Free HVAC sizing tool
BTU Calculator — What Size Air Conditioner Do I Need?
Get the exact cooling and heating capacity your room needs in BTUs, tons and a real recommended unit size. Adjusted for ceiling height, insulation, sunlight, climate zone and occupancy — not just square footage.
- Cooling + heating BTU
- Tons & matching unit size
- Feet or metres
- No sign-up
Air conditioner BTU size calculator
Room & conditions
Floor area: 224 sq ft
Each person past two adds 600 BTU.
Your result
Cooling capacity needed
5,376
BTU per hour
- Recommended AC size
- 6,000 BTU
- In tons
- 0.5 ton
- Heating capacity
- 10,080 BTU
- Floor area
- 224 sq ft
How the number was built
Sizing looks balanced. Match the unit to this figure rather than rounding up several sizes.
Why this BTU calculator is more accurate
Most AC size calculators multiply square footage by twenty and stop. Real heat load depends on far more than floor area.
Volume, not just floor area
A room with a 12 ft ceiling holds 50% more air than the same footprint at 8 ft. This tool scales the load by actual cubic volume, which is where flat square-foot charts fail.
Climate zone weighting
The same bedroom needs very different capacity in Phoenix and in Minneapolis. Five climate bands adjust both the cooling multiplier and the heating BTU per square foot.
Insulation, sun and roof gain
Draughty walls, west-facing glass and a room sitting straight under the roof can swing the requirement by 30% or more. Each is a separate, visible adjustment.
Occupancy and kitchen load
People and cooking appliances are real heat sources. Extra occupants add 600 BTU each and a kitchen adds 4,000 BTU, matching standard HVAC practice.
A real unit size, not a raw number
Raw BTU output is useless at the shop. You also get the nearest standard capacity actually sold — 5,000, 8,000, 12,000, 18,000, 24,000 and up — plus the tonnage.
Oversizing warnings
An oversized air conditioner short-cycles, never dehumidifies properly and wears out early. If your inputs push you near a size boundary, the tool says so instead of quietly rounding up.
How to use the BTU calculator
Four inputs get you a usable number. The rest fine-tune it.
Measure the room
Enter length and width in feet or metres. Measure wall to wall at floor level. For an L-shaped room, split it into two rectangles and add the results together.
Set the ceiling height
Standard is 8 ft (2.4 m). Vaulted, loft and older homes often run 10–12 ft, which meaningfully raises the load. Use the average height if the ceiling slopes.
Pick your climate zone and insulation
Choose the band that matches your summers. If you are unsure about insulation, pick average — homes built before the 1990s without a retrofit usually count as poor.
Add sunlight, people and room type
Flag heavy sun exposure, a kitchen, or a top-floor room under the roof. Set how many people are typically in the space at once.
Read the recommended unit size
Use the recommended AC size, not the raw BTU figure, when you shop. Compare it with the tonnage if you are quoting central air or a mini split.
BTU chart by room size
Quick reference for a standard 8 ft ceiling, average insulation and moderate climate. Use the calculator above for your actual conditions.
| Room size | Cooling BTU | Unit size | Tons | Typical room |
|---|---|---|---|---|
| 100–150 sq ft | 3,000 | 5,000 BTU | 0.4 | Small bedroom, home office |
| 150–250 sq ft | 5,000 | 6,000 BTU | 0.5 | Bedroom, den |
| 250–350 sq ft | 7,000 | 8,000 BTU | 0.7 | Large bedroom, studio |
| 350–450 sq ft | 9,000 | 10,000 BTU | 0.8 | Living room |
| 450–550 sq ft | 11,000 | 12,000 BTU | 1.0 | Open lounge, small flat |
| 550–700 sq ft | 14,000 | 14,000 BTU | 1.2 | Large open plan area |
| 700–1,000 sq ft | 20,000 | 24,000 BTU | 2.0 | One-bedroom apartment |
| 1,000–1,500 sq ft | 30,000 | 30,000 BTU | 2.5 | Small house zone |
| 1,500–2,000 sq ft | 40,000 | 42,000 BTU | 3.5 | Whole-home central air |
Why correct BTU sizing matters
Getting the BTU rating right is the single biggest lever on how well an air conditioner performs, and it is the one most people skip. An undersized unit runs continuously on hot afternoons, never reaches the set temperature and pushes your electricity bill up while delivering worse comfort.
Oversizing causes the opposite and less obvious problem. A unit with too much capacity cools the air quickly, hits the thermostat setpoint and shuts off before it has run long enough to pull moisture out. The room ends up cold and clammy, the compressor short-cycles through dozens of starts an hour, and that start-stop pattern is what shortens equipment life more than anything else.
Correct sizing also protects the numbers on the label. SEER2 and EER efficiency ratings are measured under steady running conditions, so a short-cycling oversized system never reaches the efficiency you paid a premium for. Sizing the unit to the real heat load is what turns a rating on a sticker into a lower bill.
For window and portable units, matching capacity to the room is usually all you need. For central air, ducted systems or a multi-zone mini split, treat the result here as a well-informed starting figure and confirm it with a full Manual J load calculation before you buy, since duct losses and whole-house airflow enter the picture.
BTU calculator FAQs
How many BTU do I need per square foot?
The industry baseline is 20 BTU per square foot of floor area for cooling, assuming an 8 ft ceiling and average conditions. A 300 sq ft room starts at 6,000 BTU. That figure then moves up or down for ceiling height, insulation, sun exposure, climate and occupancy, which is exactly what the calculator above applies.
What size air conditioner do I need for a 12x12 room?
A 12x12 room is 144 sq ft, which works out to roughly 2,900 BTU at the baseline rate. In practice the smallest window units sold are 5,000 BTU, so that is the size to buy. If the room is a sunny top-floor bedroom with poor insulation, the requirement rises to around 4,000 BTU and 5,000 BTU is still the right unit.
How many BTU is a 1 ton air conditioner?
One ton of cooling equals 12,000 BTU per hour. So a 1.5 ton unit is 18,000 BTU, a 2 ton unit is 24,000 BTU and a 3 ton unit is 36,000 BTU. The term dates from the amount of heat needed to melt one ton of ice in 24 hours, and tonnage is the standard way central air and mini splits are sold.
Is it better to oversize or undersize an air conditioner?
Neither, and oversizing is usually the more damaging mistake. An oversized unit short-cycles, leaves the room humid because it never runs long enough to dehumidify, and wears out its compressor early. An undersized unit runs constantly and struggles on peak days. Aim to match the calculated load, and if you fall between two standard sizes, choose the nearer one rather than jumping up two steps.
How many BTU do I need to heat a room?
Heating typically needs 25 to 55 BTU per square foot depending on your climate and insulation, so a 300 sq ft room needs roughly 7,500 BTU in a warm climate and up to 16,500 BTU in a cold one. Heating loads are almost always higher than cooling loads for the same room, which is why the calculator reports both figures separately.
Does ceiling height change how many BTU I need?
Yes, significantly. Air conditioners cool a volume of air, not a floor. Going from an 8 ft to a 10 ft ceiling adds 25% more air to condition, and a 12 ft vaulted ceiling adds 50%. Any calculator that ignores height will undersize rooms in older homes, loft conversions and open-plan spaces.
Do I still need a Manual J calculation?
For a window unit, portable or single-room mini split, this estimate is normally enough. For whole-house central air or a ducted replacement, get a Manual J load calculation from a licensed contractor. It accounts for duct leakage, window orientation, air infiltration and local design temperatures at a level of detail no online tool can reach. Use this result to sanity-check the quote you are given.
Related calculators
Planning the same room from other angles.
Square Footage Calculator
Work out the floor area of odd-shaped or multi-part rooms before you size the unit.
Electricity Cost Calculator
Estimate what your air conditioner actually costs to run per hour, day and month.
Unit Converter
Convert between feet and metres, watts and BTU, and other measurement units.
Size it once, and get it right
Enter your room dimensions and conditions above to get the cooling BTU, heating BTU, tonnage and the exact unit size to shop for — free, instant and with no sign-up.