❄️ BTU Calculator: What Size AC or Heater Do I Need?
By Shihab Mia · Updated 2026-06-27
Add 600 BTU for each person beyond the first two.
This is a rough sizing estimate. Real needs also depend on ceiling height, insulation, climate, window type and how airtight the room is. For a permanent or central system, have a contractor run a full load calculation.
This BTU calculator estimates the cooling capacity an air conditioner needs for a room. Enter the room size (or its length and width) and tick a few adjustments for strong sun, extra people, or a kitchen, and it returns a recommended BTU per hour figure plus the matching air conditioner size in tons. It is a quick sizing guide, not a substitute for a full load calculation, but it gets you close enough to shop with confidence.
What is the BTU Calculator?
BTU stands for British Thermal Unit, the amount of heat needed to raise one pound of water by one degree Fahrenheit. For air conditioners and heaters the number that matters is BTU per hour (BTU/h), which describes how much heat the unit can move into or out of a room each hour. A bigger room, or one that gains or loses more heat, needs a higher BTU rating to stay comfortable. Getting that number right is the whole point of sizing: too small and the unit runs flat out and never reaches the target temperature, too big and it short-cycles, wastes energy and leaves the room cold and clammy.
For cooling, the most common rule of thumb is roughly 20 BTU per square foot of floor area. So a 180 square foot room starts at about 3,600 BTU/h. That base figure assumes an 8 foot ceiling, reasonable insulation and a moderate climate, then you nudge it up or down for the room's real conditions. A room that bakes in afternoon sun gains extra heat, so add about 10%. A heavily shaded room can come down about 10%. Each person in the room adds body heat, so add roughly 600 BTU for every occupant beyond the first two. A kitchen runs hot from appliances and cooking, so add about 4,000 BTU. Ceilings taller than 8 feet, large or single-pane windows and extra exterior walls all push the requirement higher.
Heating is sized the same way but with bigger numbers, because keeping a room warm in winter fights a larger temperature gap than cooling it in summer. A useful range is about 30 to 60 BTU per square foot of heated space, driven almost entirely by climate zone and insulation. A mild southern climate might need 30 to 35 BTU per square foot, a moderate climate 40 to 45, and a cold northern climate 50 to 60. A well-insulated, tightly sealed home sits at the low end of its band, while an older home with single-pane windows and little wall insulation sits at the top. Furnace output is quoted in BTU/h too, so a 1,500 square foot home in a moderate climate typically wants a furnace rated around 60,000 to 70,000 BTU/h of output.
Air conditioner capacity is often quoted in tons rather than BTU. One ton of cooling equals 12,000 BTU per hour, a figure that comes from the heat needed to melt one ton of ice in 24 hours. So a 12,000 BTU unit is a 1 ton air conditioner, and 24,000 BTU is 2 tons. Window and portable room units are usually sold by their BTU rating directly (5,000, 8,000, 12,000 and so on), while central systems and mini-splits are sold in tons, so it helps to be fluent in both. Heaters and furnaces are almost always rated in BTU/h, and note that furnaces list both an input rating and a lower output rating after combustion efficiency, so always size by the output figure.
The simple square-foot method this tool uses is ideal for a single room, a window or portable AC, or a quick mini-split estimate. It is not a Manual J load calculation, the detailed room-by-room method that HVAC professionals run for whole-home central systems. Manual J accounts for every window's size and orientation, duct losses, infiltration, wall and roof construction and the local design temperature. For a central air conditioner or a whole-home furnace, treat this calculator's answer as a sanity check on a contractor's proposal rather than the final spec, and ask any contractor who skips the load calculation why.
A quick word on what BTU does not tell you. Two units with the same BTU rating can differ a lot in running cost, because efficiency is measured separately: SEER2 and EER2 for cooling and AFUE for gas furnaces. A 12,000 BTU mini-split at 22 SEER2 removes the same heat as a 12,000 BTU window unit at 11 EER but uses far less electricity to do it. Size on BTU first to get comfort right, then compare efficiency ratings within that size class to control the energy bill.
When to use it
- Sizing a window or portable air conditioner for a single bedroom, office or living room before you buy.
- Checking whether the unit a salesperson recommended is roughly the right capacity for your space.
- Comparing rooms in a home to see which needs the most cooling power.
- Estimating the ton rating of a mini-split for a sunroom, garage conversion or home gym.
- Getting a ballpark heating BTU figure for a space heater, gas log set or supplemental heater.
- Sanity-checking an HVAC quote so you can tell whether a proposed unit is undersized or oversized.
How to use the BTU Calculator
- Choose whether to enter the room dimensions or a known floor area, and pick feet or metres.
- Enter the length and width (or the total floor area).
- Tick any adjustments that apply: very sunny, heavily shaded, or a kitchen.
- Set the number of regular occupants.
- Read off the recommended BTU per hour and the equivalent air conditioner size in tons.
- Round up to the next standard unit size when you shop, since AC and heaters come in fixed steps.
Formula & method
Worked examples
A 15 ft by 12 ft bedroom with 2 occupants and no special conditions.
- Floor area = 15 x 12 = 180 sq ft
- Base BTU = 180 x 20 = 3,600 BTU/h
- No sun, shade or kitchen adjustment, and only 2 people, so nothing is added
- Tons = 3,600 / 12,000 = 0.3 ton
Result: About 3,600 BTU/h, roughly a 0.3 ton unit (a small 5,000 BTU window AC covers this with margin)
A 12 ft by 12 ft sunny home office used by 3 people.
- Floor area = 12 x 12 = 144 sq ft
- Base BTU = 144 x 20 = 2,880 BTU/h
- Very sunny, so multiply by 1.1: 2,880 x 1.1 = 3,168 BTU/h
- One person beyond two adds 600 BTU: 3,168 + 600 = 3,768 BTU/h
- Round up to the nearest 100: 3,800 BTU/h
- Tons = 3,800 / 12,000 = about 0.32 ton
Result: About 3,800 BTU/h, roughly a 0.32 ton unit
A 200 sq ft kitchen with 2 cooks.
- Floor area = 200 sq ft (entered directly)
- Base BTU = 200 x 20 = 4,000 BTU/h
- Kitchen, so add 4,000 BTU: 4,000 + 4,000 = 8,000 BTU/h
- Only 2 people, so no occupant addition
- Tons = 8,000 / 12,000 = about 0.67 ton
Result: About 8,000 BTU/h, roughly a 0.67 ton unit
Heating a 300 sq ft room in a cold northern climate with average insulation.
- Pick a heating factor for a cold climate: about 50 BTU per sq ft
- Base heating BTU = 300 x 50 = 15,000 BTU/h
- Insulation is only average, so stay at the chosen factor rather than dropping to the low end
- A space heater or gas log set in the 15,000 to 18,000 BTU/h range suits this room
Result: About 15,000 BTU/h of heating, satisfied by a 15,000 to 18,000 BTU heater
Rough cooling BTU by room size (at 20 BTU per sq ft, before adjustments)
| Floor area | Base BTU/h | Approx. tons | Typical AC type |
|---|---|---|---|
| 100 to 150 sq ft | 2,000 to 3,000 | 0.2 to 0.25 | Small window unit (5,000 BTU class) |
| 150 to 250 sq ft | 3,000 to 5,000 | 0.25 to 0.42 | Window unit (6,000 to 8,000 BTU) |
| 250 to 350 sq ft | 5,000 to 7,000 | 0.42 to 0.58 | Window or portable unit |
| 350 to 550 sq ft | 7,000 to 11,000 | 0.58 to 0.92 | Large window or mini-split |
| 550 to 700 sq ft | 11,000 to 14,000 | 0.92 to 1.17 | 1 ton mini-split |
| 700 to 1,000 sq ft | 14,000 to 21,000 | 1.17 to 1.75 | 1.5 ton mini-split or small central |
Heating BTU per square foot by climate zone (whole-home estimate)
| Climate zone | BTU per sq ft | 1,500 sq ft home | Typical example |
|---|---|---|---|
| Hot / mild (south) | 30 to 35 | 45,000 to 53,000 BTU/h | Gulf Coast, Florida, Arizona |
| Warm | 35 to 40 | 53,000 to 60,000 BTU/h | Carolinas, Tennessee |
| Moderate | 40 to 45 | 60,000 to 68,000 BTU/h | Mid-Atlantic, lower Midwest |
| Cold | 45 to 55 | 68,000 to 83,000 BTU/h | Upper Midwest, New England |
| Very cold | 55 to 60 | 83,000 to 90,000 BTU/h | Northern Plains, Maine, Minnesota |
BTU to ton conversion (12,000 BTU/h = 1 ton)
| BTU per hour | Tons of cooling |
|---|---|
| 6,000 BTU/h | 0.5 ton |
| 12,000 BTU/h | 1 ton |
| 18,000 BTU/h | 1.5 tons |
| 24,000 BTU/h | 2 tons |
| 36,000 BTU/h | 3 tons |
| 60,000 BTU/h | 5 tons |
Common mistakes to avoid
- Buying the biggest unit you can afford. An oversized air conditioner cools the air fast then shuts off before it removes much humidity, so the room feels cold and damp and the compressor short-cycles. Matching capacity to the room is better than overshooting.
- Forgetting the heat sources in the room. Sun, people and appliances all add heat. Skipping the kitchen or occupant adjustments can leave the estimate well below what the room actually needs on a busy day.
- Ignoring ceiling height and insulation. The 20 BTU per square foot rule assumes a standard 8 foot ceiling and average insulation. Add roughly 1,000 BTU for each foot of ceiling above 8 feet, and raise the figure for single-pane windows or poor insulation. Treat the base number as a starting point.
- Confusing BTU with tons or watts. BTU per hour is a rate of heat movement. Tons (12,000 BTU/h each) and watts are just other units for the same thing. Compare units on the same scale so you do not size by a factor of ten.
- Using a furnace input rating as if it were output. Gas furnaces list a higher input BTU and a lower output BTU after efficiency losses. An 80,000 BTU input furnace at 80% AFUE only delivers about 64,000 BTU of heat, so always size and compare by the output figure.
- Ignoring climate when sizing heat. A cooling estimate is fairly climate-neutral, but heating is not. The same room needs far more heating BTU in Minnesota than in Florida, so always apply a climate-zone factor before picking a heater or furnace.
Glossary
- BTU
- British Thermal Unit, the heat needed to raise one pound of water by one degree Fahrenheit.
- BTU per hour (BTU/h)
- The rate at which a unit adds or removes heat from a room. This is the number used to size cooling and heating.
- Ton of cooling
- A unit of air conditioner capacity equal to 12,000 BTU per hour, based on the heat to melt one ton of ice in a day.
- Cooling load
- The total amount of heat that must be removed to keep a room at the target temperature.
- Heat loss
- The heat a room sheds to the outdoors in winter, which the heating system must replace. It rises with cold climate and poor insulation.
- Short-cycling
- When an oversized unit turns on and off rapidly, cooling the air but not running long enough to remove humidity.
- Manual J
- The industry-standard room-by-room load calculation HVAC professionals use to size whole-home systems precisely.
- SEER2 / AFUE
- Efficiency ratings: SEER2 (and EER2) measure cooling efficiency, AFUE measures the share of fuel a furnace turns into usable heat.
Frequently asked questions
How many BTU do I need to cool a room?
As a starting point, use about 20 BTU per square foot of floor area, so a 180 square foot room needs roughly 3,600 BTU/h. Then add for strong sun, extra people and kitchen heat, and bump it up for tall ceilings or poor insulation. This calculator does that arithmetic for you.
What size air conditioner do I need for a 12x12 room?
A 12 by 12 room is 144 square feet, so the base requirement is about 144 x 20 = 2,880 BTU/h. A 5,000 BTU window unit comfortably covers it, with margin for sun or extra people. Add roughly 10% for a very sunny room and 600 BTU per occupant beyond two.
How many BTU is a 1 ton air conditioner?
One ton of cooling equals 12,000 BTU per hour. So a 1 ton unit is 12,000 BTU, a 1.5 ton unit is 18,000 BTU and a 2 ton unit is 24,000 BTU. The figure comes from the heat needed to melt a ton of ice in 24 hours.
How many BTU do I need to heat a room?
Heating needs more BTU than cooling. Plan on roughly 30 to 60 BTU per square foot depending on climate: about 30 to 35 in a mild southern climate, 40 to 45 in a moderate one, and 50 to 60 in a cold northern climate. A 300 square foot room in a cold climate needs around 15,000 BTU/h.
Is it bad to buy an air conditioner that is too big?
Yes. An oversized unit cools the air quickly then shuts off before it can pull humidity out, leaving the room cold but clammy, and the frequent on-off cycling wears the compressor and wastes energy. Matching capacity to the room is best.
Does a sunny room need more BTU?
It does. A room that gets strong, direct sun gains extra heat through the windows, so add roughly 10% to the base figure. A heavily shaded room can come down about 10%. The calculator applies these when you tick the boxes.
Why does a kitchen need extra BTU?
Ovens, cooktops, refrigerators and other appliances throw off a lot of heat, so a kitchen carries a higher cooling load than a similar-sized bedroom. A common allowance is an extra 4,000 BTU for a kitchen.
How does ceiling height affect BTU?
The 20 BTU per square foot rule assumes an 8 foot ceiling. Taller ceilings mean more air volume to condition, so a common adjustment is to add about 1,000 BTU per hour for each foot of ceiling height above 8 feet, or to scale the whole estimate up proportionally.
What is a Manual J calculation and do I need one?
Manual J is the detailed, room-by-room load calculation HVAC professionals use to size whole-home systems. It accounts for window orientation, insulation, ductwork, infiltration and local design temperatures. For a window or portable unit this calculator is enough, but for central air or a furnace you want a Manual J.
Is this calculator accurate enough to size a central system?
It is a quick estimate, good for window and portable units or a rough mini-split size. For central air or a whole-home furnace, have an HVAC contractor run a full Manual J load calculation that accounts for insulation, ductwork, windows and local climate, then use this tool to sanity-check their proposal.