How the AC Size Calculator Works
The AC size calculator estimates central air conditioner capacity in BTU/hr and tonnes using your city's NBCC 2020 cooling design temperature, floor area, insulation level, and sun exposure. It produces a preliminary sizing range suitable for getting quotes — not a replacement for a Manual J calculation, which is required for most Canadian HVAC permits.
Step 1 — Cooling design temperature
The calculator uses each city's 2.5% July dry-bulb cooling design temperature from NBCC 2020 Appendix C. This is the temperature that is exceeded only 2.5% of the time during July — representing the hottest afternoon your AC needs to handle. The higher the cooling design temperature, the larger the cooling load for the same building.
| City | Province | Cooling design temp (°C) |
|---|---|---|
| Toronto | ON | 33°C |
| Ottawa | ON | 31°C |
| Hamilton | ON | 32°C |
| London | ON | 31°C |
| Windsor | ON | 33°C |
| Kingston | ON | 30°C |
| Barrie | ON | 29°C |
| Sudbury | ON | 28°C |
| Thunder Bay | ON | 27°C |
| Bracebridge | ON | 26°C |
| Montreal | QC | 30°C |
| Quebec City | QC | 28°C |
| Vancouver | BC | 29°C |
| Victoria | BC | 25°C |
| Kelowna | BC | 34°C |
| Prince George | BC | 29°C |
| Calgary | AB | 28°C |
| Edmonton | AB | 27°C |
| Lethbridge | AB | 31°C |
| Winnipeg | MB | 31°C |
| Brandon | MB | 30°C |
| Saskatoon | SK | 31°C |
| Halifax | NS | 27°C |
| Fredericton | NB | 29°C |
| Charlottetown | PE | 27°C |
Source: NBCC 2020 Appendix C — 2.5% July design dry-bulb temperatures for Canadian cities.
Step 2 — Base load from floor area
The calculator starts with a base cooling load derived from floor area and the city's cooling design temperature. The base formula scales linearly with both — a 200 m² home in Windsor (33°C design temp) has a significantly larger load than the same home in Victoria (25°C design temp). All floor area inputs should include only conditioned, finished space — heated basement levels included; unfinished basement and garage excluded.
Step 3 — Insulation multiplier
Insulation quality determines how fast heat enters the building through the envelope. The calculator applies a multiplier to the base load:
- Well insulated (RSI-3.5+ walls, RSI-7+ attic, modern windows) — multiplier 0.85. Typical post-2000 construction built to provincial energy codes.
- Average insulation (RSI-2.1–3.5 walls, RSI-5–7 attic) — multiplier 1.0. Most 1980–2000 construction.
- Poorly insulated (RSI below 2.1 walls, older single-pane or minimal windows, leaky construction) — multiplier 1.25. Pre-1980 homes with no major envelope upgrades.
Step 4 — Solar gain from sun exposure
Solar radiation through glazing is the largest variable in residential cooling load — south and west-facing windows with no external shading can double the peak afternoon load versus a well-shaded home of the same size. The calculator adds a solar gain component based on the exposure level selected:
- Low — mostly north/east-facing windows, heavy tree cover or overhangs, or interior blinds consistently closed on hot days.
- Average — mixed orientation, some shading. Typical Canadian suburban house.
- High — many south/west-facing windows, minimal shading, open-plan main floor. New construction with large glazing areas or a house with a large south-facing addition.
Why Canadian sizing differs from US rules of thumb
The common US rule of thumb — 1 tonne (12,000 BTU/hr) per 500–600 sq ft — was derived from hot, humid US climates like Georgia and Texas. It over-sizes AC units for most of Canada:
- Vancouver's cooling design temperature (29°C) is similar to Seattle, not Phoenix. The same 200 m² house in Vancouver needs roughly 30–40% less cooling capacity than the US rule of thumb suggests.
- Even Toronto (33°C) and Windsor (33°C) — the hottest major Canadian cities — are cooler than Atlanta (35°C) or Dallas (39°C), and Canadian homes tend to have better insulation than comparable US homes due to energy code requirements driven by cold winters.
- An oversized AC unit short-cycles — it cools the space so quickly it shuts off before removing humidity. The result is a cold, clammy house, faster compressor wear, and higher electricity bills.
Limitations of this calculator
This calculator produces a preliminary estimate, not a Manual J calculation. It does not account for:
- Exact window area and glazing type (U-value and SHGC)
- Ceiling height — rooms taller than 2.4 m have more air volume to cool
- Internal heat gains from people, appliances, and lighting
- Duct losses if the ductwork runs through an unconditioned attic or crawlspace
- Multi-zone systems or room-by-room load differences
For equipment selection and permit submission, a certified HVAC contractor should perform a full Manual J calculation. Most provinces require a load calculation for any new HVAC installation permit. The cost is typically $200–$500 and is often bundled into the contractor's installation quote.