How to calculate your birth chart, step by step
The data you need, where to find your birth time, what happens during the calculation and how to get your natal chart interpreted for free in a few minutes.
5 min read · Updated
What a birth chart is and what gets calculated
The map of the sky at your birth
A birth chart (or natal chart) is a circular diagram showing the position of the Sun, Moon and planets, seen from where you were born, at the exact minute of your birth. The circle is divided into the twelve zodiac signs and twelve houses.
Calculating it means obtaining four kinds of data: the sign and degree of each planet, your rising sign (ascendant) and midheaven, the twelve house cusps, and the angles (aspects) the planets form with each other.
- Planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, plus the lunar nodes
- Angles: ascendant, descendant, midheaven and imum coeli
- Houses: the twelve areas of life, starting from the ascendant
- Aspects: conjunctions, sextiles, squares, trines and oppositions
The data you need
Three details are enough. The accuracy of your birth time matters most.
Birth date
- Why it matters
- Sets the sign and degree of every planet.
- Where to find it
- ID document or birth certificate.
Birth time
- Why it matters
- Sets the rising sign, midheaven and houses, and refines the Moon position. Four minutes of difference move the ascendant about one degree.
- Where to find it
- Long-form birth certificate, hospital records, baby book or relatives.
Birth place
- Why it matters
- Provides latitude, longitude and the time zone in effect that day, including daylight saving time.
- Where to find it
- The city or town where you were born; if it is very small, the nearest city.
| Detail | Why it matters | Where to find it |
|---|---|---|
| Birth date | Sets the sign and degree of every planet. | ID document or birth certificate. |
| Birth time | Sets the rising sign, midheaven and houses, and refines the Moon position. Four minutes of difference move the ascendant about one degree. | Long-form birth certificate, hospital records, baby book or relatives. |
| Birth place | Provides latitude, longitude and the time zone in effect that day, including daylight saving time. | The city or town where you were born; if it is very small, the nearest city. |
The historical time zone and daylight saving time are resolved automatically from the city and date; you do not need to work them out.
How to calculate your birth chart for free on Aspectados
The full process, from gathering your data to reading the interpretation.
- 1
Gather your birth details
Have your birth date, your birth time as accurately as possible and your birth city at hand.
If you are torn between two times, note both: you can compare which one describes your rising sign better.
- 2
Create your free account
Sign up on Aspectados with your email or Google. No credit card required.
- 3
Save your details in your profile
Enter date, time and city. Aspectados resolves the coordinates and time zone and keeps them for all your questions.
- 4
Pick the "Natal chart" technique
On the chat welcome screen, choose "Natal chart" and one of the suggested questions, or type your own.
- 5
Read and go deeper
You will get an interpretation of your placements. Ask about any planet, house or aspect to expand it.
What happens during the calculation
What software does in milliseconds, and what used to be done by hand with tables.
1. Local time to Universal Time
Your birth time is converted to Universal Time (UT) using the time zone and daylight saving rules in force at that place and date. This is where most manual errors happened, because time-zone rules changed a lot during the 20th century.
2. Planet positions from the ephemeris
With the moment in UT, the ephemeris is consulted: astronomical tables giving each planet’s ecliptic longitude. Software computes them to fractions of an arc second.
3. Sidereal time, ascendant and midheaven
From the local sidereal time, which depends on geographic longitude, the software obtains the zodiac degree rising on the eastern horizon (the ascendant) and the one culminating on the meridian (the midheaven).
4. Houses and aspects
With the ascendant, midheaven and latitude, the twelve house cusps are calculated according to the chosen system (Placidus is the most widespread). Finally, the angular distances between planets are measured to find the aspects within orb.
If you do not know your birth time
What you can know without the time
Without a time, the chart is usually cast for noon. The Sun and slower planets are placed correctly by sign; the Moon can vary about 6 degrees either way, so there will be doubt if it is near a sign change. The rising sign and houses cannot be determined.
Where to look for your birth time
Before giving up on it, it is worth checking:
- Your long-form birth certificate, which includes the time in many countries
- Hospital records or your baby book
- Family photos, diaries or birth announcements
- Relatives who were present, even if they recall an approximate time
Rectification
When there is no record at all, astrologers use rectification: they compare major life events with transits and progressions to estimate the most likely birth time. It is laborious, but it can recover your rising sign and houses.
How to calculate a birth chart: FAQ
Common questions about data, accuracy and results.
Yes. On Aspectados, calculating and interpreting your natal chart is included in the free plan, with no credit card.
Planets barely move, but the ascendant advances about one degree every four minutes and changes sign roughly every two hours. A 15–20 minute error can shift house cusps and sometimes the rising sign.
There is no single "correct" one. Placidus is the most used in modern Western astrology; Equal and Whole Sign houses are popular in traditional approaches. What matters is using the same one consistently.
If you were born near a sign change (around the 20th of each month), the exact date the Sun changes sign varies from year to year. Only a calculation with your time and place confirms your Sun sign.
No. The software does the calculation and the AI explains the result at the level of detail you ask for.
Calculate your birth chart now
You know what you need. Create your free account, add your details and get your chart interpreted in minutes.
Get my free birth chartNo credit card required. The natal chart is included in the free plan.