# Why 1.01 equals -0.75 in Q2

I'm trying to learn Q notation but I don't understand why 1.01 in the Q2 number format equals -0.75. How is that calculation done? I can understand that 1.1 equals -0.5 because the leading 1 means the number is negative and the first digit after the binary point is 1 which means 1*2^(-1) which is 0.5 and therefore the number is -0.5. Similarly, 1.01 should equal -0.25 and not -0.75.

It is 1.11 which is -0.75 or what is my mistake?

• What do you mean by Q2? I am only familiar with Qm.n format: en.wikipedia.org/wiki/Q_(number_format) – Atul Ingle Sep 25 '17 at 18:45
• @AtulIngle : that page says m. (optional, assumed to be zero or one) so I'm assuming the format is Q1.2? – Peter K. Sep 25 '17 at 19:09

$$1\times (-1)+0\times2^{-1}+1\times2^{-2}=-0.75$$