BTU Calculator — How Many BTU to Heat or Cool a Room

Enter your room's length and width in feet — that is all you need for a quick answer. The calculator multiplies length × width to get floor area, then applies the standard 25 BTU per square foot rule to estimate the cooling or heating capacity your space needs. Open More options to fine-tune for ceiling height, number of people, sun exposure, insulation, or a kitchen. Want a fast answer to *how many BTU to cool a 400 square foot room*? Just enter 20 by 20 and read the result.

A BTU (British Thermal Unit) measures heat energy. Air conditioners and heaters are rated in BTU/hr — how much heat they can move each hour. Undersize the unit and it runs non-stop without cooling; oversize it and it short-cycles, wasting power and leaving the room clammy. The most common rule of thumb, and the one this btu calculator uses by default, is roughly 25 BTU per square foot of floor space for cooling in a typical, moderately insulated US room with 8-foot ceilings. That means a 12x12 room (144 sq ft) needs about 3,600 BTU, while a 20x20 room (400 sq ft) needs about 10,000 BTU. Real rooms vary, so the adjustments under More options nudge the estimate for taller ceilings, extra people, strong sun, poor insulation, and heat-heavy kitchens.

Base BTU = length × width × 25

Then the calculator adds or scales for real-world factors:

- Ceiling over 8 ft: +1,000 BTU for each foot above 8.
- Occupants: +600 BTU for each person beyond the first two.
- Kitchen: +4,000 BTU for cooking heat.
- Sun exposure: +10% if very sunny, −10% if heavily shaded.
- Insulation: +10% if poor, −10% if good.

The result is rounded up to the nearest 500 BTU because air conditioners are sold in fixed sizes. Divide BTU/hr by 12,000 to get the equivalent in tons of cooling.

Example 1 — 20×20 bedroom (default settings): 20 × 20 = 400 sq ft. 400 × 25 = 10,000 BTU/hr, about 0.83 tons. That matches the 400–450 sq ft row of the sizing table below.

Example 2 — how many BTU to cool a 12x12 room: 12 × 12 = 144 sq ft. 144 × 25 = 3,600, rounded up to 4,000 BTU/hr — a small window or portable unit.

Example 3 — sunny kitchen with high ceilings: a 15×20 kitchen (300 sq ft) starts at 7,500 BTU. Add 4,000 for the kitchen and 2,000 for a 10-ft ceiling = 13,500, then ×1.1 for strong sun ≈ 15,000 BTU/hr.

- Measure to the walls, not the furniture. Use the full floor footprint for each room the unit will cool.
- Cooling an open-plan space? Add up the connected areas — kitchen, dining, and living room together — rather than sizing for one.
- Buy the next size up, not down. If your answer lands between two AC models, round up so the unit isn't straining on the hottest days.
- For heating, the 25 BTU/sq ft base is a solid starting point in mild climates, but cold northern climates often need 30–40 BTU/sq ft — bump insulation to "Poor" for a safety margin.
- Check the plug. Units above ~12,000 BTU often need a dedicated 20-amp or 240-volt circuit.