Estimate the cooling needs of a typical room or house — for example, the power of a window air conditioner for an apartment room, or a central air conditioner for an entire house.
Estimate the BTUs required to heat or cool an area. The desired temperature change is the increase or decrease from the outdoor temperature needed to reach your target indoor temperature. For example, an unheated Boston home in winter might sit at −5°F; reaching 75°F requires an 80°F increase.
The BTU calculator on this page helps you estimate how much heating or cooling power a space needs, measured in British Thermal Units per hour (BTU/hr, usually shortened to "BTU"). Choosing the right BTU rating is one of the most important decisions when buying an air conditioner, heater, heat pump, mini-split, or furnace. An undersized unit will run constantly and never reach a comfortable temperature, while an oversized unit cycles on and off too quickly, wastes energy, and fails to remove humidity properly. This calculator gives you a reliable starting estimate so you can shop with confidence.
There are two calculators above. The first, the AC BTU Calculator, estimates the cooling load of a room or house using its size and a few real-world factors such as the number of occupants, the room's purpose, insulation quality, sun exposure, and local climate. The second, the General Purpose AC or Heating BTU Calculator, estimates the BTUs needed to heat or cool a space for any desired temperature change, based on its dimensions and insulation. Together they cover the vast majority of residential sizing questions.
A British Thermal Unit is a basic unit of energy. One BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In the context of heating and cooling, equipment is rated in BTUs per hour, which describes how much heat it can add to or remove from a space each hour. A small window air conditioner might be rated at 5,000 BTU, a typical central air system at 24,000 to 60,000 BTU, and a large furnace at 100,000 BTU or more.
BTUs are often converted into other units. Watts are the metric measure of power: 1 watt equals about 3.412 BTU/hr, so the calculators above show the equivalent wattage alongside the BTU figure. For larger cooling systems, capacity is frequently expressed in tons: one ton of cooling equals 12,000 BTU/hr, a number that dates back to the amount of heat needed to melt one ton of ice in a day. The general-purpose calculator shows the result in tons as well, which is handy when comparing central air conditioners and heat pumps.
The AC BTU calculator starts from the floor area of the space and the height of the ceiling, then adjusts the estimate for the conditions that most affect cooling load:
By layering these adjustments on top of the base size estimate, the AC BTU calculator produces a realistic figure for the cooling capacity a room actually needs, rather than a one-size-fits-all rule of thumb.
The general-purpose calculator is based on the physics of heat transfer. The amount of energy needed to maintain a temperature difference between the inside and outside of a space depends on three things: the total surface area of the space (through which heat escapes or enters), how big a temperature change you want, and how well the space is insulated. You enter the width, length, and ceiling height of the room or house, choose an insulation level, and specify the desired temperature increase or decrease. The calculator then estimates the heating or cooling BTUs required.
The desired temperature change is the gap between the starting outdoor temperature and your target indoor temperature. For heating, if it is 20°F outside and you want 70°F inside, the desired increase is 50°F. For cooling, if it is 95°F outside and you want 75°F inside, the desired decrease is 20°F. Because heating in cold climates often involves a much larger temperature swing than cooling, heating BTU requirements are frequently higher than cooling requirements for the same building.
A widely used starting point for cooling is roughly 20 BTU per square foot of living space, assuming standard eight-foot ceilings. By that guide, a 300-square-foot room needs about 6,000 BTU of cooling. However, rules of thumb ignore the real factors that shift the requirement substantially, which is why the calculators above adjust for ceiling height, occupancy, sun, insulation, and climate. The following rough cooling guidelines illustrate the relationship between area and capacity:
These figures are only a guide. A sunny top-floor kitchen of 300 square feet could easily need far more than the 8,000 BTU the table suggests, while a shaded, well-insulated basement bedroom of the same size might need less. Use the calculators to refine the estimate for your specific situation.
Both undersizing and oversizing carry real penalties. An air conditioner that is too small simply cannot keep up on the hottest days; it runs without stopping, drives up energy bills, wears out faster, and still leaves the space uncomfortable. An oversized unit creates a different problem: it cools the air so quickly that it shuts off before it has run long enough to pull humidity out of the air. The result is a space that feels cold and clammy, with frequent on-off cycling that stresses the compressor and shortens the equipment's life. Right-sizing avoids both extremes, delivering steady comfort, efficient operation, and proper humidity control.
Insulation is one of the biggest levers on heating and cooling load. A tightly sealed, well-insulated home can require dramatically fewer BTUs than a drafty one of the same size. Before upgrading equipment, it is often more cost-effective to seal leaks around windows, doors, and ducts and to add insulation in the attic and walls. Doing so lets you install a smaller, cheaper unit and lowers operating costs for the life of the system.
Windows are weak points for heat transfer. Large windows, especially single-pane or south- and west-facing ones, let in significant solar heat in summer and lose heat in winter. Rooms with heavy sun exposure need more cooling capacity, which is why the AC BTU calculator includes a sun-exposure adjustment. Shading, reflective films, and modern double- or triple-pane windows can meaningfully reduce the load.
Standard sizing rules assume eight-foot ceilings. Rooms with vaulted or higher ceilings contain more air volume and more wall surface, increasing the load. Both calculators account for ceiling height, so a great room with a twelve-foot ceiling will correctly show a higher requirement than a standard room of the same footprint.
People and equipment add heat. Each occupant contributes body heat, and appliances such as ovens, stovetops, computers, and televisions release additional warmth. Kitchens are the most demanding rooms because of cooking heat, which is why they receive the largest room-type adjustment. Spaces with many occupants, such as conference rooms or home theaters, also need extra capacity.
Local climate sets the baseline. A home in a hot, humid region faces a larger cooling load than an identical home in a mild coastal climate, while a home in a cold region faces a much larger heating load. The AC BTU calculator's climate setting shifts the estimate to reflect these regional realities.
Although both heating and cooling are measured in BTUs, the requirements are not interchangeable. Heating load depends heavily on how cold it gets and how much heat the building loses, while cooling load depends on outdoor heat, humidity, sun exposure, and internal heat gains from people and appliances. In cold climates, the heating BTU requirement usually exceeds the cooling requirement because winter temperature differences are larger. When sizing a system that does both, such as a heat pump, it is important to consider both loads and choose equipment that handles the more demanding of the two, often with supplemental heat for the coldest days.
Once you know how many BTUs you need, the next consideration is how efficiently a unit delivers them. Cooling equipment is rated by its SEER (Seasonal Energy Efficiency Ratio) and EER (Energy Efficiency Ratio), which describe how many BTUs of cooling you get per watt-hour of electricity consumed. A higher SEER means lower running costs for the same cooling output, so two air conditioners with identical BTU ratings can have very different electricity bills. Heating equipment, particularly heat pumps, is rated by HSPF (Heating Seasonal Performance Factor), with higher numbers again indicating greater efficiency. When you compare two units of the correct BTU size, choosing the one with a higher SEER, EER, or HSPF can save a substantial amount of money over the equipment's lifetime, often offsetting a higher purchase price within a few years.
It is worth emphasizing that efficiency ratings only matter once the size is right. A highly efficient unit that is badly oversized or undersized will still perform poorly, cycling improperly or running constantly. Always determine the correct BTU capacity first using the calculators above, then compare efficiency ratings among the appropriately sized options. Pairing correct sizing with a high efficiency rating, good insulation, and proper installation is the surest path to comfortable, low-cost heating and cooling.
As a rough cooling guideline, about 20 BTU per square foot works for a room with standard eight-foot ceilings under average conditions. However, the real requirement changes with ceiling height, sun exposure, insulation, occupancy, room type, and climate. The AC BTU calculator above adjusts for all of these, giving a more accurate figure than a flat per-square-foot rule.
They all measure the same thing — the rate of heat transfer — in different units. One watt equals about 3.412 BTU per hour, so you can convert BTU to watts by dividing by 3.412. One ton of cooling equals 12,000 BTU per hour, a unit commonly used for central air conditioners and heat pumps. The calculators show all three so you can compare equipment rated in any of them.
Neither. An undersized unit runs constantly and never quite cools the space, while an oversized unit cools too fast, shuts off before removing humidity, and cycles on and off in a way that wastes energy and wears out the compressor. The goal is to match the unit's capacity to the calculated load as closely as possible.
Most sizing rules assume eight-foot ceilings. A taller ceiling means more air volume and more wall area, so the space takes more energy to heat or cool. Both calculators factor in ceiling height, so a room with a high or vaulted ceiling will correctly show a larger BTU requirement than a standard room of the same floor area.
Yes. Kitchens generate substantial heat from ovens, stovetops, and other appliances, so they need extra cooling capacity. The AC BTU calculator adds a significant boost when you select "Kitchen" as the room type to account for this additional heat load.
The calculators provide solid estimates suitable for selecting residential equipment, and they reflect the same factors used by common sizing methods. For a precise figure on a large or complex project, a professional Manual J load calculation, which accounts for every window, wall, and duct in detail, is recommended. Use these tools to understand the ballpark and to compare options before consulting an installer.
This BTU Calculator is provided for educational and general informational purposes only and produces rough estimates. Actual heating and cooling requirements depend on many additional factors, including window quality, building orientation, ductwork, local building codes, and detailed climate data. For equipment selection on significant projects, consult a qualified HVAC professional and request a detailed load calculation.