How do we say km2 in words? Do we use km exponent by two, square kilometer, kilometer squared or something else?
When talking about it as in a formula, you would say "kilometer squared". You would typically use "square kilometer" when discussing area, as in "his farm was three square kilometers".
Depends on the context.
edit: Checked the SI recommendations.
Although in general you would say kilomtres-squared when reading a formula, they recommend that if the length^2 represents an area eg. Pressure = N/m^2, then you read it as newtons-per-square-metre, since it is the area of a real square metre. I can't off-hand think of any units that are length^2 where it isn't an area.
However if the unit has a different quantity squared, such as Acceleration = m/s^2 then you read it as per-second-squared since a square second has no meaning on it's own.
The correct terms when written in a formal context are:
Remember these are different.
Three kilometres squared is three kilometres on one side and three kilometres on the other side which is 9 square kilometres.
Three square kilometres is three kilometres on one side and one kilometre on the other side.
In a formulaic context 3 kilometres squared is written
And 3 square kilometres is written like this
with the two being an exponent (supertext).