How to Calculate a Composite Chart (Step by Step)

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A midpoint composite chart takes two birth charts and turns each pair of matching planets into one point halfway between them. The arithmetic is simple once two traps are handled: the circle wraps around at 0° Aries, and every pair of positions has two possible midpoints. This guide works through both with a real pair of charts, then finds the aspects between the composite planets and checks what changes when a birth time is missing. What a composite means, and how it differs from synastry and from a Davison chart, is covered in composite charts.

Educational guide: Astrologist.com does not currently calculate synastry, inter-chart aspects, house overlays or composite charts. The compatibility tool provides a sign-level estimate based on the Sun, Moon and Rising signs; known birth times are required for that tool. The contacts discussed here must be identified separately from the two charts' actual positions.

In particular, the site does not calculate midpoint composites, Davison charts, composite aspects, composite angles or composite houses; the compatibility tool remains a sign-level Big Three estimate and the chat is not a composite calculator, so the free chart supplies only each person's own positions and every composite figure below is hand arithmetic from them.

Step 1: get both sets of positions

Run the free birth chart once for each person and copy the ten planets from the Sun to Pluto from the table under the wheel, with sign, degree and minute. The example uses the same pair as comparing two charts by degree:

• Person A: 14 March 1988, 07:30, Amsterdam.
• Person B: 2 August 1990, 21:10, London.

Step 2: convert each position to longitude

Turn each position into a single number from 0° to 360°: add the degrees and minutes within the sign to the sign's starting point (Aries 0°, Taurus 30°, Gemini 60°, and so on up to Pisces 330°). The full table is in comparing two charts by degree. A's Sun at 23°54' Pisces becomes 330° + 23°54' = 353°54'; B's Sun at 10°16' Leo becomes 120° + 10°16' = 130°16'.

Step 3: take the shorter-arc midpoint

Two points split the circle into two arcs, a shorter one and a longer one. This guide uses the midpoint of the shorter arc for every planet. That is the explicit convention of these worked examples, not a universal rule; Step 4 covers the alternative.

1. Measure the gap the short way round: subtract the smaller longitude from the larger, and if the answer is more than 180°, subtract it from 360°.
2. Halve the gap.
3. Start from the position at the beginning of the short arc (the one you reach first going forward through the signs along that arc) and add the half gap. If the result passes 360°, subtract 360°.
4. Convert back to a sign: 62°05' is 60° (Gemini) plus 2°05', so 2°05' Gemini.

The Sun, across 0° Aries. 353°54' − 130°16' = 223°38', more than 180°, so the short gap is 360° − 223°38' = 136°22'. The short arc runs forward from A's Sun at 353°54' through 0° Aries to B's Sun at 130°16'. Half of 136°22' is 68°11'. 353°54' + 68°11' = 422°05', minus 360° is 62°05': the composite Sun is at 2°05' Gemini.

Saturn, a simple case. 290°39' − 271°55' = 18°44'. Half is 9°22'. 271°55' + 9°22' = 281°17', which is 11°17' Capricorn.

The Moon, with half a minute. 300°49' − 266°58' = 33°51', so the short arc runs from B's Moon at 266°58' to A's. Half of 33°51' is 16°55½'. 266°58' + 16°55½' = 283°53½', or 13°53½' Capricorn.

The quick shortcut of averaging the two longitudes fails whenever the short arc crosses 0° Aries. For the Suns, (353°54' + 130°16') ÷ 2 = 242°05', which is 2°05' Sagittarius: the midpoint of the long arc, exactly opposite the right answer.

Step 4: the other midpoint, and when it matters

Every pair also has the opposite midpoint, 180° away, in the middle of the longer arc. Most of the time it is far from both planets and nobody would choose it. The closer the two planets come to an exact opposition, the closer the two arcs are in length; at exactly 180° they are equal, and neither midpoint is the shorter one. Then you have to choose, and say which one you chose.

A and B's Mercuries show the effect. 327°16' − 156°01' = 171°15', just under 180°. The short arc runs from B's Mercury at 156°01' forward to A's; half the gap is 85°37½', so the shorter-arc midpoint is 156°01' + 85°37½' = 241°38½', 1°38½' Sagittarius. The alternative is 180° away: 1°38½' Gemini.

The choice changes the reading. The composite Sun is at 2°05' Gemini, so with the shorter-arc Mercury the composite has Mercury opposite the Sun; with the alternative, Mercury is conjunct the Sun. A real Mercury is never more than about 28° from the Sun, but a composite Mercury is not a real Mercury, and the shorter-arc convention allows the opposition. This guide keeps the shorter-arc Mercury, as it does for every planet, and labels the opposition as a result of that convention. It is not evidence that either choice is more correct.

Step 5: the full composite

Repeat Step 3 for all ten planets. Working from the displayed minutes gives:

The last column was calculated from the unrounded positions and cut to whole minutes. The free chart shows minutes without seconds, so a hand result can differ from it by up to about one minute, as it does here for the Moon, Mercury, Uranus and Neptune. That makes no difference to which aspects are found below.

Step 6: find the aspects between composite planets

Measure the gap between each pair of composite planets exactly as you would between two charts, and compare it with the five major aspects. These guides use 8° for the conjunction, square, trine and opposition and 6° for the sextile. That is the convention for the site's educational examples, borrowed from the orbs the calculator uses inside one natal chart; the software does not calculate composite aspects, and many astrologers use tighter orbs in composites. The reasoning is in aspect orbs, the angles themselves in major aspects.

An example: the composite Moon at 283°53½' and composite Mars at 343°55½' are 60°02' apart, a sextile 0°02' from exact.

All the contacts for A and B, from the full-precision positions (orbs from the displayed-minute version differ by at most 0.01°):

With the alternative Mercury from Step 4, the first row would read Sun conjunct Mercury, 0.43°, and nothing else would change.

Mark the generational contacts before reading anything into them. A and B were born two years apart, and each already has Saturn, Uranus and Neptune together in Capricorn, so composite Saturn conjunct Neptune, Saturn conjunct Uranus and Uranus conjunct Neptune mostly repeat what both charts share with their generation. The same goes for any contact between the composite outer planets of two people born close together.

Step 7: check the Moon when a birth time is unknown

The Moon moves roughly 12° to 16° a day, so a missing birth time leaves it uncertain. The composite Moon is only half as uncertain, because it moves half as far as the Moon that moves. When the time is unknown, the free chart draws the Moon at local noon as a date reference; noon is not a birth time and cannot settle a composite Moon placement or contact. Instead:

1. As a first screen, run the free chart for the untimed person at 00:00 and at 23:59 on the birth date, keeping the other person's known time fixed, and read only the Moon. Ignore the Rising sign and houses of these test charts. The two results show roughly how far the composite Moon can move, but two endpoints alone do not prove that a sign or contact holds in between.
2. To claim a composite Moon sign or contact, check it at many times spread densely through the whole plausible interval, or the whole birth date if nothing narrows it, close enough together that nothing can change between checks. The examples below were checked at every minute of the date.
3. Publish a composite Moon sign or contact only if it holds at every one of those checks; otherwise call it unresolved.

If A's time were unknown: A's Moon ran from 26°17' Capricorn to 10°55' Aquarius over 14 March 1988, so the composite Moon would lie between about 11°38' and 18°57' Capricorn. It stays in Capricorn, and a check at every minute of the date found the same four contacts all day: sextile Mars, conjunct Saturn, conjunct Neptune and sextile Pluto.

If B's time were unknown: B's Moon ran from 16°22' to 28°24' Sagittarius over 2 August 1990, so the composite Moon would lie between about 8°36' and 14°37' Capricorn. The same four contacts hold all day. A fifth does not: the composite Moon comes within 8° of composite Uranus for birth times up to about 11:56 and not after, so a composite Moon conjunct Uranus would be unresolved. The noon chart would show no conjunction, which is exactly why one time of day cannot stand in for the whole date.

The other composite planets move far less. For A and B, every other composite planet keeps the same sign and the same aspects across either whole birth date. If both birth times are unknown, both ranges apply at once and the composite Moon's uncertainty grows. Checking each date on its own, or only the ends of the two dates, is not enough: every plausible time for one person has to be combined with every plausible time for the other. Publish a composite sign or contact only if it holds across that full joint range; otherwise call it unresolved. The Moon without a birth time explains the daily range in more detail.

Step 8: angles and houses

A composite Ascendant, Midheaven and houses exist in several versions. Some astrologers take the midpoints of the two Ascendants and the two Midheavens; others derive the houses from the composite Midheaven for a chosen place. The versions can disagree. All of them need precise birth times for both people and the exact degrees of each person's angles, which the free chart does not print as numbers. This guide therefore stops at the planets: it gives no composite angles, cusps or house placements, and none can be found without both birth times. Composite houses are also a different construct from house overlays, where one person's planets are placed in the other person's houses.

Common mistakes

• Averaging without the wrap-around. A plain average gives the long-arc midpoint whenever the short arc crosses 0° Aries, as the Suns showed in Step 3.
• Not saying which midpoint you used. Near an opposition the two midpoints give different aspects. State the convention.
• Mixing methods. Midpoint composite positions and Davison positions are different charts; do not combine planets from one with planets from the other.
• Reading a composite contact as a synastry contact. Composite Venus square composite Mars is not one person's Venus square the other's Mars. The synastry method is in comparing two charts by degree.
• Using the noon Moon as if it were timed. Check the whole birth date instead.
• Treating generational contacts as personal. Outer-planet contacts in the composite of two people of similar age mostly reflect their generation.

Reading the result

The finished composite is read as the relationship's own chart: shared themes and tendencies the two people may recognise, not a forecast or a verdict on the relationship. Composite charts covers how astrologers read the composite planets and their aspects, and why no placement decides how long a relationship lasts or marks a fated bond. As with any comparison, use another person's birth data only with their consent.

Related articles

• Composite Charts
• Comparing Two Charts by Degree
• Synastry Basics
• House Overlays and Birth Time
• Aspect Orbs
• The Moon Without a Birth Time
• All compatibility guides