Birth Chart Calculator

What is a birth chart?

A birth chart is a diagram of where every planet stood in the zodiac at the exact moment and place of your birth, cut into twelve houses by your local horizon and meridian. The Sun sign you read horoscopes for is one entry on it. The Moon adds how you feel, Mercury how you think, Venus how you love, Mars how you fight; the houses say in which area of life each of those plays out, and the aspects say which of them talk to each other. That combination is why two people with the same Sun sign can be nothing alike, and it is what reading a birth chart actually means.

How is a birth chart calculated?

Three steps: convert your local birth time to universal time, look up where each planet was at that instant, and divide the sky into houses for your birth place. This calculator does all three in your browser. Your city's time zone, with the daylight-saving rules of that year, gives the universal time. An astronomical ephemeris gives the apparent geocentric longitude of each body on the tropical zodiac. The sidereal time at your longitude and your latitude give the ascendant and midheaven, and the Placidus semi-arc method gives the remaining cusps. No part of it is estimated from your Sun sign.

Method: Astronomy Engine ephemeris (open source, MIT); tropical zodiac; true obliquity of date; Placidus houses with whole-sign fallback beyond about 66° latitude; aspect orbs 8° for conjunction, trine and opposition, 7° for square, 5° for sextile.

What if I don't know my birth time?

You lose the houses and the rising sign, and you keep everything else. Tick the box and the calculator uses noon; the Sun and the planets from Mercury to Pluto move slowly enough that their signs and degrees are right for any hour of the day. The Moon travels about 13° a day, so it may or may not be right; the tool checks whether it changed sign on your birthday and, if it did, shows both possibilities with the local time of the change. The Moon sign calculator goes deeper on that one question, and the rising sign calculator shows how wide the window for your ascendant is once you do have a time.

What each planet stands for

The slower a planet moves, the more people share its sign; everyone born within a few years of you has the same Pluto sign, which is why the outer planets describe generations more than individuals, and why the fast ones (Sun to Mars) plus the Moon and rising sign carry most of the personal information.

What do the twelve houses mean?

Houses are the areas of life; a planet's sign says how it acts and its house says where. The first house begins at the rising sign and the houses run counter-clockwise from there. The four angular houses (1st, 4th, 7th, 10th) start at the ascendant, the lowest point, the descendant and the midheaven, and planets in them are the loudest in the chart.

How do I read my birth chart once I have it?

Start with the Big 3, then the tightest aspect, then whatever sits in an angular house. That order covers most of what a professional would say in the first ten minutes. Our beginner's guide to reading a birth chart walks through it, powerful placements explains what makes one planet dominate, and the "Copy chart as text" button above gives you a plain-text version to paste anywhere you like.

Frequently asked questions

What is a birth chart?

A birth chart, or natal chart, is a map of where the Sun, Moon and planets stood in the zodiac at the exact minute and place you were born, divided into twelve houses by the horizon and the meridian at that place. Every placement is a fact of astronomy; the meanings astrology attaches to them are the interpretation. Your Sun sign is one line of it.

Is this birth chart calculator accurate?

Yes. Planet positions come from an astronomical ephemeris, the Astronomy Engine library, and agree with professional chart software to within a few arcseconds. House cusps use the Placidus system, the default in most software, computed with the standard semi-arc iteration and checked against reference implementations to a ten-thousandth of a degree. Your birth city's time zone, including historical daylight-saving rules, converts your local time to universal time.

Can I calculate a birth chart without my birth time?

Partly. Tick "I don't know my birth time" and the calculator gives the planets for noon on your birthday. The Sun, and every planet from Mercury outward, will be correct; the Moon may be off, so the tool tells you if it changed sign that day and shows both. What cannot be calculated without a time are the rising sign, the midheaven and the houses, and any tool that shows them anyway is guessing.

What house system does this calculator use?

Placidus, because it is what most apps and astrologers use and therefore what your chart elsewhere is most likely to match. Placidus is undefined inside the polar circles (roughly beyond 66° latitude); for those birthplaces the calculator switches to whole-sign houses and says so. The planets' signs are the same in every house system; only their house numbers change.

What are aspects in a birth chart?

Aspects are specific angles between two planets as seen from Earth: conjunction (0°), sextile (60°), square (90°), trine (120°) and opposition (180°). Two planets within a few degrees of an exact angle, called the orb, are said to be in aspect, and the planets' meanings combine. This calculator lists the major aspects sorted by how exact they are and marks each as applying (still tightening at birth) or separating.

Why do other sites show my planets in different houses?

Different house systems. Placidus, Koch, Equal, Whole Sign and others divide the chart differently, so a planet near a cusp changes house between them. If your rising sign or Moon sign differs as well, the cause is a time-zone or birth-time discrepancy, not the house system: check that the other site converted your local time correctly.

What does retrograde mean in my birth chart?

A planet marked ℞ appeared to move backwards against the stars on the day you were born, an optical effect of Earth and that planet moving at different speeds. Roughly one person in five has Mercury retrograde natally; it is common and not a flaw. Astrologers read a natal retrograde as that planet's themes being turned inward or arriving on a delay.