BTU Calculator
Size an air conditioner or heater correctly. An undersized unit runs constantly and never quite works; an oversized one cycles and dehumidifies poorly. This calculator uses room size, ceiling height, insulation, and sun exposure.
BTU Calculation Results
How the BTU Calculator Works
BTU (British Thermal Units per hour) measures how much heat an air conditioner can move or a heater can produce. The baseline rule is simple, and the adjustments are where accuracy comes from:
BTU = floor area (ft²) × 20 × ceiling factor × insulation × sun exposure + 600 per extra occupant + 4,000 if kitchen
Twenty BTU per square foot suits a standard-height room with average insulation. Tall ceilings add air volume, so the figure scales with heights above 2.4 m. Poor insulation (older buildings, single glazing) adds 20%; good modern insulation subtracts 15%. A sunny, south-facing room carries a heat load a shaded one doesn't, every person beyond two adds roughly 600 BTU of body heat, and kitchens get a flat 4,000 BTU surcharge for appliance heat. The result is matched to the nearest standard unit size — because you buy an 8,000 or a 12,000 BTU unit, not a 9,350.
How to Size an Air Conditioner (and Why Bigger Isn't Better)
Measure the room you're conditioning, not the whole floor — an AC unit sized for "the apartment" but installed in the bedroom is oversized for the bedroom. For open-plan spaces, include every connected area air flows into, and expect one unit to struggle beyond about 60 m² regardless of rating.
Resist the instinct to oversize "to be safe." An oversized air conditioner cools the air quickly and shuts off before it dehumidifies — the result is a cold, clammy room and short-cycling that wears the compressor. Slightly over the calculated figure is fine (that's why the calculator rounds up to the next standard size); double is a mistake. Inverter units deserve preference: they modulate output continuously instead of switching on and off, holding both temperature and humidity steadier at lower running cost.
Insulation honesty matters more than any other input — drafty single-glazed rooms genuinely need 20% more capacity, and no amount of BTU fixes an open window. Heat rises, so rooms above conditioned floors run cooler; attic rooms under the roof run hotter. For year-round comfort, pair the AC with ceiling circulation — our ceiling fan calculator sizes that — and good ambient light planning via the lighting calculator keeps heat-heavy halogens out of the load.
BTU Quick Reference by Room Size (standard ceiling, average conditions)
| Room size | BTU needed | Typical unit |
|---|---|---|
| 10 m² (110 ft²) | ~2,500 | 5,000 BTU (smallest common) |
| 15 m² (160 ft²) | ~3,500 | 5,000 BTU |
| 20 m² (215 ft²) | ~4,500 | 5,000–6,000 BTU |
| 30 m² (325 ft²) | ~6,800 | 8,000 BTU |
| 45 m² (485 ft²) | ~10,000 | 10,000–12,000 BTU |
| 60 m² (645 ft²) | ~13,500 | 14,000 BTU or two units |
Common Mistakes to Avoid
- Oversizing 'to be safe' — the unit short-cycles, leaves humidity behind, and wears out faster.
- Rating the room by floor area alone when it has 3+ m ceilings or a glass wall.
- Ignoring the kitchen surcharge; an oven can out-heat a small AC.
- Buying a fixed-speed unit when an inverter model costs slightly more and runs far better.
Frequently Asked Questions
How many BTU do I need for a bedroom?
A typical 12–15 m² bedroom with standard ceilings and average insulation needs roughly 3,000–3,500 BTU — meaning the smallest common unit (5,000 BTU) covers nearly every bedroom. Only very large, sunny, or poorly insulated bedrooms justify stepping up to 6,000–8,000 BTU.
Is it bad to have too many BTU?
Yes — an oversized AC cools the air before it has time to dehumidify it, then shuts off. You get a cold but clammy room, more compressor wear from constant restarts, and higher bills. Aim for the calculated figure rounded up one standard size, never doubled.
How many BTU is a kilowatt?
One kilowatt equals about 3,412 BTU per hour. European AC units are often rated in kW: a 2.5 kW unit is roughly 8,500 BTU, a 3.5 kW unit about 12,000 BTU. The calculator shows both figures so you can shop either market.
Do ceiling height and sun exposure really matter for AC sizing?
Substantially. A 3 m ceiling adds 25% more air volume than a 2.4 m one — the calculator scales for it. A sunny, south-facing room with big windows carries around 15% extra heat load, while a shaded room needs 10% less. Skipping these adjustments is how rules of thumb fail.