BTU Calculator - Room Air Conditioning & Heating Needs

How to use this tool

1. Enter the room length, width, and ceiling height in feet.
2. Choose sun exposure and occupant count; check the kitchen option when applicable.
3. Select a heating climate band to set its BTU-per-square-foot midpoint.
4. Read the point estimates for room cooling and heating BTU per hour.

What is a BTU?

A British Thermal Unit (BTU) is the amount of energy needed to raise 1 pound of water by 1°F. In HVAC, BTUs measure the cooling or heating capacity of air conditioners and furnaces.

This calculator gives a point estimate for one room using a simple rule of thumb. It reports room area and volume, but cooling and heating use separate formulas: the cooling estimate applies sun, occupancy, kitchen, and tall-ceiling adjustments, while heating uses a selected climate rate per square foot.

Room sizing guide

Adjustments

Cooling starts with areaFt2 * 20 * sunFactor, where low, medium, and high sun use factors of 0.9, 1.0, and 1.1. After the sun adjustment, occupancy, kitchen, and ceiling additions are added to form this cooling point estimate: areaFt2 * 20 * sunFactor + max(0, occupants - 2) * 600 + (kitchen ? 4000 : 0) + max(0, heightFt - 8) * 1000. Kitchen and ceiling additions affect cooling only; ceilings at or below 8 feet do not reduce BTU.

• Low sun reduces the area baseline by 10%; high sun raises it by 10%.
• Add 600 cooling BTU per occupant beyond two regular occupants.
• Add 4,000 cooling BTU for cooking heat when the room includes a kitchen.
• Add 1,000 cooling BTU/hr per ceiling foot above 8 feet.

Heating BTU guide

Heating uses room area multiplied by the midpoint of the selected climate band. The selector defaults to Moderate (17.5 BTU/sq ft); Mild uses 12.5, Cold uses 25, and Very cold uses 35 BTU/sq ft. Ceiling height is included in the displayed room volume, but it does not change this simplified heating estimate. Actual heating loads also depend on insulation, air leaks, windows, and local design temperatures.

For example, a 180 sq ft room in the default Moderate band has a heating point estimate of 180 × 17.5 = 3,150 BTU/hr. A 10-foot ceiling would add 2,000 BTU/hr to cooling, but does not change that heating estimate. These climate bands are broad guidance, not a location-specific heat-loss calculation.

EER and SEER

BTU capacity tells you how powerful a unit is; efficiency ratings tell you how much electricity it uses to produce that output.

• EER (Energy Efficiency Ratio): BTU output divided by watts consumed at a single test condition. Higher is more efficient. A window unit with EER 12 uses less power than one with EER 8 to cool the same space.
• SEER (Seasonal Energy Efficiency Ratio): similar to EER but measured over a range of temperatures representing a full cooling season. Central air conditioners are rated by SEER. The US federal minimum is SEER 14 (2023+). High-efficiency units reach SEER 20–25.

HVAC sizing warning

The tables and formulas here are rough estimates for single-room window or portable units, not authoritative HVAC sizing. Results are point estimates rather than equipment-size ranges. For a whole-home central HVAC system, a proper Manual J calculation performed by a licensed HVAC professional is the correct sizing method. Manual J accounts for local climate data, insulation values, window area and orientation, ceiling height, infiltration rates, and internal heat gains. Oversized systems short-cycle (turn off before fully dehumidifying), while undersized systems run continuously and cannot maintain setpoint on peak days.