BTU Calculator — Free Online Tool
An undersized AC runs forever and never cools; an oversized one short-cycles and leaves the air clammy. This BTU calculator sizes cooling capacity to your room with the industry's standard 20 BTU per square foot rule.
What is the BTU Calculator?
The baseline rule: 20 BTU per square foot for rooms with 8-foot ceilings and average insulation — a 12×12 bedroom needs about 2,880 BTU. Poor insulation, big south-facing windows, or hot climates push it toward 30 BTU/sq ft, which is why this calculator shows both figures. Ceiling height matters too: vaulted rooms need proportionally more.
Tonnage is just BTU ÷ 12,000 — a “ton” of cooling is an old ice-melting reference, not weight. For whole-house sizing, a Manual J load calculation by an HVAC pro beats any rule of thumb, since ductwork, windows, and occupancy all matter. This BTU calculator is ideal for sizing window units, mini-splits, and portable ACs for single rooms.
How to use this calculator
- Measure the room's length and width in feet.
- Enter ceiling height (8 ft is standard).
- Click Calculate BTUs and compare standard vs poor-insulation figures.
- Choose a unit at or slightly above the recommended BTUs.
Formula
Frequently asked questions
What is a BTU?
The heat needed to raise one pound of water one degree Fahrenheit — the standard unit of cooling capacity. This BTU calculator converts your room into BTUs.
What does a 'ton' of air conditioning mean?
12,000 BTU per hour — a historical reference to melting a ton of ice, not the unit's weight.
Is an oversized AC bad?
Yes — it short-cycles, cooling too fast to dehumidify, leaving rooms cold but clammy.
Do kitchens need extra BTUs?
Yes — add roughly 4,000 BTU for a kitchen's appliances and heat load.