HVAC Load Calculator (Manual J Simplified)
Estimate heating and cooling loads and equipment size for a dwelling.
About HVAC Load Calculator (Manual J Simplified)
Fabric and ventilation losses are calculated separately because insulation and airtightness are independent properties. Cooling load includes internal gains from people and equipment, which heating load does not credit. Oversized equipment short-cycles, which worsens humidity control and efficiency, so a load calculation matters more than adding a safety margin.
Formula
HVAC Load Calculator (Manual J Simplified)
Load = fabric loss + ventilation loss, where ventilation loss = 0.33 × ACH × volume × ΔT
- 0.33
- Volumetric heat capacity of air in watt hours per cubic metre per kelvin
- one ton
- 3.517 kW of cooling capacity
Worked example
With the default values below, HVAC Load Calculator (Manual J Simplified) returns Heating Load of 14.696. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Conditioned floor areaminimum 1 m2
- 140 m2
- Ceiling heightminimum 1.5 m
- 2.4 m
- Envelope qualityone of Poor, uninsulated, Average, Good, well insulated, Passive standard
- Average
- Design temperature differenceminimum 1 C
- 22 C
- Occupantsminimum 0
- 4
Results
- Heating Load
- 14.696 kW
- Cooling Load
- 6.805 kW
- Heating Load
- 50,145 BTU/h
- Cooling Capacity
- 1.935 tons
- Specific Heat Loss
- 104.97 W/m2
Step by step
- 1Values used
- 2HVAC Load Calculator (Manual J Simplified)
- 3Heating Load= 14.696 kW
- 4Cooling Load= 6.805 kW
- 5Heating Load= 50,145 BTU/h
- 6Cooling Capacity= 1.935 tons
- 7Specific Heat Loss= 104.97 W/m2
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is HVAC Load Calculator (Manual J Simplified) calculated?
Load = fabric loss + ventilation loss, where ventilation loss = 0.33 × ACH × volume × ΔT Fabric and ventilation losses are calculated separately because insulation and airtightness are independent properties. Cooling load includes internal gains from people and equipment, which heating load does not credit.
Why does HVAC Load Calculator (Manual J Simplified) matter?
Oversized equipment short-cycles, which worsens humidity control and efficiency, so a load calculation matters more than adding a safety margin.
What values do I need to enter?
This calculator takes 5 inputs: Conditioned floor area, Ceiling height, Envelope quality, Design temperature difference, Occupants. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.