When we quote a new AC for a Mississauga home, the first question we ask is not which brand you want. It is how many BTU per hour your house actually needs. Pick a unit that is too small and it runs all day without cooling the upstairs. Pick a unit that is too big and it short-cycles, fails to remove humidity, and burns out the compressor years early. This guide shows you exactly how we size every AC we install across the GTA.
The BTU Rule of Thumb (Start Here)
A typical GTA home needs roughly 20 BTU per square foot of cooled area. A 2,000 sq ft Mississauga home in the Erin Mills area lands at about 40,000 BTU, which is a 3.5 ton AC unit. This is a starting estimate and not a final spec.
| Home size | Approximate BTU/hr | AC tonnage |
|---|---|---|
| 1,000 sq ft | 20,000 BTU | 1.5 to 2 tons |
| 1,500 sq ft | 30,000 BTU | 2.5 tons |
| 2,000 sq ft | 40,000 BTU | 3 to 3.5 tons |
| 2,500 sq ft | 50,000 BTU | 4 tons |
| 3,000 sq ft | 60,000 BTU | 5 tons |
Why the Rule of Thumb Is Only a Starting Point
Two homes of the exact same square footage can need very different AC capacity. We adjust the BTU estimate up or down based on six load factors specific to your home.
- Insulation grade: a 1970s Mississauga bungalow with R-12 attic insulation needs 15 to 25 percent more BTU than a 2015 new build with R-50.
- Window count and orientation: large west-facing windows in Oakville add up to 30 percent to the cooling load on a July afternoon.
- Ceiling height: 9 or 10 foot ceilings in Vaughan custom builds add 10 to 15 percent.
- Air leakage and ductwork: leaky ducts in unconditioned attics or basements lose 20 to 30 percent of cooled air.
- Occupancy: every full-time resident adds about 600 BTU. A family of 5 adds the equivalent of one small room.
- Appliance heat gain: kitchens, dryers, and home offices with multiple computers add 1,000 to 3,000 BTU.
The Manual J Load Calculation (The Real Answer)
ACCA Manual J is the industry-standard load calculation that every TSSA-certified contractor should run before quoting a replacement unit. We use Wrightsoft Right-J software on every Mississauga and Toronto install. The calculation factors in your home orientation, window U-value, insulation R-values, infiltration rate and local climate data for Pearson International Airport.
A proper Manual J takes about 45 minutes during the in-home estimate. Skip a contractor who guesses based on the old unit size, because the old unit was often oversized to begin with.
Common Sizing Mistakes We See in GTA Homes
Replacing like-for-like without recalculating
Your 2002 home had a 4-ton AC. After you added attic insulation and triple-pane windows in 2020, your real load is now 3 tons. Installing a like-for-like replacement wastes money and creates humidity problems.
Sizing for the worst day in July only
A unit sized for 35C peak load runs at 40 percent capacity in May and September. Variable-speed AC fixes this. Single-stage AC does not.
Ignoring duct capacity
A 5-ton AC pushing through ducts designed for 2.5 tons creates static pressure that wrecks the compressor.
EcoFrost runs a full Manual J on every Mississauga and GTA AC install. Free in-home assessment.
Book Free AC Sizing AssessmentDuctless Mini-Split Sizing Is Different
If you are adding a ductless mini-split for a single room (basement, attic conversion, garage office), the rule is much tighter. Most Mitsubishi and Daikin mini-split heads come in 6,000, 9,000, 12,000, 18,000 or 24,000 BTU. Pick the closest size above your calculated room load. Oversized mini-splits short-cycle harder than oversized central AC.
How Much Does the Right-Sized AC Cost in 2026?
Central AC installation in the GTA runs from $3,500 for a 2-ton single-stage unit to $9,000+ for a 5-ton variable-speed inverter system. Ductless mini-split single-zone installs run $3,800 to $5,500. Multi-zone systems with 3 to 5 indoor heads run $9,000 to $14,000.
See our 2026 GTA HVAC pricing for AC, furnace, heat pump and water heater installations.
View Full Pricing Guide?Frequently Asked Questions
Is bigger always better when sizing AC?
Can I use my old AC tonnage as a guide for the new one?
How long does an AC sizing assessment take?
Does AC sizing affect rebate eligibility in Ontario?
What size AC should I get for a 2,000 sq ft Mississauga home?
EcoFrost Heating & Cooling
Toronto's Trusted HVAC Experts Since 2015
Our certified HVAC technicians have served 5,000+ Toronto-area homes. We write about heating, cooling, and air quality from real field experience not marketing copy. Learn about us →
