How to EASILY size an HVAC system and mini-splits with online tools

Home Builder with Zeus Construction MA CS-114391

Using online tools to EASILY size your traditional and mini-split HVAC system

If you're looking to build a new home, build an addition, convert an existing home into a multi-family or perhaps converting an attic or garage into conditioned space, you need to know what size HVAC unit you'll need.

My next project includes converting an antique (1855) single family home into 3 separate condo units. One of the things that will be part of the conversion is to provide a new HVAC system for each of the individual condos. I've decided to go with mini-split heat pump systems to provide both heat and AC to each unit. This means I need to know the heating and cooling load requirements for each separate condo in order to size an appropriate system. The first unit is 2 bedrooms and has 2 floors, the second unit is one bedroom and is on the first floor while the third unit is also one bedroom but is on the second floor. That sounds pretty complicated to me!

There's a formal HVAC industry calculation called Manual J load calculation that provides these results to determine the appropriate equipment sizing. My HVAC subcontractor typically takes care of this as part of his quoting process. But since I'm still in the planning stage, I don't need a full ACCA approved MJ report just yet and I can make do with a rough calculation. I just want to be able to size and price out my mini-split systems.

After a quick Google search I came across a really neat online tool that provides a fairly accurate load calculation for both heat and AC that can be used for system sizing. It's called Cool Calc.

Once you sign up (it's free to sign up and free to get system sizing) and enter all the particulars of the specific unit, it does the calculations for you! It even lets you specify what floor your unit is on (remember one of mine is a 2nd story unit).

The neat part about Cool Calc is that once you've entered all your data and have a completed project, you can save the project to your account. Later when you're ready to buy the system, you can pay a small fee to have the actual Manual J Report generated. I think it's $20 for 5 ACCA Approved reports, that's $4 per report.

I have no affiliation with Cool Calc and I don't get anything for endorsing them. I just thought I'd share since it's a neat online tool to get free Manual J calcs online.


Posted by

Cameron Bagherpour
Builder, Developer, Investor
Zeus Construction
Canton, MA 02021

Comments (2)

Bill Salvatore - East Valley
Arizona Elite Properties - Chandler, AZ
Realtor - 602-999-0952 / em:


Welcome to the Rain. Enjoyed your blog page, and I added you as a friend. I would love the follow back. Bill

Apr 15, 2023 10:59 AM
John Pusa
Berkshire Hathaway Home Services Crest - Glendale, CA
Your All Time Realtor With Exceptional Service

Hello Cameron Bagherpour very good helpful report for how to easily size an HVAC system and mini splits with online tools.

Apr 15, 2023 02:02 PM