Manual J HVAC Load Calculator
Estimate a home's heating and cooling load using building-envelope, infiltration, solar, occupancy, and internal heat-gain inputs. Unlike simple square-foot BTU rules, this calculator models walls, windows, doors, ceilings, floors, air leakage, design temperatures, and latent cooling separately. It is intended for preliminary HVAC planning and is not an official ACCA Manual J report.
- Estimated Heating Load
- 23,504.656 BTU/h
- Nominal Cooling Equivalent
- 2.303 tons
- Sensible Cooling Load
- 24,121.862 BTU/h
- Latent Cooling Load
- 3,520 BTU/h
- Sensible Heat Ratio
- 87.266%
- Heating Envelope Load
- 16,304.656 BTU/h
- Heating Infiltration Load
- 7,200 BTU/h
- Cooling Envelope Load
- 6,521.862 BTU/h
- Window Solar Gain
- 10,800 BTU/h
- Estimated Infiltration Airflow
- 133.333 CFM
- Occupant Sensible Gain
- 920 BTU/h
- Occupant Latent Gain
- 800 BTU/h
- Heating Load per Square Foot
- 11.752
- Cooling Load per Square Foot
- 13.821
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Heating Load estimates the rate of heat that must be supplied during the entered winter design condition. Total Cooling Load estimates the rate of sensible and latent heat that must be removed during the entered summer design condition. Sensible Cooling Load represents temperature-related cooling, while Latent Cooling Load represents moisture removal. Nominal Cooling Tons converts the total cooling load using 12,000 BTU/h per ton. Do not automatically select equipment solely from the tonnage result; equipment selection also depends on actual performance at design conditions, airflow, latent capacity, duct design, and other system factors.
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How to Use This Calculator
Enter the conditioned floor area and ceiling height, followed by the exposed areas and thermal properties of the home's walls, windows, doors, ceiling or roof, and floors over unconditioned space. Enter indoor and outdoor design temperatures, estimated air changes per hour, window SHGC and solar gain factor, humidity difference, occupants, and other internal sensible heat gains. The calculator estimates heating load, sensible cooling load, latent cooling load, total cooling load, and the corresponding nominal cooling tonnage.
Formula & Methodology
Envelope conduction is estimated using Q = U × A × ΔT, where U is the inverse of R-value for insulated assemblies. Infiltration airflow is estimated from building volume and air changes per hour: CFM = Volume × ACH ÷ 60. Sensible infiltration load is estimated as 1.08 × CFM × temperature difference. Latent infiltration load is estimated as 0.68 × CFM × humidity difference in grains per pound. Window solar gain is estimated from window area, SHGC, and the entered solar heat gain factor. Occupant and internal sensible gains are added to the cooling load. Total cooling load is the sum of sensible and latent loads, and nominal cooling tons equal total cooling BTU/h divided by 12,000.
Example: 2,000 sq ft Home with Moderate Insulation
For a 2,000 sq ft home, the calculator separately evaluates conductive heat transfer through exterior walls, windows, doors, the ceiling, and exposed floors. It then adds infiltration based on the home's volume and estimated air leakage. During cooling conditions, window solar gain, occupants, internal equipment, and latent moisture load are added. The result may differ substantially from a simple square-foot sizing rule because two homes of the same floor area can have very different insulation, glazing, leakage, climate, and solar exposure.
This is a simplified residential load-estimation model and is not a certified or official ACCA Manual J calculation. A complete professional load calculation may account for room-by-room orientation, detailed fenestration properties, shading, construction assemblies, duct gains and losses, ventilation systems, infiltration methodology, local design conditions, adjacent-space temperatures, appliance schedules, fireplace effects, slab and basement conditions, and other factors not fully modeled here. Solar gain is particularly sensitive to window orientation, shading, latitude, season, and glazing characteristics. Inputs should be based on reliable building and climate data whenever possible.
For preliminary HVAC planning and educational use only. This calculator is not affiliated with or endorsed by ACCA and does not produce an ACCA-certified Manual J report. Do not use the result as a substitute for required code-compliance calculations, permit documentation, professional HVAC design, or manufacturer equipment-selection procedures.
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Frequently Asked Questions
- What is a Manual J load calculation?
- Manual J is an ACCA residential HVAC load-calculation methodology used to determine heating and cooling requirements from building construction, climate, infiltration, glazing, occupancy, and other factors rather than relying only on floor area.
- Is this an official ACCA Manual J calculator?
- No. This Calciro tool provides a preliminary load estimate using common heat-transfer and HVAC load principles. It does not produce an ACCA-certified Manual J report.
- Why is a square-foot HVAC rule not enough?
- Homes with identical floor areas can have very different loads because of insulation, windows, air leakage, ceiling height, climate, orientation, shading, occupancy, and construction.
- How is heating load calculated?
- The calculator estimates conductive heat loss through walls, windows, doors, ceilings, and exposed floors, then adds sensible heat loss associated with outdoor-air infiltration.
- How is cooling load calculated?
- Cooling load includes envelope heat gain, infiltration, solar gain through windows, occupant heat, internal sensible gains, and latent moisture load.
- What is sensible cooling load?
- Sensible load is the heat that changes indoor air temperature. It includes conduction, solar gain, infiltration temperature load, occupants, lighting, appliances, and other heat sources.
- What is latent cooling load?
- Latent load represents moisture that the cooling system must remove from the indoor air. Outdoor humidity entering through infiltration is one contributor.
- How do I convert HVAC cooling load to tons?
- One nominal ton of cooling equals 12,000 BTU per hour. Divide total cooling load in BTU/h by 12,000 to calculate nominal cooling tons.
- Should I buy the next larger AC size than the calculated tonnage?
- Not automatically. Equipment should be selected using its actual performance at the relevant indoor and outdoor design conditions. Oversizing can affect cycling, efficiency, comfort, and humidity control.
- What R-values should I enter?
- Use the best available estimate of the effective thermal resistance of each building assembly. Construction documents, insulation records, energy audits, or professional inspection may provide better information than guessing.
- What is ACH in an HVAC load calculation?
- ACH means air changes per hour. It describes how frequently the building's indoor air volume is replaced by outdoor air and is used here to estimate infiltration airflow.
- What should I use for outdoor design temperatures?
- Use appropriate local heating and cooling design conditions rather than record-high or record-low temperatures. Professional HVAC calculations normally use recognized climatic design data for the property's location.
Published 8/8/2026