HVAC Sizing Calculator

Conditioned area:

Use the room, connected zone, or small-home area you want to cool.

Average ceiling height:

Use the average height across the conditioned area.

Climate load:

Insulation and air sealing:

Sun and window exposure:

Regular occupants:

Extra internal load:

Leave 0 for no extra internal load.

Optional sizing reserve:

Leave 0 for the modeled load; add reserve only for explicit uncertainty.

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Selection notes

This is a transparent screening model, not Manual J, Manual S, duct design, ventilation design, or a permit-ready equipment selection.

Load contributors

See the running cooling load after each modeled contributor is applied.

Two rooms with the same floor area can need very different cooling capacity. A shaded bedroom with a low ceiling gains less heat than a west-facing sunroom, while computers, cooking equipment, and regular occupants add heat from inside. Cooling size is a heat-removal rate, so square footage is only the starting point.

A room envelope with heat gain and cooling capacity shown as balancing flows — Cooling capacity must balance heat entering through the room envelope and internal sources.

Capacity is commonly stated in British thermal units per hour (BTU/h), thermal kilowatts, or refrigeration tons. One refrigeration ton is 12,000 BTU/h. These are cooling-rate units, not electrical power consumption. A 3 kW thermal cooling output does not mean the equipment necessarily draws 3 kW from the supply.

Both undersizing and oversizing have consequences. Too little capacity can leave the space above its setpoint during peak conditions. Too much fixed capacity can shorten cycles and weaken humidity removal, particularly in humid climates. Variable-capacity equipment changes that tradeoff, but it still needs a sound load calculation and equipment selection.

Sensible load: Heat that changes air or surface temperature.
Latent load: Moisture removal needed for humidity control.
Design load: The cooling rate needed under chosen indoor and outdoor design conditions.
Nominal size: A listed equipment-capacity class, which may differ from delivered capacity at actual conditions.

A rough room or zone estimate helps compare early options, but it does not replace a residential Manual J calculation or the manufacturer performance data used for final equipment selection. Duct gains, infiltration, ventilation, detailed window properties, orientation, humidity, and local design weather are outside a simple screening model.

How to Use This Tool:

Model one room, connected zone, or small-home area at a time, using an average ceiling height and conditions representative of the hottest expected period.

Enter the Conditioned area and Average ceiling height in metres or feet. Do not mix the room area with the whole-building height.
Select the climate, insulation and air-sealing, and sun/window profiles that most closely describe the space. These are broad multipliers, so avoid choosing a severe profile merely to create extra capacity.
Enter the number of regular occupants and any known Extra internal load from equipment in watts. Leave the equipment value at zero when no additional load is known.
Keep Optional sizing reserve at 0% unless you have a specific uncertainty to represent. The accepted range is 0% to 30%.
Read the target load and nearest listed capacity together, then check the fit label and load contributors before using the result for product comparison.

Interpreting Results:

The Target cooling load is the modeled heat-removal rate after every selected multiplier, occupant load, equipment load, and reserve. Nearest listed capacity is only the closest value in the calculator's standard-capacity list; it is not an automatic equipment recommendation.

HVAC capacity fit labels
Difference from target	Label	Meaning
Below −10%	Undersized	The listed capacity is materially below the modeled load.
−10% to below 0%	Slight undersize	The listed capacity is close but still below target.
0% through 15%	Target band	The listed capacity is at or modestly above target.
Above 15% through 30%	Oversize caution	Review cycling and humidity risk.
Above 30%	Heavy oversize	Do not select from this screening estimate alone.

Check actual equipment performance at the local design temperature, not only its nominal label. A fit result cannot establish dehumidification performance, airflow, duct suitability, electrical requirements, or code compliance.

Technical Details:

The screening model begins with 20 BTU/h for each square foot at an 8 ft reference ceiling. It scales that base for ceiling height, then applies climate, envelope, and solar multipliers in sequence. Heat from occupants beyond the first two and entered equipment watts is added afterward. Reserve is applied to that subtotal.

Formula Core:

The target is a staged estimate rather than one flat area factor. Multiplication handles the building and exposure profiles; people and equipment are additive heat sources.

\begin{array}{l} Q_{base} = 20 \times A_{ft²} \times \frac{H_{ft}}{8} \\ Q_{profiled} = Q_{base} \times M_{climate} \times M_{envelope} \times M_{solar} \\ Q_{target} = (Q_{profiled} + 600 \times \max (0, n - 2) + 3.412142 \times P_{W}) \times (1 + \frac{r}{100}) \end{array}

Here, area is in square feet, height is in feet, profile values are ratios, n is the regular occupant count, entered equipment power is in watts, and r is reserve percent. One watt is converted to 3.412142 BTU/h. Thermal kilowatts divide BTU/h by the same conversion constant and refrigeration tons divide by 12,000.

HVAC screening profile multipliers
Factor	Available multipliers
Climate	Cool or marine 0.90×; mixed or typical 1.00×; hot summer 1.15×; very hot or humid 1.25×
Envelope	Efficient and well sealed 0.88×; average 1.00×; older or moderately leaky 1.15×; poor insulation or high leakage 1.30×
Sun and windows	Mostly shaded 0.90×; average 1.00×; very sunny or west-facing 1.10×; large glass area or sunroom 1.20×

The nearest standard capacity is selected by absolute BTU/h difference from 5,000 through 60,000 BTU/h. If two sizes are equally distant, the lower value encountered first is retained. Fit boundaries are exact: −10% belongs to Slight undersize, 0% through 15% belongs to Target band, and 30% belongs to Oversize caution.

Conditioned area must resolve to 5 through 1,850 m², ceiling height to 1.8 through 6.1 m, occupants to a whole number from 0 through 200, equipment load to 0 through 50,000 W, and reserve to 0% through 30%.

Accuracy Notes:

This is a transparent screening estimate, not ACCA-approved Manual J software. It does not model room-by-room construction assemblies, detailed fenestration, infiltration tests, ventilation, duct location and leakage, design humidity, or manufacturer performance at outdoor conditions.

Use a professional load calculation for final residential equipment sizing, permits, and whole-system design.
Do not convert the thermal-kilowatt result into operating cost without the equipment's electrical input and efficiency data.
Do not use reserve to hide uncertain area, ceiling, envelope, or exposure measurements; correct those inputs first.

References:

Manual J Residential Load Calculation, Air Conditioning Contractors of America.
ENERGY STAR Certified Room Air Conditioners, U.S. Environmental Protection Agency.