You only use 25%-30% (estimated) for ACTIVE thought/memory/etc, basically, the things that make up YOU as a person only require about a quarter of your actual brain power and mass.
The entire rest of your brain is used for non-active maintenance like breathing, heart beat, organ regulation, hormone release, sensation, etc; all the shit you have zero control over that just happens because it's essential that you live. Sensory input is a HUGE part of that because your brain is in a constant feedback loop with every nerve in your body, which is why people who loose a limb often still "feel" the limb there. Your coordination and motor skills also take up a lot of processing power because humans muscles require ridiculous amounts of precision and fine tuning. Hand control is a significant portion of your brain power all by itself.
tl;dr, you use 100% of your brain, your intelligence/memory/critical thinking is like 25% of that roughly.