BTU Calculator

Calculate BTU needed for air conditioning or heating, convert between BTU and watts/kW, and find the right AC unit size for any room

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AC BTU Calculator

Estimate the BTU cooling capacity needed for a room or house air conditioner. Accounts for room size, ceiling height, insulation, sun exposure, climate, and occupants.

Floor area of the room or space
People regularly in the room
Each window adds ~1,000 BTU
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Heating BTU Calculator

Calculate the BTU heating capacity needed for a room or house. Based on room dimensions, insulation, climate, and desired indoor temperature.

Outdoor to desired indoor temp difference
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General Purpose BTU Calculator

General heating or cooling BTU estimator using room volume. Enter room dimensions and the desired temperature change from outdoor to indoor.

Difference between outdoor temperature and desired indoor temperature
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BTU / Watts / kW Converter

Convert between BTU/hour, Watts, Kilowatts, Tons of refrigeration, Horsepower, and Calories. Based on: 1 Watt = 3.41214 BTU/hr.

Conversion Reference
1 BTU/hr = 0.2931 W  |  1 W = 3.4121 BTU/hr
1 kW = 3,412 BTU/hr  |  1 Ton = 12,000 BTU/hr
1 hp = 2,545 BTU/hr  |  1 kcal/hr = 3.9683 BTU/hr
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BTU Size Chart Reference

EnergyStar.gov recommended BTU capacity by room size. Values assume standard 8 ft ceilings, average insulation, and moderate sun exposure.

Area (sq ft)BTU/hr NeededTonskWTypical Use
Adjustment Factors
+10% if room is very sunny or south/west facing
-10% if room is heavily shaded or north/east facing
+10% for each additional occupant beyond 2
+10-25% for poor insulation / older home
+25% for kitchen (stove heat generation)
+10% per foot above 8 ft ceiling height
SEER Rating Guide
SEER 18+ Highly Efficient — Premium units, lowest operating cost
SEER 15-17 High Efficiency — Good balance of cost and efficiency
SEER 13-14 Standard — Minimum federal efficiency requirement

SEER formula: Annual energy cost = BTU/hr ÷ SEER ÷ 1000 × hours × electricity rate
What is a BTU?
One British Thermal Unit (BTU) is the amount of energy needed to raise 1 pound of water by 1°F. In HVAC: 1 BTU/hr = 0.293 watts. Higher BTU rating = stronger heating or cooling power. For AC, BTUs measure how much heat the unit can remove from the air per hour.