To calculate the size of your HVAC unit: Add all the equipment that is on at once (in watts), then add in how many people (~500 btu per person lets say) then get the measurement of the exterior in your room, as wall, and window. For the wall if you know the insulation U value, it's calculation is area * U value * delta T = btu.
Window U value is calculated the same way, it simply is a different value.
Windows also have solar radiation, which can vary quite a bit.
So In short:
Lighting: How many watt bulbs do you have? Take Watts * 3.412 = btu
Equipment: Think Computers, toasters, TV's, etc, There is some details if you'd like some help for kitchens let me know: Watts * 3.412 = btu
People: In general 500 btu per person, if it's an exercise room, can go up to 2000 per exercising person.
Walls: Wall insulation, if unknown look at the insulation code for walls for your area. or err on the side of caution and use an R-8 (or 0.125 U value) U value * SQFT * Delta T=btu (This is your worst case temp outside compared to indoors, Think if it's 100F outside, and you want it to be 70 inside, you have a delta T of 30)
Windows: Same as wall, just a different U value. Worst case, use a U value of 1.0. if you have double glazed windows, maybe think of using 0.5 or so. U value * SQFT * Delta T=btu
Windows: Windows are also transparent, they have solar radiation. This is calculated by an SHGC value (solar heat gain coefficient) This is a judgement call for where it is, some windows don't have any sun go in them and are tinted, some have a lot and are clear. and it can be drastically different. if unknown, maybe try below with the latitude of your location, SHGC of around 0.8, orientation of your windows, and change output format to btu/sqft. (remember to multiply by the sqft of your glass with the worst case you see on there)
https://susdesign.com/windowheatgain/Roof: U value of roof is same as wall, just for horizontal surface rather than vertical U value is usually worse than what the installed R value is, so I'd assume to be safe an R-15. about a 0.08 after thermal bridging or so. SQFT * U * ( Delta T) = btu
Floor: Usually can ignore, as ground temp is usually not a huge differentiator.
Add all your BTU's together, multiply by 1.15 (standard safety factor, add more if you want this is your house not mine)
That is the BTU you need for your AC unit. I assumed we were cooling, if heating only count the Roof/Windows/Wall and ignore the radiation, and use cold temperatures, and you should get the heating requirement as well(safety factor for heating is 1.25).
These are numbers to actually calculate, rather than trusting a "oh, well, just do it by rule of thumb" numbers. I will state for a normal situation, rule of thumb works well, it's there for a reason and all contractors do is rule of thumb. (and afterward check it with their required calcs, which are equally as so-so.)
