The Low Gallifreyan Number System is a base-7 system, meaning there are only seven basic digits. The digit 0 is a simple hexagon, and the other six digits are shown below:
With a base-7 number system, the value of each digit within a number is different to the decimal (base-10) system. This means that 10 in Gallifreyan is 7 in decimal, and 100 in Gallifreyan is 49 in decimal, and 1000 in Gallifreyan is 343 in decimal. So, the year 2012 would be written as 5603 in Gallifreyan.
Gallifreyan numbers are constructed by placing the above symbols into a hexagon, with the most-significant digit at the top. It is usual to precede the number with zeros to pad it out so that the hexagon contains six digits. As the symbols for two and five are larger than the others, the preceding hexagons are moved so that each digit remains touching the adjacent ones, but a clear chain can be seen from the top to the bottom of the hexagon.
Below are the chapter headings taken from the New Series Adventures books. These show how each number is constructed, and how the preceding hexagons are shuffled so that the digits fit. It is worth noting that in all New Series Adventures books, there is a 'typo' in the number system for the number 13. It shows the digits 14, when it should be 16 (in base 7) - 14 in Gallifreyan is 11 in Human decimal, and if you compare the images for 11 and 13 below, you will see they are identical.