I’m Chris, senior HVAC tech at Calgary Air Heating and Cooling Ltd, and I’ve been on enough summer calls in Calgary to know this topic trips people up. You walk into a big-box store or scroll online and you see a bunch of numbers and “tons” and it starts to feel like you’re guessing. And honestly, guessing is how you end up with a system that short-cycles all day, or one that runs forever and still leaves you sweaty in the upstairs bedroom. Neither is fun. Usually anyway.

I’ve seen both sides of it in real houses. A newer two-storey in Evanston with huge west windows and not much shade can roast in the afternoon, even if the square footage looks “normal” on paper. Then you go to an older bungalow in Marlborough with decent attic insulation and smaller glass and it cools down nicely with less cooling capacity than you’d expect. That’s why I don’t love the “just go by floor area” advice. It gets you close sometimes. Other times it sends you straight into regret.

There’s also the way people actually live in the place. Doors left open, blinds always up, five people cooking and showering back-to-back, or the opposite where it’s quiet and shaded and you barely generate heat inside. And then there’s ductwork, which gets ignored until it bites. I’ve walked into basements where the supply trunks are leaking like crazy, returns are undersized, and the poor air conditioner is blamed like it’s the problem. Well, it is part of the problem, but it’s not the whole story. Most of the time, at least.

So in this article I’m going to talk about how to land on a cooling system that fits your place properly, using real factors like insulation, windows, sun exposure, layout, and airflow. Not sales talk. Just the stuff that decides whether you’re comfortable in July or standing over a vent wondering why the air feels “kinda cool” but not enough.

Measure your cooled area and use a BTU-per-square-foot rule by climate zone

The first part is boring, but it saves headaches later: measure the spaces you actually cool. Not the whole building, not the garage, not that storage room with the door closed all summer. Grab a tape, sketch a rough floor plan, and add up the square footage of the rooms that get supply air and that you want comfortable. I’ve walked into plenty of places where the basement is counted, but the supply registers are shut because someone thinks it “keeps the cold upstairs”. Then the numbers are off before you’ve even picked equipment.

If your ceilings are way higher than 8 feet, pause and think. A tall bonus room over the garage in Calgary can act like a little heat battery, especially with big west windows. Square footage still matters, but volume starts to show up in how it feels at 5 p.m. on a sunny day. Most of the time, at least, people notice that room first and assume the whole system is wrong, when it is really one space that needs more attention.

Quick measuring rules I use on site

• Measure length × width per room, then total the rooms you cool.
• Exclude garages, unfinished mechanical rooms, and areas with no supply air.
• If a room is open to another (no door, big opening), treat it as one combined area.

Once you have the cooled area, apply a BTU-per-square-foot estimate based on climate zone. This is not a load calculation, it’s a rough starting point so you’re not wildly off. Hotter, stickier climates generally use a higher BTU per square foot. Drier or milder spots can use less. Here in Alberta, most residential cooling is more about short hot stretches and sun load than constant humidity removal, so the number is often on the lower end compared to southern Ontario or the U.S. Gulf states.

Here’s a simple set of ranges that won’t get you laughed out of a mechanical room. Use the lower end if your place is newer, shaded, and well sealed. Use the higher end if you’ve got older windows, lots of sun, or air leaks you can feel in winter.

• Cool/marine climates: 18–22 BTU per sq ft
• Mixed/temperate: 22–28 BTU per sq ft
• Hot/dry: 24–30 BTU per sq ft
• Hot/humid: 28–35 BTU per sq ft

Example math is plain: 1,200 sq ft of cooled space in a mixed/temperate area at, say, 25 BTU per sq ft gives 30,000 BTU/h. That points you in the right ballpark. Then you sanity-check it against the real world: does your place roast in late afternoon, do you have big glass, is the attic insulation decent, do you run a bunch of computers, do you cook a lot. I’ve seen a 1,100 sq ft townhouse with brutal west exposure act like a much larger place, and a shaded bungalow stay comfortable with less cooling than you’d guess.

One thing I’ll gently say because I see it: people love closing vents and blocking returns with furniture. Then they complain the upstairs is warm and the main floor is freezing. Air has to move. Your BTU estimate assumes the air distribution is not being sabotaged by a sofa jammed against the only return grille. Well, usually anyway.

If your estimate lands you between two options, don’t rush the “bigger is safer” mindset. Overshooting can mean short run times, noisier airflow, and poorer moisture control in humid regions. Undershooting can mean long run times on the hottest days. The climate-zone BTU-per-square-foot method is a rough map, not the destination, but it’s a decent way to keep the conversation grounded before someone sells you something that does not match how your space actually behaves.

Adjust tonnage for ceilings, insulation, windows, shade, occupancy, and heat-producing appliances

Tonnage is the quick shorthand people use, but real capacity comes from the building. Square footage gets you in the ballpark, then ceilings, insulation, glass, shade, and the way you live inside the place pull it up or down. I’ve seen two houses on the same street, same floor area, and one struggles all July while the other cruises. The difference was mostly attic insulation and a big west window that cooked the living room every afternoon.

Ceiling height is a sneaky one. Those 9 ft ceilings are fine, but once you get into 10, 12, vaulted great rooms, you are not just cooling floor area, you are cooling a bigger air volume, and stratification becomes a thing. Warm air sits up high, the thermostat is down low, and the system keeps running trying to catch up. If you are already close on capacity, tall ceilings can push you over the line and you end up calling residential air conditioning repair thinking something is broken, when it is really just working past its comfort zone.

Insulation and air sealing are the opposite story. A tight attic hatch, decent blown-in, sealed top plates, weatherstripped doors. That stuff actually lets you step down a bit on capacity because you are not trying to cool the outdoors all day. Most of the time, at least. The tricky part is people assume “newer house” equals “tight house,” and I’ve opened plenty of newer attic accesses that might as well be a window left open.

Windows and orientation matter more than people like to admit. Big south and west facing glass, especially without decent low-e and without proper coverings, can add a lot of solar load. Shade helps. Trees, overhangs, exterior shades. Interior blinds help too, just not as much because the heat is already inside once it passes the glass. If you are planning upgrades, it’s a good moment to talk through air conditioning equipment installation at the same time, because changing windows can change the cooling math.

Shade and outdoor airflow

People love the idea of boxing in the outdoor condenser with lattice or shrubs so it “looks nicer.” I get it, but that can starve it of airflow and make it run hotter and longer. Shade from a tree canopy is great, as long as you still have room for the fan to breathe and you are not packing the coil with fluff and leaves. And if you’re the type who also wonders about seasonal protection, read Should I cover my outdoor commercial AC unit in winter? and then apply the same common sense to keeping it clear in summer.

People and appliances add heat fast

Occupancy is real load. A family of five with friends over on weekends is not the same as one person working nights and sleeping days. Bodies throw heat, cooking throws heat, showers throw humidity. In Calgary we don’t always think about latent load as much as humid climates do, but it still shows up as that “sticky” feeling after cooking or laundry, and the system has to remove it.

Then there are the heat-makers you forget about. Big gaming PCs, servers, older fridges in the basement, a south-facing bonus room with two monitors and a printer running all day. I’ve walked into places where the upstairs office is 4°C warmer than the rest of the floor, and the answer wasn’t “more cooling everywhere,” it was “treat that room like its own heat source and plan around it.” If you’re weighing options and want it done right the first time, that’s when people search residential air conditioning installation near me and ask about zoning, returns, and whether a slightly higher capacity or a different layout makes more sense.

Q&A:

My house is about 1,800 sq ft. Can I just pick an AC size from a square-foot chart?

Square-foot charts are a rough starting point, not a final answer. Two homes with the same floor area can need very different cooling because of insulation, window size and direction, ceiling height, duct condition, and local climate. A quick rule-of-thumb often lands around 18–24 BTU per sq ft (so roughly 32,000–43,000 BTU/h for 1,800 sq ft, or about 2.5–3.5 tons), but that range is wide for a reason. The reliable method is a Manual J load calculation, which estimates the actual heat gain of your home. If you’re between sizes, the right choice depends on humidity control, your ductwork, and whether you’ll use a variable-speed system that can run longer at lower output.

What happens if my AC unit is too big? Won’t it cool the house faster and be better?

An oversized unit often cools the air fast but leaves comfort problems. It may cycle on and off frequently, which can cause uneven temperatures and higher wear on parts. Short run times also reduce moisture removal, so the house can feel cold but damp. You can also get louder operation and more noticeable temperature swings. Bigger is not automatically better; the goal is a system that runs long enough to keep both temperature and humidity steady during typical hot weather.

I’m replacing an old 3-ton system. Should I buy another 3-ton unit to be safe?

Not automatically. Many older systems were sized using simple rules or “match what’s there,” and homes often change over time (new windows, added insulation, finished basements, air sealing, roof replacement). Any of those can reduce the cooling load, meaning a smaller unit could work better. Also, a new system may have different performance characteristics than the old one. The best path is to measure your duct sizes and airflow capability, then run a load calculation. If your old unit struggled, the issue might have been duct leakage, low airflow, poor refrigerant charge, or attic heat gain—not just capacity. A same-size replacement can repeat the same comfort issue if the root cause wasn’t the tonnage.

I live in a humid area and the upstairs feels hotter. Should I size the AC larger for the second floor?

Hot upstairs rooms are often an airflow and building-envelope problem, not a “needs more tons” problem. Warm air rises, attics radiate heat downward, and upstairs ducts can be undersized, leaky, or poorly insulated. If you simply increase capacity, you may cool the downstairs too much while the upstairs still lags, and humidity control can get worse from short cycling. Better fixes include improving attic insulation and air sealing, adding return air upstairs, balancing supply dampers, sealing ducts, or adding zoning or a separate system for the upper level. In humid climates, consider a variable-speed or two-stage unit paired with the right-sized equipment so it can run longer at lower output and remove more moisture.