BTU Calculator

Two calculators sit on this page. The first estimates the cooling a room or a whole house needs, which is the number to take shopping for a window unit or a central system. The second works out the BTUs needed to heat or cool a space through any temperature change you name, so it covers heating as well. A 18 m² bedroom with a 3 m ceiling and two people in it comes out at 6,402 BTU per hour, or 1,876 watts.

What is a BTU?

The British Thermal Unit is a unit of energy. One BTU is roughly the energy needed to raise one pound of water by one degree Fahrenheit. Put next to units you may know better, 1 BTU is about 1,055 joules, 252 calories or 0.293 watt-hours, which is roughly the energy given off by burning a single match. Going the other way, one watt is about 3.412 BTU per hour.

BTUs are widely used to compare fuels. Natural gas is sold by volume and heating oil by the barrel, but both can be expressed in BTUs according to the heat they carry, which is what makes them comparable at all. A cubic foot of natural gas holds about 1,037 BTU, so the therm your gas bill is priced in, 100,000 BTU, is about 96 cubic feet. A kilowatt-hour of electricity is 3,412 BTU.

The same unit describes what an appliance can do. The higher the BTU rating, the more heat a heater can put out. For air conditioners the rating works the other way round: it is the heat the unit can pull out of the air and dump outside. Worth knowing is that an appliance labelled "12,000 BTU" almost always means 12,000 BTU per hour, a rate rather than a quantity, and the "per hour" is simply left off the box.

Cooling equipment is also sold in tons, where one ton is 12,000 BTU per hour. The name is literal. It came from the ice trade, and it is roughly the rate of cooling you get from melting a short ton of ice over a day: 2,000 pounds at about 143 BTU per pound is 286,800 BTU, which works out near 11,950 BTU per hour and was rounded to 12,000.

Size and ceiling height

A smaller room needs fewer BTUs, which is no surprise, but the figure that matters is volume rather than floor area. Air conditioning cools air, and a room with a 3 m ceiling holds a quarter more air than the same floor plan at 2.4 m. That is why both calculators here ask for ceiling height, and why a lofted or vaulted room needs noticeably more capacity than its square footage suggests.

For a rough check based on floor area alone, the table below is the guide published by EnergyStar. It assumes an ordinary 8-foot ceiling.

Read the table as a sanity check rather than a specification. It steps in jumps because air conditioners are sold in fixed sizes, so it rounds up to the next model on the shelf. The calculator above works from actual volume and the conditions you enter, which is why its answers land between the table's steps.

Insulation condition

Thermal insulation is the reduction of heat transfer between objects in contact or within range of each other's radiation. It matters here because heat always moves from warmer to cooler until the difference is gone, and insulation is what slows that down. Poor insulation means the cooled air you paid for leaks away and has to be replaced, hour after hour.

Newer homes generally insulate better than older ones, through both improved materials and stricter building codes. Owners of older homes who upgrade often see the benefit twice: lower bills, and a higher valuation when they sell.

A material's resistance to heat flow is its R-value. The higher the R-value, the harder heat finds it to pass through, so higher-R products insulate better, though they usually cost more. Windows are the weak point in almost every wall, since even good double glazing resists heat far less than the insulated wall around it. A room with a lot of glass should be treated as poorly insulated regardless of how new it is.

When choosing the insulation setting, generalise. A beach bungalow built in the 1800s with no renovations is poor. A three-year-old house in a new development probably deserves a good rating. Anything in between, or anything you are unsure about, is average. In this calculator good and poor shift the answer by 15% either way, which is enough to change which unit you buy.

Desired temperature increase or decrease

For the second calculator, the temperature change is the gap between the untreated outdoor temperature and the indoor temperature you want. Most people are comfortable somewhere between 70 and 80 °F.

An Atlanta homeowner working out winter heating might note that Atlanta winters hover near 45 °F and occasionally fall to 30 °F. Wanting 75 °F indoors, the required increase is 75 − 30 = 45 °F. A Boston home is a harder case: unheated, it can sit at −5 °F, so reaching the same 75 °F takes a change of 80 °F, nearly twice the load for an identical house.

Homes in extreme climates swing more and use more. Heating in Alaska in winter or cooling in Houston in summer both demand far more than keeping a house comfortable in Honolulu, where the temperature stays near 80 °F all year.

Other factors worth knowing

The number of people. A body at rest gives off heat continuously, so more occupants means more cooling and slightly less heating. The standard allowance, and the one used above, is 600 BTU per hour for each person beyond the first two.

Where the condenser sits. Put the outdoor unit on the shadiest side of the house, usually north or east. The more sun falls on it, the hotter the air it is trying to dump heat into, and the harder it works. Shade improves efficiency and extends the life of the equipment. Trees can provide that shade, but leave room for airflow, since a condenser choked by vegetation is worse off than one in full sun.

Getting the size right, in both directions. An oversized unit cools the room quickly, then shuts off before it has run long enough to pull the humidity out, leaving the air cold and clammy and the compressor cycling on and off in a way that wears it out early. An undersized unit runs almost continuously and never quite catches up. Bigger is not safer.

Ceiling fans. Fans do not cool air, they move it, and moving it evens out the dead spots that every home has: the corner behind the sofa, the bathroom with a window and no vent, the laundry room. A thermostat sitting in a dead spot reads a temperature the rest of the house never sees, so circulating the air makes the whole system behave better.

Roof colour. A dark surface absorbs more radiant energy than a light one. Even the difference between a clean white roof and the same roof after a few years of grime is measurable in the cooling load underneath it.

Efficiency falling with age. Like most appliances, heaters and air conditioners lose efficiency as they age. An air conditioner running short of refrigerant can lose half its efficiency or more, which usually shows up as a unit that runs constantly without cooling much.

The shape of the building. A long narrow house has more wall for the same floor area than a square one, and walls are where heat leaves. Two homes with identical square footage can have quite different loads.

What the BTU rating costs to run

Capacity and running cost are separate questions, and efficiency ratings connect them. In the US, cooling efficiency is quoted as SEER, the seasonal ratio of cooling delivered to electricity consumed. A 12,000 BTU unit at SEER 15 draws about 800 watts while running; the same capacity at SEER 20 draws about 600. Over a season of eight hours a day for 90 days, that is 576 kWh against 432, or about 98 dollars against 73 at 17 cents a kilowatt-hour.

The takeaway is that BTUs decide whether the room gets cool and the efficiency rating decides what it costs. Sizing up beyond what the room needs raises both the purchase price and the bill without making the room more comfortable, and usually makes it less so.

Reading the results on this page

Both panels report BTU per hour first, then watts, then tons, since equipment is labelled in all three depending on where you shop. The step-by-step card under each shows how the figure was built: the volume, the base load, each adjustment applied to it, and the flat allowances for a kitchen or extra occupants. Treat the answer as a starting point for choosing a unit rather than a specification. Whole-house systems should still be sized with a proper room-by-room load calculation, since that accounts for window orientation, duct losses and local design temperatures that no single-box estimate can see.