HVAC unit converter

How to convert BTU to tons

Divide BTU/hr by 12,000 to get tons of cooling. Multiply tons by 12,000 to get BTU/hr. A 36,000 BTU/hr air conditioner is 3 tons. A 1.5-ton mini-split is 18,000 BTU/hr.

The 12,000 factor is definitional, not derived. It came from the ice industry in the 1880s, when commercial buildings paid for cooling by the block of ice delivered each morning. One short ton of ice (2,000 pounds) melting over a 24-hour day absorbs about 288,000 BTU of heat, which works out to 12,000 BTU per hour. When mechanical refrigeration replaced ice delivery in the 1890s and 1900s, the new compressors were rated in tons so building owners could compare them directly to the ice service they were replacing. The unit stuck.

Standard residential AC sizes run 1, 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons, which is 12,000 through 60,000 BTU/hr in 6,000 BTU increments. If a contractor quotes a number outside that ladder (like 28,000 BTU or 42,000 BTU), they are usually rounding from a specific model's rated capacity. A Carrier 24ABB336 condenser is rated 36,000 BTU/hr at standard test conditions, so it sells as a 3-ton unit even though the actual capacity drifts a few percent up or down depending on outdoor temperature.

For the matched sizing question, the AC tonnage calculator runs the load calc for your specific house instead of using the rough rule of one ton per 500 to 600 square feet that most contractors quote from.

How to convert BTU to kilowatts

Divide BTU/hr by 3,412.142 to get kilowatts. Multiply kilowatts by 3,412.142 to get BTU/hr. A 12,000 BTU/hr mini-split is 3.517 kW of cooling. A 24,000 BTU/hr unit is 7.034 kW.

The 3,412.142 factor comes from the joule, the SI unit of energy. One BTU (International Table) equals exactly 1,055.05585 joules. One kilowatt is 1,000 joules per second, or 3,600,000 joules per hour. Divide 3,600,000 by 1,055.05585 and you get 3,412.141633, which is where the 3,412.142 rounding comes from. The shorter 3,412 you see on most converter sites introduces a tiny error (about four thousandths of a percent) that does not matter on residential gear but compounds on commercial chiller sizing.

The most common reason a US homeowner needs this conversion is reading a European or Asian mini-split spec sheet. Daikin, Mitsubishi, LG, and Fujitsu publish their cooling capacity in kilowatts because the rest of the world uses kilowatts for HVAC capacity. A Daikin FTXR18T spec sheet lists "5.3 kW rated cooling capacity," which converts to 18,084 BTU/hr, which is sold in the US as an 18,000 BTU 1.5-ton mini-split. Same machine, same capacity, two notations.

How to convert watts to BTU per hour

Multiply watts by 3.412142 to get BTU/hr. Divide BTU/hr by 3.412142 to get watts. A 1,000-watt portable AC delivers about 3,412 BTU/hr of cooling at the rated input power. A 5,000 BTU/hr window unit draws roughly 1,465 watts.

Watts and BTU/hr both measure power, so the conversion is one for one with no efficiency penalty in the math. That sounds straightforward until you hit the catch: the watts label on a portable AC or window unit is almost always the electrical input watts, not the cooling output. A 5,000 BTU/hr window AC at 11 EER pulls 5,000 / 11 = 454 watts to deliver that cooling. The wattage rating you see on the nameplate is closer to that input number than to the BTU/hr capacity converted through 3.412142.

The cleanest way to keep them straight: the BTU/hr rating is the heat the unit can move, and the watt rating on the nameplate is the electricity it pulls from the wall. They are related by the unit's efficiency (EER or SEER2), not by the unit conversion factor. The EER vs SEER calculator walks through that efficiency math.

Portable power station and generator shoppers run this conversion in the other direction. A homeowner sizing a backup for a 12,000 BTU/hr window AC needs to know the running watts (about 1,200 to 1,400 W) and the starting watts (often 2,500 to 3,500 W during compressor startup), neither of which is a direct conversion from BTU/hr. The generator sizing calculator handles the LRA spike and soft-start kit math.

How to convert kilowatts to tons

Divide kilowatts by 3.5168525 to get tons of refrigeration. Multiply tons by 3.5168525 to get kilowatts. A 10 kW commercial chiller is 2.843 tons. A 3-ton residential AC is 10.55 kW of cooling output.

The 3.5168525 factor is the same conversion as above, expressed in tons rather than BTU. The math: one ton equals 12,000 BTU/hr, and one kilowatt equals 3,412.142 BTU/hr, so 12,000 divided by 3,412.142 gives 3.5168525 kW per ton. Most spec sheets and engineering references round to 3.517 or even 3.5 kW per ton. The rounded 3.5 is wrong by half a percent, which is fine on a 3-ton residential quote but materially off on a 200-ton chiller.

Commercial HVAC quotes in the US often mix the two units because chillers are commonly specified in tons while their electrical input is specified in kilowatts. A 200-ton water-cooled centrifugal chiller delivers 200 x 12,000 = 2.4 million BTU/hr of cooling, which is 703 kW of cooling capacity. Its electrical draw at full load is closer to 130 kW at a typical commercial efficiency. Two different kilowatt numbers on one chiller, which is the confusion the next section walks through.

Why is air conditioning measured in tons?

The short answer is the ice industry. Before mechanical refrigeration spread through commercial buildings in the early 1900s, breweries, restaurants, theaters, and meat lockers paid for cooling by the block of ice delivered each morning. The standard delivery unit was a short ton of ice (2,000 pounds), and a ton of ice melting over a 24-hour day absorbs about 288,000 BTU of heat as it phase-changes from solid to liquid at 32 F. Divide by 24 hours and you get 12,000 BTU per hour, which became the definition of a ton of refrigeration.

When Willis Carrier and his contemporaries started selling mechanical refrigeration units in the 1900s and 1910s, they rated their machines in tons so a brewery owner could decide between renewing the annual ice contract (priced in tons of ice) or buying a 50-ton compressor. The unit outlived the industry that created it. Modern residential homeowners shopping for a 3-ton heat pump are using a unit of measure that was set up to compete with a horse-drawn ice cart.

BTU vs BTU per hour: energy is not power

BTU is a unit of energy. BTU per hour is a unit of power. The converter on this page works in power units. Mixing them up is the single most common conversion mistake in HVAC marketing, including on most competing converter pages, but the distinction matters in three places:

• Air conditioning capacity: always BTU per hour, even when the marketing copy drops the "per hour." A 12,000 BTU mini-split is a 12,000 BTU per hour mini-split. Run it for two hours and it has delivered 24,000 BTU of total cooling energy.
• Furnace heating capacity: BTU per hour, same convention as AC. A 60,000 BTU furnace delivers 60,000 BTU per hour at full fire.
• Water heater input rating: tank water heaters list both an input BTU per hour and a recovery rate in gallons per hour. Tankless units list a maximum BTU per hour and a flow rate. The total energy a water heater can deliver in one cycle is BTU per hour multiplied by run time, which is why the same unit reads differently when compared to portable battery packs rated in watt-hours.

The converter on this page handles power. If you need to convert energy units (BTU to kilowatt-hours for a utility-bill comparison, for example), the math is BTU divided by 3,412.142 to get kWh, but make sure the BTU number you started with is actually energy and not power times one hour.

Reading a heat pump spec sheet: input kW vs output kW

A heat pump spec sheet often lists two different kilowatt numbers, and confusing them is what trips up homeowners shopping European or Asian mini-splits. The two numbers are the cooling or heating output (heat moved) and the electrical input (electricity drawn).

A Mitsubishi MUZ-FS18NA outdoor unit publishes a rated cooling capacity of 5.3 kW and a rated input power of 1.6 kW. The 5.3 kW is the cooling output, the heat the compressor pulls out of your house and dumps outside. The 1.6 kW is what the compressor and outdoor fan together pull from the wall. The ratio 5.3 / 1.6 is the COP, in this case about 3.3, which is what tells you the unit is a heat pump rather than a resistance heater (COP would be 1.0).

For a US homeowner converting that spec sheet, the 5.3 kW is what matches the US tonnage rating (5.3 / 3.5168 = 1.5 tons), and the 1.6 kW is what matches your electric bill. Both numbers are real, both are correct, and they live next to each other on the same page of the same spec sheet. The COP calculator converts between heating output and electrical input across the full range of outdoor temperatures.

Standard residential AC sizes at a glance

Residential split-system air conditioners in the US ship in eight standard sizes. Pick a row to see the same capacity in all four units:

• 1 ton: 12,000 BTU/hr = 3.52 kW = 3,517 W. Large bedroom or studio mini-split.
• 1.5 ton: 18,000 BTU/hr = 5.28 kW = 5,275 W. Open-plan room or small apartment.
• 2 ton: 24,000 BTU/hr = 7.03 kW = 7,034 W. Up to about 1,200 square feet of conditioned space.
• 2.5 ton: 30,000 BTU/hr = 8.79 kW = 8,792 W. Up to about 1,500 square feet.
• 3 ton: 36,000 BTU/hr = 10.55 kW = 10,551 W. The most common residential size, fits a typical 1,500 to 2,000 square foot home.
• 3.5 ton: 42,000 BTU/hr = 12.31 kW = 12,309 W. Roughly 2,000 to 2,500 square feet.
• 4 ton: 48,000 BTU/hr = 14.07 kW = 14,067 W. Roughly 2,500 to 3,000 square feet.
• 5 ton: 60,000 BTU/hr = 17.58 kW = 17,584 W. Largest residential size, 3,000 to 3,500 square feet.

The watts column is the cooling output in watts (the same physical quantity as the kW column, just multiplied by 1,000). It is not what your electric meter sees. For a 3-ton SEER2 15.2 unit, the actual wall-draw at design conditions is closer to 2,400 watts, not 10,551. The operating cost calculator runs the dollars-per-month math at your local utility rate.

Common questions about HVAC unit conversion

How many BTU is 1 ton?

12,000 BTU per hour. The definition has not changed since the 1900s. A 3-ton AC is 36,000 BTU per hour, a 5-ton is 60,000 BTU per hour.

How many BTU is 1 kW?

3,412.142 BTU per hour. Most quick-reference charts round to 3,412, which is close enough for any residential conversation but introduces a small error on commercial sizing.

How many kW is a 12,000 BTU mini-split?

3.517 kW of cooling output. That is the heat-moved capacity, not the electrical input. The wall-draw on a typical SEER2 18 mini-split at that capacity is closer to 950 watts.

How many watts does a 12,000 BTU AC use?

Around 1,000 to 1,400 watts of electrical input at the rated cooling capacity, depending on the unit's EER or SEER2. The 12,000 BTU/hr is the cooling output, not the watts pulled from the wall. Divide the BTU/hr by the EER (a 10 EER unit pulls 1,200 watts, an 11 EER unit pulls 1,091 watts) to get the actual electrical draw.

Is BTU the same as BTU per hour?

No, but HVAC marketing uses them interchangeably and gets away with it because the units the marketing describes (AC capacity, furnace output, heat pump capacity) are always rated per hour. A unit advertised as "12,000 BTU" is delivering 12,000 BTU per hour. Strictly, BTU is energy and BTU per hour is power. The converter on this page works in power.

Why does the European spec sheet use kW instead of tons?

Because the rest of the world uses SI units for HVAC capacity. The ton of refrigeration is a US-specific unit that survived because the domestic HVAC industry grew up around the ice trade. European, Japanese, and Korean manufacturers spec their equipment in kilowatts and convert to tons or BTU/hr only when targeting the US market.

What size AC do I need for 1,500 square feet?

Roughly 2.5 to 3 tons (30,000 to 36,000 BTU/hr) for a typical well-insulated home in a moderate climate, but square footage alone is a weak proxy. Climate zone, insulation, window orientation, ceiling height, and air sealing all change the answer. The Manual J calculator runs the load calc properly. Most contractors who quote by square footage alone oversize by 25 to 50 percent.

Once you have the unit conversion figured out, the natural next questions are sizing and operating cost. The AC tonnage calculator sizes the unit for your specific house, and the SEER2 savings calculator compares the dollar impact of jumping from an older SEER rating to the current 14.3 SEER2 minimum or a high-efficiency variable-speed tier.