What number are you?

I'm 81. No, not 81 years old, or 81 kg (I wish!) - I'm not a value, I'm a number.
What do I mean?
Everything about me tells me that I'm 81.

• I'm odd
• I'm a square
• I'm rational
• I consist of a lot of factors
• I'm curvy
• I'm not perfect (ie. not 100)
• in the grand scheme of things, I'm not that big

So, what number are you?