What Size Air Conditioner Do I Need? AC Sizing Guide (2026)

Quick answer: roughly 20 BTU per square foot of conditioned space. For a 2,000 sq. ft. home, that’s 40,000 BTU — or 3.3 tons, which most contractors round to a 3 or 3.5-ton unit.

Treat that as a starting point, not a decision. The actual right size for your home depends on insulation quality, ceiling height, window placement, and your climate. In Oregon specifically, well-insulated homes in the Willamette Valley often come in 10–20% below the national rule of thumb — the mild, marine summers mean a lower peak load than anywhere in the South or Midwest. The only way to nail the number is a Manual J load calculation; everything else, including this guide, gets you close enough to have an informed conversation before signing anything.

AC sizing basics: tons, BTU, and what they mean

BTU stands for British Thermal Unit — the energy required to raise one pound of water by 1°F. It’s an old unit, but the HVAC industry has used it for a century and isn’t changing.

Air conditioners are rated in tons. One ton of cooling capacity = 12,000 BTU removed from your home per hour. The term comes from early mechanical cooling, when a literal ton of ice was used to absorb heat overnight. Now it’s just the standard that stuck. Residential systems run from 1.5 tons (18,000 BTU/hr) to 5 tons (60,000 BTU/hr). Most Portland-area homes fall in the 2–4 ton range.

One ton of cooling does not mean the unit weighs a ton or uses a ton of electricity. It means it removes 12,000 BTU per hour. That’s the whole definition.

The quick rule of thumb (20 BTU per square foot)

Multiply your conditioned square footage by 20 to get an approximate BTU requirement:

• 1,000 sq. ft. × 20 = 20,000 BTU ≈ 1.5–2 tons
• 1,500 sq. ft. × 20 = 30,000 BTU = 2.5 tons
• 2,000 sq. ft. × 20 = 40,000 BTU ≈ 3–3.5 tons
• 2,500 sq. ft. × 20 = 50,000 BTU ≈ 4 tons
• 3,000 sq. ft. × 20 = 60,000 BTU = 5 tons

This rule assumes 8-foot ceilings, moderate insulation, mixed window exposure, and a typical mid-range U.S. climate. It works as a sanity check.

Here’s where it breaks down: the 20 BTU/sq. ft. standard was calibrated for warmer, more humid climates than Oregon. A well-insulated newer home in Beaverton might genuinely only need 15–17 BTU/sq. ft. — meaning the rule of thumb oversizes by 15–25%. Oversizing is actually the more common error in contractor quotes, and it creates its own problems (more on that below). If someone quotes you a system significantly larger than 20 BTU/sq. ft. suggests, ask to see their load calculation before agreeing.

AC size by square footage (chart)

These ranges apply the 20 BTU/sq. ft. rule with ±15–25% for typical real-world variation. Assumes 8-foot ceilings, moderate insulation, and mixed sun exposure. Adjust using the factors in the next section.

AC size for 1,000 sq. ft.: 1.5 tons in a standard home; 2 tons if ceilings are high or the south/west sun exposure is significant.

AC unit size for 1,500 sq. ft.: 2–2.5 tons. A well-insulated Oregon home this size often comes in closer to 2 tons on a proper Manual J.

What size AC for 2,000 square feet: 3 tons is the most common result. If the home was built before 1990 with minimal attic insulation and has west-facing glass, 3.5 tons is reasonable.

AC size for 2,500 sq. ft.: 3.5–4 tons depending on construction vintage and window placement. Don’t let a contractor jump straight to 5 tons without showing the math.

AC size for 3,000 sq. ft.: 4–5 tons. At this size, the variable differences between individual homes add up enough that a real load calculation isn’t optional — it’s the only way to get it right.

Factors that change your AC size

Square footage is just the starting point. These are the variables that actually move the needle.

Ceiling height

The 20 BTU/sq. ft. rule assumes 8-foot ceilings. Every foot above that adds roughly 10–15% to the cubic volume you’re conditioning. A 2,000 sq. ft. home with 10-foot ceilings contains 25% more air than the same footprint with 8-foot ceilings. Vaulted great rooms or open stairwells compound this further. A rough adjustment: add 10% to your BTU estimate for 9-foot ceilings, 20–25% for 10-foot, more for vaulted.

Insulation

This one matters more than most homeowners expect. An older 1970s home in Tigard — single-pane windows, 2–3 inches of attic insulation, no house wrap — can have twice the cooling load of a newer code-built home with the same square footage. R-38+ attic insulation, double-pane low-E windows, and decent air sealing don’t just reduce energy bills — they directly determine what size system your home needs. A contractor who doesn’t ask about your insulation before sizing your AC is skipping a critical input.

Climate (Oregon)

Oregon’s Willamette Valley sits in ASHRAE Climate Zone 4C — a marine climate with mild, dry summers relative to most of the country. Portland’s 99th-percentile cooling design temperature is approximately 88–91°F. Compare that to Phoenix at 107°F or Atlanta at 95°F with sustained high humidity. That lower peak temperature means a lower peak cooling load. Well-insulated homes in the Portland metro often need 15–20% less tonnage than the national rule of thumb suggests.

One note on the 2021 heat dome: Portland briefly hit 116°F. Sizing a system around that extreme would mean a grossly oversized unit that short-cycles for 350 days a year. Design around typical conditions; add a ceiling fan for the anomalies.

Windows and sun exposure

South- and west-facing glass are the biggest wildcard in load calculations. A 6×4 ft west-facing window adds roughly 500–1,000 BTU/hr of solar heat gain on a clear summer afternoon — more than some people’s entire insulation adjustment. A home with substantial west-facing glazing and no exterior shade needs more cooling than the square footage suggests. Mature trees or overhangs on the west side can meaningfully reduce the load.

Occupants and internal heat

People generate heat — roughly 250 BTU/hr per person at rest, more when active. The same 2,000 sq. ft. home runs differently with two occupants versus six. Kitchen equipment, home offices, and even lighting add up. It won’t change your sizing by a full ton, but it’s one more reason the square footage rule is an approximation.

Why correct sizing matters (oversized vs undersized)

This is where most blog posts get it wrong by treating “bigger is safer” as reasonable advice. It isn’t.

An oversized AC cools air temperature fast — the space hits setpoint quickly and the compressor shuts off. Then temperatures creep back up and it kicks on again. These short cycles (sometimes 5–7 minutes on, 5 minutes off, repeat) never give the system enough runtime to pull humidity out of the air. In an Oregon summer, where a 75°F house can feel sticky at 65% relative humidity, that’s a genuine comfort failure. The house lands at the right temperature but still feels clammy. People respond by dropping the thermostat, which makes the short-cycling worse.

There’s a mechanical cost too. Compressor startups draw significantly higher amperage than steady-state operation — every start is a mechanical stress event. A system that cycles 8–10 times per hour accumulates wear at a faster rate than one cycling 2–3 times. Short-cycling doesn’t announce itself; it shows up as a compressor failure at year 8 or 9 instead of year 15–18.

An undersized unit has a simpler failure mode: it runs continuously on hot days and can’t hold setpoint when it matters. Higher operating costs, shorter equipment life, uncomfortable house on the 10 days per year when that’s most noticeable.

The uncomfortable truth: most homeowners can’t tell from normal operation whether their system is correctly sized. If it keeps up, they assume it’s working. Persistent indoor humidity in summer — that “cold and clammy” feeling even when the thermostat is satisfied — is the primary symptom of an oversized, short-cycling system.

How to find the size of your current AC

The tonnage is embedded in the model number. No tools required.

Find the outdoor condenser unit and look for the label on the side panel. Scan the model number for a two- or three-digit number that’s a multiple of 6 or 12: 18, 24, 30, 36, 42, 48, or 60. That number represents thousands of BTU per hour. Divide by 12 to get tons.

The relevant digits will always be a multiple of 6 (18, 24, 30, 36, 42, 48, 60). If you see something like 24 in a Carrier model number — that could be the voltage phase code, not the tonnage. Context matters: find the number that fits a recognizable BTU increment. When in doubt, write down the full model number and search “[model number] BTU” or “[model number] tonnage” — every manufacturer posts spec sheets that confirm the output.

Rule of thumb vs Manual J — when you need a pro

The square footage chart above gives you a reasonable estimate. For a final purchase decision on a $6,000–$18,000 system, that estimate isn’t sufficient on its own.

Manual J is the ACCA (Air Conditioning Contractors of America) standard for residential cooling load calculations. A proper Manual J accounts for every heat source and loss point in your specific home: each room’s dimensions and ceiling height, wall and attic insulation R-values, window area by orientation, U-factors and solar heat gain coefficients, infiltration rate, duct layout and leakage, local design temperatures, and internal heat gains from occupants and equipment.

It takes a trained technician 1–2 hours. The output is a room-by-room load breakdown and a total BTU figure that is specific to your home — not a regional average, not a neighbor’s house, yours.

This matters more in Oregon than most places realize. Homes in the Portland metro span 80-plus years of construction vintage. A 1952 Cape Cod in Lake Oswego and a 2020 build in Hillsboro with identical square footage can have cooling loads that differ by 30–50%. The square footage rule treats them identically. Manual J doesn’t.

If a contractor quotes you a system size without performing a Manual J or doing a detailed room-by-room walkthrough with measurements and insulation verification, they’re making an educated guess. Sometimes they guess well. Often they upsize to protect themselves — “better too big than too small” — which is exactly backwards from a comfort and efficiency standpoint.

Here’s what AC installation in Beaverton looks like when it’s done right: a load calculation before any equipment gets quoted.

AC sizing calculator

The math behind every online AC sizing calculator is the same formula:

(Square footage × 20) ÷ 12,000 = baseline tons

Run that first, then apply the adjustments below for your actual home conditions.

Worked example — 2,000 sq. ft. Beaverton home, 10-ft ceilings, good insulation, mixed sun:

• Base: 2,000 × 20 ÷ 12,000 = 3.3 tons
• 10-ft ceilings ×1.22 → 4.1 tons
• Good insulation ×0.90 → 3.7 tons
• Oregon climate ×0.90 → 3.3 tons → call it 3 tons

That’s the manual air conditioner size calculator result. Apply the factors that match your home, land on a range, then compare it against a contractor’s quote. If their number is meaningfully higher without an explanation, that’s the conversation to have — ask to see the load calculation that justifies the upsizing.

This approach gets you close. For a calculation that accounts for your specific duct layout, window U-factors, and room-by-room load, you still need a Manual J.

FAQ

What size AC for a 2 bedroom house?

Bedroom count is a poor proxy for AC sizing — it doesn’t tell you the square footage, ceiling height, or insulation quality. A two-bedroom house in Oregon typically runs 900–1,400 sq. ft., which works out to 1.5–2.5 tons using the 20 BTU/sq. ft. rule. A two-bedroom with vaulted ceilings and a wall of west-facing glass might need more than a two-bedroom that’s well-shaded and tightly built. Measure the conditioned square footage and use the table above.

What size central air conditioner do I need for a 2,000 sq. ft. house?

At 20 BTU/sq. ft., that’s 40,000 BTU — about 3.3 tons. Most contractors quote a 3 or 3.5-ton unit. In Oregon, if the home has solid insulation (R-38 attic, double-pane low-E windows, decent air sealing), a Manual J often comes back at 2.5–3 tons. Don’t let anyone install a 4-ton system in a well-insulated 2,000 sq. ft. Willamette Valley home without showing you the load calculation that justifies it.

Can’t I just match what I have?

Sometimes yes — if the old system kept up with summer heat and the house was comfortable (not just cool but not humid), it was probably sized reasonably well. If the old unit was short-cycling or couldn’t hold temperature on hot days, replacing it with the same size repeats the same mistake. Use the system replacement as an opportunity to verify the sizing, especially if you’ve added insulation, new windows, or done major air sealing work since the original installation.

How much does AC size affect my utility bill?

An oversized unit uses more electricity per hour while it runs, and it runs in shorter, more frequent bursts — both factors that reduce efficiency. A correctly sized system runs longer, steadier cycles, which is where heat pumps and high-efficiency ACs operate best. The efficiency ratings (SEER2) on the spec sheet are tested under steady-state operation; short-cycling degrades real-world performance below the rated number.

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