How to calculate A/C requirements?

How many tons of A/C is required for 800 sqft room? Its on the third floor out of 10 floors.

I am currently thinking about 1x2 ton. Use both of them initially and when the room is chilled, turn off one of it.

I am not so aware of this, but I know one thing. When deciding, do consider from how many places your room's walls are exposed to sunlight. You are not on the top floor, so your ceiling is not exposed. What about all the 4 sides?

commercial or non commercial?

residential room got more walls so there is good insulation in place already, but if its commercial then now a days it has become common to have commercial buildings with lots of glass windows so if third floor got more windows so less insulation.

keep this in mind so same 800 sqft room could require different tonnage at different situations.