Room AC & Heater BTU Calculator
Estimate how many BTU per hour of cooling or heating a room needs based on its size, ceiling height, sun exposure and number of people.
ft
ft
ft
Sun exposure
people
Is this a kitchen?
Sizing for this room
- 1
Room area
15 × 12 = 180Floor area in square feet - 2
Height factor
8 ÷ 8 = 1Scales the load for ceilings taller or shorter than the standard 8 ft - 3
Total BTU/hr
180 × 20 × 1 × 1 = 3,600
How does this calculator work?
Multiply room area (length × width) by 20 BTU/hr per square foot, scale by ceiling height ÷ 8 and a ±10% sun factor, add 600 BTU per occupant above two and 4000 BTU for a kitchen. The result is your required capacity; divide by 12,000 for tons.
Formula
How this is calculated
The calculator starts from the room floor area (length × width) and applies a baseline of 20 BTU per hour per square foot, the rule of thumb for typical cooling loads. That base is scaled by a ceiling-height factor (your ceiling height divided by the standard 8 ft) because taller rooms hold more air to condition.
Sun exposure adjusts the result by ±10%: a very sunny room gains heat and needs about 10% more capacity, while a heavily shaded room needs about 10% less. Each occupant beyond the first two adds 600 BTU/hr to account for body heat, and a kitchen adds a flat 4000 BTU/hr for appliance and cooking heat.
The total is converted to tons (1 ton = 12,000 BTU/hr) and matched to the smallest common unit size that meets the load. The meter compares your required load against that suggested unit so you can see whether a given unit is over- or undersized. These are planning estimates; insulation quality, window area, climate and air leakage can shift the real figure, so consult an HVAC professional for a formal Manual J load calculation.
Frequently asked questions
At the 20 BTU/sq ft baseline with an 8 ft ceiling and average sun, about 6,000 BTU/hr, so a 6,000 BTU window or mini-split unit is typical. Add capacity for sunny exposure, extra occupants or a kitchen.
One ton of air-conditioning capacity equals 12,000 BTU per hour. The calculator shows your required capacity in both BTU/hr and tons.
Yes. BTU/hr is a generic measure of heating and cooling capacity, so the same load estimate sizes electric or gas heaters as well, though heating loads in very cold climates may run higher.
Also known as
TG we-Calculate Editorial Team. (2026). Room AC & Heater BTU Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/room-ac-btu-calculator
TG we-Calculate Editorial Team. "Room AC & Heater BTU Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/room-ac-btu-calculator.
TG we-Calculate Editorial Team, "Room AC & Heater BTU Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/room-ac-btu-calculator
@misc{wecalculate_room_ac_btu_calculator, title = {Room AC & Heater BTU Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/room-ac-btu-calculator}}, year = {2026}, note = {TG we-Calculate} }
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