❄️ BTU Calculator (Room Cooling/Heating)
By ToolNimba Home & Build Team · Updated 2026-06-19
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 the relevant number is BTU per hour, which describes how much heat the unit can remove from a room each hour. A bigger room, or one that gains more heat, needs a higher BTU rating to stay comfortable.
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 average ceiling height, 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.
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. Getting the size right matters: an undersized unit runs constantly and never quite cools the room, while an oversized one cools so fast it cycles off before it can pull moisture out of the air, leaving the room cold but clammy.
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.
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.
Formula & method
Worked examples
A 15 ft by 12 ft bedroom with 2 occupants and no special conditions.
- Floor area = 15 × 12 = 180 sq ft
- Base BTU = 180 × 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 × 12 = 144 sq ft
- Base BTU = 144 × 20 = 2,880 BTU/h
- Very sunny, so multiply by 1.1: 2,880 × 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 ≈ 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 × 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 ≈ 0.67 ton
Result: About 8,000 BTU/h, roughly a 0.67 ton unit
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 |
| 150 to 250 sq ft | 3,000 to 5,000 | 0.25 to 0.42 | Window unit |
| 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 |
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 |
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 ceiling and average insulation. Tall ceilings, single-pane windows or poor insulation all raise the real requirement, so treat the figure as a starting point.
- Confusing BTU with tons or watts. BTU per hour is a rate of heat removal. 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.
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 an air conditioner removes heat from a room. This is the number used to size cooling.
- 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.
- Short-cycling
- When an oversized unit turns on and off rapidly, cooling the air but not running long enough to remove humidity.
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. This calculator does that arithmetic for you.
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.
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.
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 system, have an HVAC contractor run a full load calculation that accounts for insulation, ductwork, windows and local climate.