What “size” really means: BTUs, tons, and why it matters
When we talk about AC “size,” we mean cooling capacity, not cabinet dimensions. Capacity is measured in BTU/hr (British Thermal Units per hour) or tons; 1 ton = 12,000 BTU/hr. The right size keeps your home comfortable on the hottest design days without wasting energy or wearing out parts. Too big and the system short-cycles (on/off constantly) and misses humidity control. Too small and it runs non-stop yet still can’t keep up. A good target is a unit that runs steady, not frantic. If you’re new to all this, start with a quick read of The Furnace Outlet’s straightforward Sizing Guide and keep this article open. We'll walk through the exact factors that push you up or down in capacity so your choice is more than a guess.
Square-foot rules: useful start, not the finish line
Rule-of-thumb charts are fine for ballpark estimates. A common shortcut is square footage × 20 BTU. For example, 1,800 sq. ft. × 20 ≈ 36,000 BTU (3 tons). Typical ranges:
|
Home Size (sq. ft.) |
Approx. BTU |
Approx. Tons |
|
600–1,000 |
18,000 |
1.5 |
|
1,000–1,500 |
24,000 |
2.0 |
|
1,500–2,000 |
30,000 |
2.5 |
|
2,000–2,500 |
36,000 |
3.0 |
|
2,500–3,000 |
42,000 |
3.5 |
|
3,000+ |
48,000+ |
4.0+ |
But shortcuts ignore insulation, windows, ceiling height, local weather, and ducts. Use them to sanity-check not to buy. If you want help translating “chart to real house,” our team can review details and even offer a Quote by Photo so you get guidance without a pushy sales visit.
Manual J: the gold standard for sizing in 2025
Manual J is the industry calculation that turns your home’s specifics into a precise load number. It looks at: insulation levels, window count/orientation, infiltration, duct efficiency, occupant heat, cooking loads, and local design temperatures. A few practical adjustments you’ll see in real-world calcs:
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People: add ~100 BTU per person
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Windows/doors: ~1,000 BTU each (type/orientation matters)
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Sun-soaked rooms: add ~10%
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Ceilings over 8 ft: add ~25% per extra foot
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Kitchens: add ~4,000 BTU for appliances
You can hire a pro or use reputable software, but don’t skip this step on larger homes or complex layouts. If you want a human sanity-check, The Furnace Outlet’s Design Center can help translate a Manual J into equipment options you actually recognize.
How your home changes the math (with quick examples)
Two 2,000 sq. ft. homes can need very different AC sizes. Example: House A has R-38 attic insulation, low-E windows, deep roof overhangs, and tight construction. House B has basic attic insulation, lots of west-facing glass, and leaky doors. If both start at ~3 tons by square-foot rule, House A might land at 2.5–3 tons, while House B might need 3.5 tons after Manual J adjustments. High ceilings? Vaulted great rooms can add 25% per extra foot over 8 ft. A big kitchen that’s always in use? Budget ~4,000 BTU more. These aren’t “upsells” , they're predictable heat gains. If you’re piecing together equipment, make sure your air handler and coil match the chosen condenser; browse compatible Air Handlers to keep the system engineered as a set.
Climate matters: humid vs. dry vs. cold
Climate can swing your choice a full half-ton or more. Hot & humid regions often benefit from longer run times to wring out moisture; variable-speed or inverter units shine here for steady dehumidification. Hot & dry climates focus more on sensible cooling capacity can be slightly lower than humid locales for the same square footage, with a premium on high SEER2 for energy costs. In colder regions, the heating side (if using a heat pump or dual-fuel) and insulation quality drive many decisions. Ask: “Do I need a heat pump that handles shoulder seasons well?” If yes, consider efficient R-32 Heat Pump Systems.
2025 rules of the road: SEER2 and regional minimums
In 2025, DOE’s SEER2 standards set different minimums by region: 13.4 SEER2 (North) and 14.3 SEER2 (Southeast/Southwest). What should you buy?
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Budget-minded: 14.3–15.2 SEER2 meets code and controls upfront cost
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Balanced ROI: 16–18 SEER2 fits most U.S. homes well
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Premium: 20+ SEER2 brings top comfort, modulation, and low noise
Higher SEER2 units usually bring variable-speed compressors and better humidity control especially important in sticky climates.Need to spread the investment out? Review our straightforward HVAC Financing options.
Symptoms of a wrong-size AC (and what to check first)
Oversized units cool fast but shut off before dehumidifying rooms feel cool yet clammy, electric bills spike from short-cycling, and parts wear out faster. Undersized units run endlessly, struggle on peak afternoons, and still miss the setpoint. Before blaming the condenser, check basics:
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Filter & coil cleanliness
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Duct leaks or disconnected runs
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Supply/return balance (closed doors can starve returns)
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Thermostat location (not over a supply or in direct sun)
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Refrigerant charge (pro check)
If red flags persist, revisit the load calculation. Our Help Center covers many quick fixes, and you can find needed accessories if a simple component is the culprit.
Ductwork, airflow, and installation details that change size
Ducts are part of the “system,” not just tubing. Undersized or leaky ductwork can steal capacity, forcing larger equipment than you’d otherwise need. A solid install will verify static pressure, seal major leaks, and confirm CFM per ton (commonly ~350–450 CFM/ton) to move heat off the coil efficiently. Outdoor units need clearance (about 2 ft) for airflow; indoor line lengths and properly sized line sets protect compressor life and efficiency. If you’re refreshing the refrigeration circuit, match components and lengths with quality Line Sets. Bottom line: two identical condensers can perform wildly differently with good vs. poor airflow. Choose a contractor who measures, not guesses and documents those readings.
Picking equipment: single-stage, two-stage, or variable-speed?
Once your load is known, pick how you want the system to modulate:
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Single-stage: On or off. Lower upfront cost; least precise humidity/temperature control.
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Two-stage: Low most of the time, high on hot spikes. Comfort step-up without top-tier price.
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Variable-speed (inverter): Broad output range for steady temps and quiet, long runs great for humidity.
In 2025, many buyers land on two-stage or variable for comfort and bill savings. Reliability-focused brands with strong track records include Carrier, Trane, Lennox, plus solid values like Amana and Goodman. If you want matched, space-saving installs, look at Residential Packaged Units.
A clear, step-by-step path to the right size (plus pro tips)
Here’s a simple workflow you can follow or hand to your contractor:
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Gather home facts: square footage, ceiling heights, window count/orientation, insulation levels, typical occupants.
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Account for climate: local design temps, humidity, shading.
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Run Manual J: professional software or a qualified contractor.
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Match equipment: capacity to load, pick staging (two-stage/variable for humidity control), confirm SEER2 fit for your region.
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Verify install plan: electrical, pad/clearances, duct condition, CFM/ton targets, and line set sizing.
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Document startup readings: static pressure, supply/return temps, superheat/subcool.
Want a hand turning numbers into a parts list? Start with our Design Center or contact us directly via Contact Us.
