Manual J Heat Load Calculator

Estimate heating and cooling loads using a simplified Manual J method based on building envelope, windows, and occupancy.

Inputs

Floor Area

sq ft

Typical rule of thumb: 400–600 BTU/hr per ft² for cooling; Manual J provides precise calculation

Ceiling Height

Window Area

sq ft

Wall Insulation R-Value

IECC 2021: walls R-20 zones 3–4; R-20+5 zones 5–8; attic R-49 zones 4–8

Design Temperature Difference

°F

Miami, FL: ~20°F ΔT; Denver, CO: ~60°F; Minneapolis, MN: ~80°F heating design ΔT

Occupants

Primary Window Direction

Tab to move between fields

Results

Heating Load

22,063 BTU/hr

Cooling Load

28,467 BTU/hr

Cooling Tonnage2.37 tons

Required Airflow949 CFM

How to Use This Calculator

Enter Floor Area, Ceiling Height, and Window Area.
Set Wall Insulation R-Value, Design Temperature Difference, and Occupants.
Adjust Primary Window Direction as needed.
Review Heating Load (BTU/hr) and Cooling Load (BTU/hr).
Use Cooling Tonnage (tons) and Required Airflow (CFM) to inform your decision.

How the result changes with Design Temperature Difference

Design Temperature Difference	Heating Load	Cooling Load
19	8,384 BTU/hr	18,878 BTU/hr
42	18,533 BTU/hr	25,993 BTU/hr
69	30,447 BTU/hr	34,345 BTU/hr
91	40,155 BTU/hr	41,151 BTU/hr

What each input means

Floor Area: Total conditioned floor area of the building or zone. ACCA Manual J is the ANSI-recognized standard for residential load calculations; ASHRAE 90.1 requires load calculations for commercial systems.
Ceiling Height: Average ceiling height. Higher ceilings increase heating and cooling loads.
Window Area: Total window and glass door area. Major source of heat gain and loss.
Wall Insulation R-Value: Thermal resistance of wall insulation. ASHRAE 90.1 minimum R-values vary by climate zone: Zone 1–2: R-13; Zone 3–4: R-13+5 or R-20; Zone 5–8: R-20+5. R-13 for 2×4 walls, R-19 for 2×6, R-30+ for attic.
Design Temperature Difference: Difference between indoor setpoint and outdoor design temperature for your climate zone. ASHRAE Fundamentals Chapter 14 provides 99.6% (heating) and 1% (cooling) design conditions by location.
Occupants: Number of occupants. Each person adds approximately 250 BTU/hr of sensible heat.
Primary Window Direction: Primary direction windows face: 1 = North, 2 = South, 3 = East, 4 = West. Affects solar gain.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Floor Area = 2000, Ceiling Height = 8, Window Area = 200, Wall Insulation R-Value = 13 = 7 input(s) provided
Calculate Heating Load
Heating Load
22063 = 22063
Calculate Cooling Load
Cooling Load
28467 = 28467
Calculate Cooling Tonnage
Cooling Tonnage
2.37 = 2.37
Calculate Required Airflow
Required Airflow
949 = 949

Engine last updated July 28, 2026. Checked against 1 independently-derived test — how we verify calculators.

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