Heat Load Calculation: Manual J Made Easy 

Have you used any heat load calculation software that is accurate?

I’d suggest using the average ceiling height, assuming you place your mini split indoor units in an optimal spot. E.g. the average between 5 and 9 is 7. A little geometry will show you this yields the correct volume. You may want to add 10% to the height to account for convection, so 7.7’ in this example.

Just Google the address Zillow usually pops up first or it's competitors but you'll find the year and compare at least 2 to make sure year is around the same

Don’t use this for that

you make the lines according to your construction plan

In one of the steps, there is a part where is Data Source. Select the text mode where you can enter the size of the rooms individually.

Manual J calcs use ceiling area, not roof area and R-value of insulation. It also uses wall area, overall sqft area, window and door areas, etc.

Licensed Mechanical/HVAC Engineer and Master Plumber here, worked 10 years for Carrier Corp before, How can the software provide an answer? Simple. The developers used reasonable assumptions plus built-in safety factors within the program. You are correct Manual J is complex and specific although not as complex and specific as an ASHRAE cooling load calculation which an hvac design ngineer does. Because the purpose of the software is to provide quick easy to use results that are nevertheless reliable enough. The expert developers of the app simplify the process by selecting reasonable , "typical" values for the various inputs. For example, thermal resistance values for light weight concrete walls in strictly speaking vary depending on specific construction, but the values should range around X to Y, based on properly done, industry practices. The same for other inputs. Based on occupancy density from code or manual input, you can also estimate average heat load from a human occupant, around 300-400 watts per person. Another critical factor they coded in for example is the temperature and humidity data for the location. However , based on extensive ASHRAE research, there is an "average climactic condition per location", ashrae usually updates every 4 year cycle. With all those reasonable estimates, educated guests nonetheless, there is bound to be errors to the software output. So this is where technical studies comparing the results from various cooling load calculation methodologies are used, from the simplest to the most detailed across various building types. In one study, we actually done in-house, Equivalent Temperature Differential (ETD) Method which is the method used in Manual J underestimates the Results if you use the most accurate method, Radiant Time Series (RTS) by 30% and other hand say, an actual test model house has 10% higher cooling load than RTS. So let say, the software selects a 40% safety factor. Now you have a more reliable but simplified estimate for the Software User without going thru hell for a simple residential project. Hope that helps. I forgot to add, peak peak peak cooling load over an 8760hrs in a year, happens only 1% of the time in the year or 88 hrs annually, based on Climate Data, So Manual J results + Fixed Speed Systems will get you an AC system that definitely meets the cooling load assuming you input the "right data" but is nevertheless less efficient than using an variable speed /inverter system that can match the load with capacity. But hey, one step at a time.

Shit, nah nuckle