AC BTU Calculator: Find the Right Size Air Conditioner

Size a window unit, portable, mini split, or whole-house central AC in under a minute. Built on the Energy Star sizing method with climate zone adjustments and an oversizing check, because the wrong size costs you comfort and money every single summer.

By D. Mercer, Indoor Air Quality Consultant
Window, portable, and mini splitCentral AC by climate zoneEnergy Star methodOversizing checked
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What are you sizing?

Pick the option that matches your situation. Tap to continue.

What type of unit?

Portables play by different rules than window units, so this matters. Tap to continue.

How big is the room?

Length times width. A 12 x 12 bedroom is 144 sq ft. Include open areas the AC must also cool.

How much sun does the room get?

Energy Star adjusts sizing 10 percent either way for sun exposure. Tap to continue.

How many people use the room regularly?

Each person past two adds 600 BTU of body heat. Tap to continue.

What kind of room is it?

Kitchens get a flat 4,000 BTU bump for appliance heat, straight from the Energy Star method. Tap to continue.

How tall is the ceiling?

The standard chart assumes 8 foot ceilings. Taller rooms hold more air to cool. Tap to see your result.

How big is the house?

Use the finished, cooled square footage. Skip the garage and unfinished basement.

Where in the US is the house?

Climate decides BTU per square foot more than anything else. Tap your zone.

How is the house insulated?

A tight 2015 build and a drafty 1965 ranch need very different equipment. Tap to see your result.

What kind of AC are you checking?

Tap one to continue.

What size is the unit?

Use the BTU on the label. For portables, use the DOE SACC number if you can find it. Tap to see coverage.

How many tons is the system?

It is on the outdoor unit label, or divide the model BTU by 12,000. Tap to see coverage.

Match the size first, then buy smart

The right BTU beats any feature on the box. As an Amazon Associate, this site earns from qualifying purchases.

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Quick answers How it works BTU chart Central AC chart Climate zones Window vs portable Oversizing Conversions FAQ

AC Sizing Rules of Thumb (Quick Answers)

These are the numbers I check every sizing question against. The calculator above refines them for your exact room, sun, and climate.

  • Room rule: roughly 20 BTU per square foot, stepped by the Energy Star chart.
  • The conversion: 12,000 BTU equals 1 ton of cooling.
  • People: add 600 BTU for every regular occupant past two.
  • Kitchens: add a flat 4,000 BTU for appliance heat.
  • Whole house: 16 to 26 BTU per square foot depending on US climate zone.
  • Portables: size by the DOE SACC rating, which runs about 60 percent of the marketing number.

How This AC BTU Calculator Works

For single rooms, this air conditioner sizing tool uses the Energy Star method: a base BTU from the official square footage chart, then the published adjustments for sun, occupancy, ceiling height, and kitchens. It is the same procedure printed on the back of every Energy Star buying guide, just done for you.

For whole houses, it applies climate zone factors that approximate what an ACCA Manual J load calculation produces for typical construction. Manual J remains the standard for an equipment purchase, and any contractor quoting you a system without one is guessing.

The coverage mode runs the math in reverse, working as a BTU to square feet converter for a unit you already own. Every chart on this page is generated from the same formulas the calculator uses, so the page can never contradict its own tool.

AC BTU Chart by Room Size (Energy Star)

This is the official Energy Star base chart for an average room with 8 foot ceilings. Run the calculator for sun, people, and ceiling adjustments.

Room sizeBTU neededTypical unit
Up to 150 sq ft5,0005,000 BTU window unit
150 to 250 sq ft6,0006,000 BTU window unit
250 to 300 sq ft7,0008,000 BTU window unit
300 to 350 sq ft8,0008,000 BTU window unit
350 to 400 sq ft9,00010,000 BTU window unit
400 to 450 sq ft10,00010,000 BTU window unit
450 to 550 sq ft12,00012,000 BTU or 1 ton mini split
550 to 700 sq ft14,00014,000 BTU unit
700 to 1,000 sq ft18,00018,000 BTU or 1.5 ton mini split
1,000 to 1,200 sq ft21,00024,000 BTU or 2 ton mini split
1,200 to 1,500 sq ft23,000 to 24,0002 ton mini split
1,500 to 2,000 sq ft30,0002.5 ton mini split
2,000 to 2,500 sq ft34,0003 ton system

Central AC Size Chart by House Size and Climate Zone

Bookmark this AC tonnage chart for quick estimates, computed with the same zone factors as the calculator at average insulation. Above 5 tons, residential practice is two systems, usually one per floor.

House sizeZone 1Zone 2Zone 3Zone 4Zone 5
1,000 sq ft2 tons2 tons1.5 tons1.5 tons1.5 tons
1,500 sq ft3.5 tons3 tons2.5 tons2.5 tons2 tons
2,000 sq ft4.5 tons4 tons3.5 tons3 tons2.5 tons
2,500 sq ftTwo systems5 tons4 tons4 tons3.5 tons
3,000 sq ftTwo systemsTwo systems5 tons4.5 tons4 tons

US Climate Zones for AC Sizing, Explained

Cooling load tracks summer design temperature and humidity, which is why Florida and Minnesota size so differently. Zone 1 covers the hot, humid Deep South: Florida, the Gulf Coast, and south Texas at about 26 BTU per square foot.

Zone 2 takes in the warm South and the desert Southwest, including Georgia, coastal Carolinas, Arizona, and north Texas at about 23. Zone 3 is the middle band through Tennessee, Oklahoma, Missouri, Virginia, and Kentucky at about 20.

Zone 4 covers the Northeast and upper Midwest, from Pennsylvania and Ohio through Illinois, New York, and New Jersey at about 18. Zone 5 is the cool North, including Minnesota, Wisconsin, Michigan, Maine, and the mountain states at about 16, where dehumidification matters less and oversizing hurts more.

Window vs Portable vs Mini Split: Why the Same BTU Behaves Differently

Window units are the honest benchmark: the BTU on the box is the BTU you get, and modern CEER ratings make them cheap to run. They cool the most square footage per dollar of any option.

Portable ACs are where buyers get burned. The big number on the box is the old ASHRAE rating, but federal law also requires the DOE SACC rating, which accounts for the hot exhaust hose and infiltration and typically lands around 55 to 65 percent of the marketing number.

That is why a portable labeled 14,000 BTU delivers roughly 8,000 BTU SACC and cools like a mid-size window unit. The calculator above sizes portables by SACC automatically, which is the only honest way to do it.

Ductless mini splits are sized like window units but run far more efficiently, with inverter compressors that modulate instead of cycling. Single heads come in 6,000 to 36,000 BTU, and past 24,000 BTU for one space, two smaller heads mix air better than one giant one.

Compare units in your calculated size

Filter by your BTU number and ignore everything bigger. As an Amazon Associate, this site earns from qualifying purchases.

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Oversized vs Undersized AC: Both Cost You

An undersized AC runs nonstop on hot days and never quite gets there, which is miserable but at least obvious. An oversized AC fails sneakily: it blasts the thermostat down in ten minutes and shuts off before it has run long enough to wring moisture out of the air.

That short cycling leaves rooms cold but clammy, and sustained indoor humidity above 60 percent is exactly where dust mites and mold thrive. As an indoor air quality consultant, more than half the musty-house calls I see trace back to a generously oversized system.

Aim for a unit within about 30 percent above your calculated load. If you are between sizes in a humid climate, take the smaller one.

Fix the humidity an oversized AC leaves behind

A smart controller adds longer, smarter cycles, and a dehumidifier covers the gap. As an Amazon Associate, this site earns from qualifying purchases.

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BTU to Tons, kW, and Watts (Conversion Chart)

Cooling capacity converts at 12,000 BTU per ton and 0.293 watts of heat moved per BTU per hour. Running watts below assume a typical EER of 11; high efficiency units draw less.

BTU per hourTonsThermal kWTypical running watts
5,0000.421.5455
8,0000.672.3727
12,0001.03.51,091
18,0001.55.31,636
24,0002.07.02,182
36,0003.010.53,273

Watts matter for circuits: most units up to 14,000 BTU run on a standard 115V outlet, while 18,000 BTU and up usually need a dedicated 230V circuit. Check the plug type before you buy, not after.

Common AC Sizing Mistakes I Get Asked to Untangle

  • Buying the portable by the box number. Match the DOE SACC rating to your load, always.
  • Going one size up to be safe. Oversizing trades comfort for humidity. Right sized and running long is the goal.
  • Ignoring sun and ceilings. A west-facing room with 10 foot ceilings needs 25 percent more than the chart says.
  • Skipping Manual J on a central system. A 5 minute square footage quote on a 10 year purchase is a bad trade.
  • Forgetting the circuit. Large units need 230V, and discovering that on install day is no fun.
  • Leaky window installs. Gaps around the unit leak hot, humid air straight back in. Seal the side panels properly.
  • Dirty filters. A clogged filter can cost 10 percent or more of rated capacity, which mimics an undersized unit.

Small parts that protect the install

Sealing and support are the difference between rated capacity and real capacity. As an Amazon Associate, this site earns from qualifying purchases.

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Frequently Asked Questions

How many BTU do I need for a 12×12 room?
A 12 x 12 room is 144 square feet, which the Energy Star chart puts at 5,000 BTU for average conditions. A 5,000 or 6,000 BTU window unit handles it comfortably. Bump to 6,000 or 8,000 if it is very sunny, has tall ceilings, or sleeps more than two people.
How many square feet does a 12,000 BTU AC cool?
A 12,000 BTU unit covers about 450 to 550 square feet in an average room, closer to 500 if very sunny and 600 if heavily shaded. For a portable AC, those numbers only hold for its DOE SACC rating, and a portable marketed at 12,000 BTU ASHRAE delivers roughly 7,200 BTU SACC.
How many BTU per square foot do I need?
Single rooms run roughly 20 BTU per square foot, stepped through the Energy Star chart rather than a straight line. Whole houses range from 16 BTU per square foot in the cool North to 26 in the hot, humid Deep South. Sun, ceilings, occupancy, and insulation move the number from there.
What size AC do I need for a 2,000 square foot house?
At average insulation, a 2,000 square foot house needs about 2.5 tons in the cool North, 3.5 tons in the middle of the country, and 4.5 tons on the Gulf Coast. That spread is exactly why square footage alone is not enough. Get a Manual J calculation before signing a contract.
Is 8,000 BTU enough for a bedroom?
Yes, generously. An 8,000 BTU unit covers up to about 350 square feet in average conditions, and most American bedrooms run 120 to 200 square feet. For a typical bedroom, a 5,000 or 6,000 BTU unit is actually the better-matched, quieter choice.
What is SACC on a portable air conditioner?
SACC stands for Seasonally Adjusted Cooling Capacity, the DOE test that measures what a portable actually delivers after its hot exhaust hose and air infiltration are counted. It typically lands at 55 to 65 percent of the old ASHRAE marketing number. Always size a portable by its SACC.
Can an air conditioner be too big for a room?
Yes, and it is the most common sizing mistake in America. An oversized unit short cycles: it hits the setpoint fast, shuts off before dehumidifying, and leaves the room cold but clammy. Sustained humidity above 60 percent then invites dust mites and mold. Stay within about 30 percent of your calculated load.
How many tons is 36,000 BTU?
36,000 BTU is exactly 3 tons, using the standard conversion of 12,000 BTU per ton. The same math gives you 24,000 BTU as 2 tons and 18,000 BTU as 1.5 tons. The term comes from the cooling delivered by melting one ton of ice in 24 hours.
What size mini split do I need for 500 square feet?
About 12,000 BTU, which is the most common single-head size sold. Go to 18,000 BTU if the space is very sunny, has vaulted ceilings, or is a bonus room over a garage. Mini split inverters modulate down, so a slightly generous head hurts less than an oversized window unit.
Do portable air conditioners need more BTU than window units?
Effectively yes, by label. The single exhaust hose pulls already-cooled room air out, which drags hot outdoor air in through every gap, so a portable needs a much larger marketing BTU to deliver the same real cooling. Dual hose models reduce the penalty. Compare SACC to window BTU and the playing field levels.
How many BTU do I need for a kitchen?
Take the normal number for the square footage and add a flat 4,000 BTU, per the Energy Star method. Cooking appliances dump serious heat, and a range hood vented outdoors helps the AC more than people expect. A 250 square foot kitchen lands around 10,000 BTU.
What SEER2 rating should I buy for central AC?
Federal minimums since 2023 are 14.3 SEER2 in the South and Southwest and 13.4 SEER2 in the North. In hot climates with long cooling seasons, stepping up to 16 or 17 SEER2 usually pays back. Right sizing matters more than the rating, because an oversized 18 SEER2 system still short cycles.

This calculator provides estimates based on the Energy Star room sizing method and standard climate zone factors for planning and educational purposes. Actual cooling loads depend on windows, insulation, air sealing, and ductwork. For central systems, get an ACCA Manual J load calculation from a licensed HVAC contractor. As an Amazon Associate, this site earns from qualifying purchases.