HVAC Load Calc: Manual J

Estimate a home's heating and cooling loads with a simplified whole-house block calculation. Use the results as an initial estimate. If your project requires a certified room-by-room Manual J, you'll need a separate calculation.

HVAC Load Calc requires a subscription for all features below. Eligible subscribers can start with a free trial.

Enter the house details

• Floor area, ceiling height, and stories
• Use the site's ZIP code to find the nearest US weather station. You can also search by town, state name, or state code, or enter your own design temperatures.
• Insulation presets for walls, ceiling, and floor (or your own U-values)
• Window area and glazing type, air tightness, and occupants

See the results

• Design heating load (BTU/h)
• Design cooling load, split into sensible and latent loads (BTU/h)
• Equivalent cooling tonnage, with a plausibility check for unusually high or low loads

Save the job

• Start a new job, save changes to the current job, or save an independent copy
• Reopen a saved calculation later with every input intact
• See each job's loads and tonnage in the saved list without opening it
• Jobs stay on your device. You don't need an account.

Share a report

• A one-page PDF with design loads, equivalent tonnage, and the job inputs
• Review the PDF before sharing it like any other file

On iPad, the form and report sit side by side and update as you work.

How the calculation works The calculation uses textbook ASHRAE heat transfer: envelope conduction, infiltration, solar and internal gains. ACCA Manual J block loads use the same basis. Design temperatures come from ASHRAE tables where available and from fifteen years of hourly NOAA records elsewhere.

You can adjust insulation, glazing, air tightness, occupants, and outdoor design temperatures. The estimate assumes a square footprint, fixed indoor design temperatures, and 40 ft² of exterior doors. You can't change those assumptions.

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Engineering reference only. This is a whole-house block load, not a substitute for a certified room-by-room Manual J or professional engineering judgment.

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