AC Size Calculator for Canadian Homes — What Size Air Conditioner Do I Need?

Full formula and methodology: How this calculator works →

What is cooling load?

Cooling load is the rate at which heat must be removed from a conditioned space to maintain a comfortable indoor temperature during peak summer conditions. It is measured in BTU/hr or tons of refrigeration (1 ton = 12,000 BTU/hr). Like heat load in winter, the cooling load determines the minimum capacity your air conditioning system must provide.

Cooling load has two components: sensible load (heat that raises the air temperature — sun through windows, conduction through walls, internal equipment) and latent load(moisture that must be removed to control humidity — occupants breathing, cooking, infiltration of humid outdoor air). In Canada's humid summer climate, the latent load is significant and is a major reason why oversizing causes comfort problems.

Why AC sizing matters in Canada

Canada's humid continental climate (eastern provinces) and semi-arid climate (Prairies) create different sizing challenges. In Ontario and Quebec, summer humidity is high — a properly sized AC runs long cycles, removing moisture and maintaining dry, comfortable air. An oversized unit blasts cold air, satisfies the thermostat temperature setpoint in minutes, and shuts off — leaving humidity high and air clammy.

In Alberta and Saskatchewan, summers are hot but dry. Cooling load is dominated by sensible heat (temperature) rather than latent heat (humidity). This means AC sizing in Calgary is more straightforward — oversizing is still wasteful but the humidity problem is less pronounced.

In coastal BC, summers are mild and the cooling season is short. A properly sized unit — often just 1–1.5 tons for a typical home — handles the modest summer heat without the oversizing trap that affects eastern Canada.

How cooling load is calculated

The CSA F280 and ASHRAE simplified method accounts for the following heat gain sources:

• Conduction through envelope: Heat flowing in through walls, ceiling, floor, windows. Q = U × A × ΔT, where ΔT = (outdoor design temp) − (indoor setpoint, typically 24°C)
• Solar gain through windows: Direct solar radiation through glazing — highest on west and south faces in afternoon. This is often the largest single cooling load component in modern homes with large windows
• Infiltration: Hot, humid outdoor air leaking in through cracks and gaps, adding both sensible and latent load
• Internal gains: Occupants, lighting, appliances, electronics — all contribute heat to the conditioned space
• Latent load: Moisture from occupants and infiltration that the AC must condense and remove

Worked examples

Example 1 — 140 m² home in Toronto (Ontario):

• Summer design: 33°C dry-bulb, 24°C wet-bulb. ΔT = 33 − 24 = 9°C
• Envelope conduction (walls + roof + floor): approximately 1,500 W
• Window solar gain (20 m² of glazing, south and west): approximately 3,500 W peak
• Infiltration + occupants + appliances: approximately 2,000 W
• Total sensible: ~7,000 W; add latent ~2,000 W
• Total cooling load: ~9,000 W = ~30,700 BTU/hr ≈ 2.5 tons

Example 2 — 140 m² home in Calgary (Alberta):

• Summer design: 28°C dry-bulb, 17°C wet-bulb (low humidity). ΔT = 28 − 24 = 4°C
• Lower ΔT reduces conduction gains by ~55% vs Toronto
• Minimal latent load due to low humidity
• Total cooling load: approximately 15,000–18,000 BTU/hr ≈ 1.5 tons

Summer design temperatures by Canadian city

Frequently asked questions

Code reference

Based on CSA F280-12 (R2017) (cooling load methodology) and ASHRAE Handbook of Fundamentals simplified cooling load method, adapted for Canadian climate data from NBCC 2020 Appendix C. Have cooling system sizing performed or verified by a licensed HVAC contractor.