that activate a decan ruler. When you profect into a house ruled by the planet that also rules your Sun's decan, expect a year where identity and circumstance reinforce each other. That is a doubled signal, and most people read it as fated.

All three are 36-fold subdivisions of the zodiac. They are not the same.

Decans are the 10-degree thirds we have been discussing. Three per sign, equal sized, ruled by the Chaldean cycle. A planet in its own decan gets +1 essential dignity.

Bounds (or terms) are unequal subdivisions, five per sign, used by Ptolemy and especially by the Egyptian tradition Valens preferred. A planet in its own bound gets +2 dignity. See the Egyptian Bounds Calculator.

Faces is sometimes used as a synonym for decans, sometimes for the older Egyptian 36-fold star system that predates the planetary attribution. In strict Hellenistic vocabulary, face and decan are interchangeable. In esoteric and magical contexts, faces usually means the older star-decan system with its image traditions.

Treat them as different layers of resolution on the same circle. Decans give you a +1 dignity. Bounds give you a +2. Both feed the almuten figuris calculation.

Classical authorities: Vettius Valens Anthology(Chaldean order and the decan's role in essential dignity), Hephaistio of Thebes Apotelesmatika, the Hermetic and Egyptian sources preserved in fragments by Firmicus Maternus.

Modern reception: Robert Hand, Chris Brennan, and the Project Hindsight translations for the Hellenistic revival. Austin Coppock, Gary Caton, and the magical lineage carrying the Egyptian star-decan tradition.

Computational defaults: ephemerides from NASA's ANISE toolkit with JPL DE-440 kernels for arc-second precision. Whole sign houses for placement context. Chaldean decan ordering for ruler lookups. For the rest of the dignity stack, see the essential dignities calculator.