In university I learned it this way:
- Multiply by two
- take decimal as the digit
- take the fraction as the starting point for the next step
- repeat until you either get to 0 or a periodic number
- read the number starting from the top – the first result is the first digit after the comma
Example:
0.1 * 2 = 0.2 -> 0
0.2 * 2 = 0.4 -> 0
0.4 * 2 = 0.8 -> 0
0.8 * 2 = 1.6 -> 1
0.6 * 2 = 1.2 -> 1
0.2 * 2 = 0.4 -> 0
0.4 * 2 = 0.8 -> 0
0.8 * 2 = 1.6 -> 1
0.6 * 2 = 1.2 -> 1
Result: 0.00011(0011) periodic.