How do I calculate heat load for a house?

To calculate heat load, enter your floor area, ceiling height, indoor and outdoor design temperatures, wall and window areas with their insulation R-values or U-factors, and infiltration rate. The calculator sums heat loss through each envelope component plus infiltration and ventilation to give total BTU/hr or kW.

What is a good heat load per square foot?

For well-insulated modern Canadian homes, a typical heat load is 15–25 BTU/hr per square foot. Older poorly insulated homes may be 35–50 BTU/hr per square foot. Commercial buildings vary widely. Always calculate from actual building data rather than rules of thumb for accurate equipment sizing.

What R-value should I use for walls in Canada?

NBC Canada recommends minimum effective R-values of R-22 to R-30 for walls depending on climate zone. Most new construction uses R-20 to R-24 for 2x6 framing with continuous insulation. Older 2x4 walls with fiberglass batts are typically R-12 to R-14 effective.

What outdoor design temperature should I use?

Use the ASHRAE 99% heating design temperature for your location — meaning outdoor temperatures are at or above this value 99% of all hours in a year. For Canadian cities this ranges from about -5°C (Vancouver) to -33°C (Edmonton). Use our Design Temperature Lookup tool to find the correct value for your city.

🌡 Load Calculations

Heat Load Calculator

Calculate the total heating load for any residential or commercial building in BTU/hr or kW. Accounts for all envelope components, infiltration, and ventilation — following ASHRAE-based load calculation principles.

Unit System:

🏛 Building Details

Building Type i

Floor Area * i

ft²

Heated floor area only — exclude garages, unheated spaces

Ceiling Height *

Average ceiling height throughout the space

🌡 Design Temperatures

Indoor Setpoint * i

°F

Outdoor Design Temp * i

°F

Look up design temp for your city →

ΔT (auto-calculated)

°F

Temperature difference (Indoor − Outdoor)

▢ Walls

Gross Wall Area * i

ft²

Wall R-Value * i

hr·ft²·°F/BTU

Common values: R-12 (old 2x4), R-20 (2x6+ci), R-28 (high-perf)

Wall U-Factor (auto)

BTU/hr·ft²·°F

U = 1/R — calculated automatically

🪟 Windows & Doors

Total Window Area * i

ft²

Window U-Factor * i

BTU/hr·ft²·°F

NBC/Energy Star: ≤0.30 for most Canadian zones

Door Area i

ft²

▲ Roof / Ceiling

Ceiling / Roof Area * i

ft²

Ceiling R-Value * i

hr·ft²·°F/BTU

Common: R-20 (older), R-40 (code min), R-60 (high-perf)

■ Floor

Floor Type i

Floor / Slab R-Value i

hr·ft²·°F/BTU

Slab Perimeter i

Required for slab-on-grade only

🌫 Infiltration & Ventilation

Construction Tightness i

Infiltration ACH i

ACH

Natural ACH (not blower door ACH50)

Mechanical Ventilation i

CFM

Calculate required ventilation →

HRV/ERV Efficiency i

0% = no HRV. Typical HRV: 65–80%. Premium ERV: 80–95%.

💡

An HRV/ERV can reduce ventilation heat load by 65–80%. See our payback calculator to evaluate the investment.

📊 Heating Load Results

BTU/hr

Heat Loss by Component

📊 Heat Loss Breakdown

Detailed Results Table

Component	Area / Factor	U-Value / ACH	ΔT	Heat Loss	% of Total

Export:

How to Use the Heat Load Calculator

Select Your Unit System

Choose Imperial (°F, ft², BTU/hr) or Metric (°C, m², kW) using the toggle at the top. All input fields and results update instantly. Use our temperature converter or BTU/kW converter if you need to convert existing data.

Enter Building & Temperature Data

Enter your floor area, ceiling height, indoor setpoint (typically 70°F / 21°C), and the outdoor winter design temperature for your city. Look up the correct ASHRAE 99% design temperature using our design temperature lookup tool — do not use the coldest day on record.

Enter Envelope Data

Input the gross wall area, total window area, roof/ceiling area, and their respective R-values or U-factors. Use the U-value / R-value calculator to determine the effective R-value of your wall assemblies if needed. Don't forget doors — a standard 3'×7' door is about 21 ft².

Set Infiltration & Ventilation

Select a construction tightness category or enter a custom ACH value from a blower door test (divide ACH50 by 20 for natural ACH). If your building has mechanical ventilation (HRV/ERV), enter the outdoor air flow rate and HRV efficiency. Our ventilation rate calculator helps determine the required outdoor air per ASHRAE 62.2.

Calculate & Review Results

Click Calculate Heating Load. Results appear instantly with a full breakdown by component — walls, windows, doors, roof, floor, infiltration, and ventilation. A bar chart visualizes each component's share of total heat loss. Use this to identify where improvements have the greatest impact.

Export & Use Your Results

Export a full PDF report for permit applications or client presentations, download CSV data for spreadsheet analysis, or copy results to clipboard. Then use the heating load to size your equipment with the furnace sizing calculator, heat pump sizing calculator, or boiler output calculator.

Understanding HVAC Heat Load Calculations

A heat load calculation — also called a heating load calculation or heat loss calculation — is the process of quantifying how much heat energy escapes a building during winter. This value, expressed in BTU/hr (British Thermal Units per hour) or kilowatts (kW), defines the minimum output capacity required from your heating system to maintain comfortable indoor temperatures at the coldest outdoor design conditions.

Why Accurate Heat Load Calculations Matter

Guessing or using rules-of-thumb for HVAC sizing leads to serious problems. An oversized furnace or heat pump will short-cycle — turning on and off rapidly — which causes temperature swings, excess humidity in summer, accelerated equipment wear, and higher energy bills. An undersized system will run continuously on the coldest days and still fail to reach setpoint, leaving occupants uncomfortable. Our load estimator follows standard ASHRAE-based methodology and eliminates both problems by producing an accurately sized load figure.

The Five Components of Building Heat Loss

This calculator accounts for all five primary heat loss pathways recognized by ASHRAE Fundamentals:

Conduction through walls: Heat flows through every exterior wall at a rate determined by the wall's U-factor (1/R-value) and the temperature difference. Use the wall heat loss calculator to break this down by wall section.
Conduction through windows and doors: Windows are typically the weakest thermal link in the envelope. A double-pane window (U-0.30) loses heat roughly 10× faster than a well-insulated wall (R-20). The window heat loss calculator provides component-level analysis.
Conduction through the roof/ceiling: Heat rises, making the ceiling/attic boundary critically important. The roof and ceiling heat loss calculator handles vaulted, flat, and attic configurations.
Conduction through the floor: Floors over unconditioned crawlspaces or slabs lose significant heat. See the floor heat loss calculator and basement heat loss calculator for detailed analysis.
Air infiltration and ventilation: Cold air leaking in through cracks and gaps must be heated to room temperature, adding directly to heating load. The infiltration load calculator isolates this component. Mechanical ventilation adds a controllable, but recoverable (with an HRV/ERV), heating load.

What to Do With Your Heat Load Result

Once you have your total heating load in BTU/hr or kW, the next steps depend on your system type:

Gas furnace: Use the furnace sizing calculator to convert load to required furnace output, accounting for AFUE efficiency.
Heat pump: Use the heat pump sizing calculator to select capacity, accounting for COP at low temperatures.
Mini-split: See the mini-split sizing calculator for zone-by-zone capacity selection.
Hydronic/boiler: Feed results into the boiler output calculator and pipe sizing calculator.
Duct system: Use the CFM calculator and duct sizing calculator to design your air distribution.
Energy analysis: Compare heating costs using the payback period calculator and energy savings estimator.

Imperial vs. Metric Heat Load Values

In Canada, heat loads are commonly expressed in both systems. HVAC equipment is often rated in BTU/hr (Imperial) while engineering calculations may use kW or MJ/hr (Metric). Our calculator outputs both simultaneously. Use the BTU to kWh converter for energy conversion and the temperature converter for design temperature conversion between °F and °C.

Frequently Asked Questions

Enter your floor area, ceiling height, indoor setpoint (70°F / 21°C is standard), and outdoor design temperature for your city. Then enter wall, window, roof, and floor dimensions with their insulation values. Select your construction tightness for infiltration. Click Calculate — the tool sums all heat loss pathways and gives you total BTU/hr or kW. For a more detailed result, use the load estimator.

Modern well-insulated Canadian homes (built after 2012 to NBC standards) typically run 15–25 BTU/hr per square foot. Older homes (pre-1980) with minimal insulation can be 40–60 BTU/hr per square foot. The wide range is why rules-of-thumb are unreliable — climate zone alone can double the load. Edmonton homes need roughly 2× the heating capacity of Vancouver homes of the same size and construction. Always calculate from actual building data.

NBC Canada Part 9 minimums vary by climate zone. As a general reference: walls R-20 to R-28 (effective), ceilings R-40 to R-60, windows U-0.30 or better. For older homes: 2×4 walls with R-12 batt are typically R-10 to R-13 effective after framing correction, 2×6 with R-20 batt are approximately R-16 to R-19 effective. Use the U-value / R-value calculator to determine the effective R-value of your specific wall assembly.

Use the ASHRAE 99% heating dry-bulb design temperature for your location. This is the temperature that is equalled or exceeded 99% of all hours in the heating season — meaning your heating system only needs to handle worse conditions 1% of the time (about 88 hours/year). Do not use the historical record low — that would massively oversize your equipment. Key Canadian values: Vancouver -5°C, Victoria -4°C, Calgary -28°C, Edmonton -33°C, Winnipeg -33°C, Toronto -17°C, Ottawa -22°C, Montreal -23°C, Quebec City -25°C, Halifax -15°C. Use our design temperature lookup for your specific city.

A Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV) pre-warms incoming fresh air using outgoing stale air before it reaches the heating system. A typical HRV with 75% efficiency reduces the ventilation heating load by 75% — so if your ventilation load is 10,000 BTU/hr, the HRV reduces it to 2,500 BTU/hr. Enter your HRV efficiency percentage in the calculator to see this reflected in results. This can reduce total heating load by 5–15% in tight, well-ventilated buildings. See the payback period calculator to evaluate whether an HRV upgrade makes financial sense.

Yes — select Commercial, Retail, or Industrial from the Building Type dropdown. The calculator uses the same heat loss methodology for all building types. For commercial buildings with significant internal heat gains from people, lighting, and equipment, also use the internal heat gain calculator, lighting heat gain calculator, and equipment heat gain calculator. For a complete commercial project, use the commercial load calculator which uses the ASHRAE block load method.