How many BTU do I need per square foot?

For cooling, typical BTU-per-square-foot ranges are: 15 BTU/sq ft in very cold climates, 20 in cold climates, 25 in moderate climates, 30 in mild climates, and 35 in hot climates. These are base values before adjustments for insulation, sun exposure, and occupancy.

How do I convert BTU to watts?

1 BTU/hr equals 0.29307 watts. To convert BTU/hr to watts, multiply by 0.29307. To convert watts to BTU/hr, divide by 0.29307 (or multiply by 3.412). For example, a 12,000 BTU/hr air conditioner draws about 3,516 watts (3.5 kW).

What is a ton of refrigeration in BTU?

1 ton of refrigeration equals 12,000 BTU/hr. The term comes from the cooling capacity needed to melt one ton of ice in 24 hours. Residential air conditioners are typically 1.5 to 5 tons (18,000 to 60,000 BTU/hr).

How much BTU does a person generate?

A sedentary person generates approximately 400 BTU/hr of sensible heat and 200 BTU/hr of latent heat (moisture). A person doing light activity generates about 700 BTU/hr sensible and 400 BTU/hr latent. In crowded spaces, occupant heat gain can be a significant portion of the total cooling load.

BTU Calculator

Estimate room BTU for HVAC sizing, convert between energy and power units, and look up appliance heat output ratings. Three tools in one.

Room Dimensions

Length(ft)

Width(ft)

Ceiling Height(ft)

Occupants

Climate & Construction

Climate Zone

Insulation Quality

Sun Exposure

Exterior Walls

Understanding BTU in HVAC

What is a BTU?

A British Thermal Unit (BTU) is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTU/hr measures the rate of heating or cooling. A typical home might need 30,000-60,000 BTU/hr for heating and 24,000-48,000 BTU/hr (2-4 tons) for cooling.

Why Room Size Alone Is Not Enough

Square footage is the starting point, but climate zone, insulation quality, window exposure, and occupancy all significantly affect the load. A 200 sq ft room in Phoenix with poor insulation and south-facing windows needs far more cooling than the same room in Portland with good insulation and north exposure.

Internal Heat Gains

Every appliance, light, and person in a space adds heat. A kitchen with an oven running can add 5,000+ BTU/hr. A server room with 10 servers might generate 30,000 BTU/hr or more. The Appliances tab shows typical ratings for common equipment to help estimate internal gains.

This Tool vs. Full Manual J

This calculator provides a simplified BTU estimate for quick sizing. A full ACCA Manual J analysis adds hour-by-hour solar gain modeling, window orientation and shading coefficients, duct losses, latent loads, and ASHRAE design conditions by ZIP code. Use this tool for ballpark estimates — use Manual J for equipment selection.

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This tool provides calculations for informational purposes only and does not constitute professional engineering, construction, or safety advice. Verify all results independently before use in design, construction, or compliance decisions. See our Terms of Service.

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