How to calculate load calculation for hvac system?

It’s definitely not my area of expertise, but this looks like it might be helpful.

Welcome to Fluther.

Aside from the theory and principles of “how to” contained in @syz‘s link, you can also access various “HVAC calculators” (and here is one) using that simple Google search.

Most calculators that I’ve seen will have you select a geographical area, to get an idea of the climate you’re making calculations for, the square footage of the space you’re trying to heat or cool or both, the construction type and insulating characteristics of the structure, and (optionally, sometimes) the room temperatures desired (otherwise I think the calculators will just select for a “typical” winter/summer room temperature).

This will give a response of “heating or cooling BTUs required”, and may additionally include recommendations as to type and size of heating or cooling units, but a calculator that does that would also request inputs in terms of fuel oil cost expected, whether natural gas is available in your area, and electricity costs, in order to make a better recommendation and annual cost estimate.

600 SqFt/Ton, w/ regular Batt insulation.