How the rashis are numbered
The rashis are numbered from 1 to 12, starting with Mesha as 1 and ending with Meena as 12. In the tools the same numbers are used with 0 to 11 for array indexing.
What it is
The numbering follows the sidereal zodiac, so Mesha begins at 0 degrees sidereal. The rashi index of a longitude is the whole part of the longitude divided by 30, counted from zero.
How it is worked out or read
When a result says sign 7, it means the seventh sign, Tula, counted from Mesha. Keep track of whether a list starts at 0 or 1, since the tools and some books differ.
Where it appears on this site
The rashi index is used throughout the birth chart. The rashi index lists the signs.
What the number tells you
A sign's number carries several of its properties. Odd-numbered signs, Mesha, Mithuna, Simha and so on, are masculine; even-numbered signs are feminine. Counting in threes gives the qualities: signs 1, 4, 7 and 10 are movable, 2, 5, 8 and 11 fixed, and 3, 6, 9 and 12 dual. Counting in fours gives the elements: 1, 5 and 9 are fire, 2, 6 and 10 earth, 3, 7 and 11 air, and 4, 8 and 12 water. So knowing that Vrishchika is sign 8 tells you at once that it is feminine, fixed and watery.
Numbers in the North Indian chart
In the North Indian chart style, the houses stay in fixed positions and the sign number is written in each house. The number in the top centre house is the Lagna. A reader who knows the numbering can read the signs of all twelve houses from those numbers alone. In the South Indian style, by contrast, the signs stay fixed and the Lagna is marked, so the numbers are not usually written.
Counting houses from the Lagna
House numbers are counted from the Lagna, while sign numbers are counted from Mesha. For a Simha Lagna, sign 5, the seventh house is sign 11, Kumbha. Adding six to the Lagna's number, and subtracting twelve if needed, finds the seventh sign quickly.
Two numbering conventions
Some sources number the rashis from one to twelve and others from zero to eleven. The zero-based numbering is convenient for calculation, because the sign index is the whole part of the longitude divided by thirty. The one-based numbering is convenient for reading, because it matches the common phrase the seventh sign. A reader who moves between a calculator and a book should check which convention each one uses, since an off-by-one error places every planet in the wrong sign.
A worked check
Take a sidereal longitude of 200 degrees. Divided by thirty, it gives six with a remainder of twenty, so the zero-based sign index is six, which is Tula, and the one-based number is seven. Tula is the seventh sign counted from Mesha, so both conventions agree once the offset is known. A reader can use this check any time a sign number looks wrong.