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BTU Calculator
Use this free BTU calculator to find the heating and cooling capacity a room needs before you buy an air conditioner or a heater. Enter the room length and width, the ceiling height, your climate zone, the insulation level, and how much sun the room gets, and the calculator shows the cooling BTUs and heating BTUs required, plus the air conditioner size in tons. Picking the right size keeps the room comfortable, controls humidity, and avoids wasted energy.
How it works
A BTU, or British Thermal Unit, is a unit of heat. One BTU is the heat needed to raise one pound of water by one degree Fahrenheit, and air conditioners and heaters are rated by how many BTUs per hour they can move. This calculator estimates how many BTUs per hour a room needs for cooling in summer and for heating in winter, using a simple rule based on floor area, then adjusting that rule for the things that make a real room easier or harder to heat and cool.
The method is a BTU-per-square-foot rule. For cooling, the rule is 20 BTU per square foot in a cold climate, 25 BTU per square foot in a moderate climate, and 30 BTU per square foot in a hot climate. For heating, the rule flips: 30 BTU per square foot in a hot climate, 40 in a moderate climate, and 50 in a cold climate. Multiply the room area by the rule for your climate and you get a base number. A 400-square-foot room in a moderate climate therefore starts at 10,000 BTU for cooling and 16,000 BTU for heating before any adjustments.
Three adjustments then refine the base number. Ceiling height matters because taller rooms hold more air: the calculator scales the result by the ceiling height divided by 8 feet, so a 10-foot ceiling adds 25 percent and a 7-foot ceiling cuts about 12 percent. Insulation matters because a drafty room leaks heat in winter and lets heat in during summer: poor insulation raises cooling by 20 percent and heating by 25 percent, while good insulation cuts cooling by 15 percent and heating by 20 percent. Sun exposure matters because a sun-baked room gains extra heat: full sun adds 15 percent to cooling, partial sun adds 5 percent, and shade adds nothing, while full sun trims the heating need by 10 percent because the sun helps warm the room.
The cooling result also converts into tons, which is how larger air conditioners are sold. One ton of cooling equals 12,000 BTU per hour, so a 10,500 BTU result is about 0.88 tons. Air conditioners are sold in half-ton steps, so the practical choice for 0.88 tons is a 1-ton unit. The tonnage is a guide for shopping, not a rule that you must buy exactly that size: a room between sizes is usually better served by the smaller unit, because a slightly smaller air conditioner runs longer and removes more humidity.
Room size is the biggest driver, but shape and use matter too. Open-plan spaces behave like one big room, so add the connected areas together instead of sizing for one corner. Kitchens run hot because ovens, stoves, and dishwashers add heat, so a kitchen needs roughly 4,000 BTU extra for cooling. Rooms with large west-facing windows, rooms above a garage, and top-floor rooms under a dark roof all run hotter than the rule suggests and deserve the full-sun setting even if the sun only hits them in the afternoon.
Keep in mind that this is a planning estimate, not a professional load calculation. A full Manual J calculation also weighs window area and type, wall and roof construction, air leakage, the number of people in the room, and local design temperatures. This calculator gets you into the right size range for a standard room so you can compare units and prices. For a whole house, a new system, or a room with unusual features, have a heating and cooling contractor run a proper load calculation before you buy.
Worked example
A bedroom measures 20 feet long by 20 feet wide with an 8-foot ceiling. The climate is moderate, the insulation is average, and the room gets partial sun.
Step 1: Find the floor area — 20 × 20 = 400 square feet.
Step 2: Apply the cooling rule for a moderate climate — 400 × 25 = 10,000 BTU.
Step 3: Adjust for an 8-foot ceiling — 10,000 × (8 ÷ 8) = 10,000 BTU.
Step 4: Adjust for average insulation — no change, 10,000 BTU.
Step 5: Adjust for partial sun — 10,000 × 1.05 = 10,500 BTU cooling.
Step 6: Apply the heating rule for a moderate climate — 400 × 40 = 16,000 BTU, then partial sun — 16,000 × 0.95 = 15,200 BTU heating.
The room needs 10,500 BTU of cooling and 15,200 BTU of heating. The cooling load is about 0.88 tons, so a 1-ton air conditioner (12,000 BTU) is the practical pick, and a heater rated at 15,000 BTU or higher covers the heating side.
FAQ
What does BTU mean?
BTU stands for British Thermal Unit. One BTU is the amount of heat needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTUs per hour, which tells you how much heating or cooling power they deliver.
How many BTUs do I need per square foot?
For cooling, a common planning rule is 20 BTU per square foot in a cold climate, 25 in a moderate climate, and 30 in a hot climate. For heating, the rule is 30 BTU per square foot in a hot climate, 40 in a moderate climate, and 50 in a cold climate. Ceiling height, insulation, and sun exposure then adjust those numbers up or down.
What happens if my air conditioner is too small?
An undersized air conditioner runs all the time and still cannot reach the set temperature on hot days. It also struggles to remove humidity, so the room can feel sticky even when the air coming out is cold. The unit wears out faster because it never gets a break.
What happens if my air conditioner is too big?
An oversized air conditioner cools the room so fast that it shuts off before it has time to pull humidity out of the air, leaving the room cool but clammy. It also cycles on and off constantly, which wastes energy and wears out the compressor. Slightly undersizing is usually better than oversizing.