How to Do a Heat Load Calculation (Manual J)

Looking to do a heat load calculation for your home? Doing a Manual J is easier — and cheaper — than you think. This tutorial will walk you through the steps for doing a heat load calculation that is simple, inexpensive, and accurate.

What is a heat load calculation?

A heat load calculation is essential to sizing the heating or cooling equipment that you’re going to put into your space. If you don’t do a proper heat load calculation, you’ll wind up guessing at the sizing — and, oftentimes, you’re going to install the wrong size equipment, and it won’t function the way that you want it to.

A heat load calculation has historically been doing by way of a Manual J. The Manual J is a very long process, but it gets you very accurate sizing of your space. But if you’re only doing a single project, paying for a whole Manual J program can be unnecessarily expensive.

Fortunately, you can use a free program called Cool Calc, which will make your heat load calculation much easier.

What is Cool Calc?

Cool Calc is a low-cost alternative to a performing a full Manual J. Cool Calc’s calculations are based on the ACCA Manual J 8th Edition and are approved by ACCA. Which means if you need to install a mini-split system, for example, this program will give you highly accurate information, at a fraction of the price.

Let’s jump in to see how this program works, and how to use it.

How to get a heat load calculation with Cool Calc

1. Create an account

Cool Calc is a free program, but you do need to create an account to use it. Signing up is easy. Enter your name, username, password, and email to get started.

2. Create a new project

Once you have access to your account, you must add a project. Enter the project name, then enter the address of your property.

3. Load calculation type

Once you’ve entered the correct information, click to choose your load calculation type. Here, you have the option to choose between Block Load (recommended for existing homes) and Room by Room (recommended for new construction or ductwork). Block Load will calculate the heat load for the entire house.

4. Enter home information

To deliver an accurate heat load calculation, the program requires as much information about your home as you can provide. This includes the year the home was built, the number of above-grade stories, the number of bedrooms, and the type of home you’re living in. The more information you enter, the better. So take the time to read the drop-downs and options in order to fill out all of fields correctly.

Ultimately, this is where this program truly shines. If you were to perform a full Manual J, you would need to measure walls, windows, attic space, insulation, and a lot of other things that you may not know. But Cool Calc automatically accounts for the codes that were required in the year your home was built — which will save you a lot of time and effort.

Tip #1: If you have an older home, some of the structural information featured on Cool Calc may be outdated. So be sure to update it! If your home’s siding is no longer brick, for example, but is now stucco, make sure that information is current.

Tip#2: if you do not know when your home was built, search for it on Zillow. All the necessary information should be there.

5. Calculate our home’s square footage

Now you’ve entered all of the information, you’re ready to trace your house to calculate the square footage. Using the program’s built-in map, zoom in on your house so that it fills the majority of the screen. Then click “Start drawing” to begin tracing the building’s outer edges (if you are doing a room by room calculation, you will need to trace the individual rooms).

Tracing is simple. Click on the each corner of the building to place a marker; click on the first marker to complete the trace. Once you’ve finished tracing, give the space a name. If you live in a multiple story home, repeat the process for each floor.

6. Create an HVAC System

You’re almost ready to get your heat load calculation. But first, you will need to create an HVAC system. Once you’ve named the system, you will need to enter some final information. Critical details include whether the system serves every room, if it is ducted or ductless, and where the system will be located. Again, be accurate: this is the final step to calculating the head load for your home.

7. Get your head load calculation

Cool Calc will now automatically generate a report of the heat load calculation for your home. You have two options regarding the report: to print it (for a $15 fee) or to manually record the information yourself (free).

You’re done! You now have information for a heating or cooling load for your home, which will ensure you size your system correctly.


Cool Calc is a great alternative to a performing a full Manual J, and will properly prepare you for purchasing the correct-sized equipment into your home. The next step is installation. Need help? Our technicians are at the ready to install your new heating and cooling system. Contact us today.

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