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Calcimator

Manual J Heat Load Calculator

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

Inputs

sq ft

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

ft
sq ft

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

°F

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

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