Cooling · Heating · BTU/h · kW

BTU calculator

Estimate the heating or cooling capacity for a room with clear adjustments and unit conversions.

Free · Private · Instant

Enter room detailsValues stay in your browser.

Choose calculationCooling and heating use different methods.

Capacity type

Measurement system

Room dimensionsMeasure the conditioned floor area and average ceiling height.

Room length

Room width

Ceiling height

Cooling table basis: 8 ft.

Cooling adjustmentsAccount for sun, people, cooking and known internal loads.

Sun exposure

Regular occupants

people

Adds 600 BTU/h for each person above two.

Is this a kitchen?

Extra known cooling load

BTU/h

Optional allowance for unusual equipment or process heat.

Room capacity guide

Calculate BTU per hour for cooling or heating

BTU per hour describes how quickly equipment can move heat. Cooling mode follows the published room-air-conditioner capacity table and adjustments. Heating mode provides a transparent preliminary estimate from volume, design temperature difference and insulation condition.

Capacity is not energy use: A result of 12,000 BTU/h equals about 3.52 kW of thermal capacity. It does not mean the appliance continuously consumes 3.52 kW of electricity.

Room air-conditioner sizing table

Cooling mode starts with the ENERGY STAR floor-area table, which assumes an 8-foot ceiling. The calculator then scales for the entered ceiling height and applies the published sunlight, occupancy and kitchen adjustments.

Area to cool	Base capacity	Area to cool	Base capacity
100–150 ft²	5,000 BTU/h	450–550 ft²	12,000 BTU/h
150–250 ft²	6,000 BTU/h	550–700 ft²	14,000 BTU/h
250–300 ft²	7,000 BTU/h	700–1,000 ft²	18,000 BTU/h
300–350 ft²	8,000 BTU/h	1,000–1,200 ft²	21,000 BTU/h
350–400 ft²	9,000 BTU/h	1,200–1,400 ft²	23,000 BTU/h
400–450 ft²	10,000 BTU/h	1,400–1,500 ft²	24,000 BTU/h
1,500–2,000 ft²	30,000 BTU/h	2,000–2,500 ft²	34,000 BTU/h

Cooling BTU calculation methodology

Measure floor area

Length × width

Select table capacity

Match the area with the published room-AC bracket.

Adjust ceiling height

Base × actual height ÷ 8 ft

Apply room loads

Adjust sunlight, occupants, kitchen use and optional known loads.

−10%Heavily shaded room

+10%Very sunny room

+600BTU/h per person above two

+4,000BTU/h for a kitchen

Heating BTU estimate methodology

Heating mode uses a simplified volumetric heat-loss model. It converts the room to cubic metres, calculates the indoor-to-outdoor design temperature difference and multiplies by the selected loss factor. The optional margin is applied last.

Heat loss (W) = volume (m³) × temperature difference (K) × loss factor (W/m³·K)BTU/h = watts ÷ 0.2930711

This simple model cannot measure the actual U-values of walls, roof, floor and glazing or the real ventilation and infiltration rate. Use it for early comparison only; final system and emitter selection needs a detailed room-by-room survey.

BTU calculator example

Room12 × 15 ft

→

Area180 ft²

→

Base cooling6,000 BTU/h

With an 8-foot ceiling, average sun, two occupants and no kitchen load, the calculated and recommended nominal capacity remains 6,000 BTU/h, approximately 1.76 kW of cooling capacity.

BTU, watts, kilowatts and tons

1 BTU/h≈ 0.2930711 W

1 kW≈ 3,412.14 BTU/h

1 cooling ton= 12,000 BTU/h

12,000 BTU/h≈ 3.52 kW capacity

Why correct equipment size matters

Undersized equipment may struggle to reach the design temperature. Oversizing is not a reliable safety strategy: an oversized room air conditioner may cycle frequently, use more energy and remove humidity poorly. Correct installation, airflow, sealing, controls and equipment efficiency also affect comfort and operating cost.

Frequently asked questions

What does BTU mean?

BTU means British thermal unit. Heating and cooling equipment capacity is normally stated as BTU per hour, which is a rate of heat transfer rather than a quantity of electricity consumed.

How does the air-conditioner BTU calculator work?

It selects the published ENERGY STAR room-AC capacity for the floor area, scales for ceiling height, applies the official shade, sun, occupancy and kitchen adjustments, adds any entered equipment load and selects the next suitable listed capacity.

Is a larger air conditioner always better?

No. An oversized room air conditioner can cycle on and off too quickly, use more energy and remove humidity poorly. Capacity should be matched to the room and verified where conditions are unusual.

How accurate is the heating estimate?

It is a transparent planning estimate based on room volume, the indoor-to-outdoor design temperature difference and a selected heat-loss factor. Final heater, radiator, boiler or heat-pump sizing should use a room-by-room heat-loss calculation with measured fabric and ventilation data.

How many watts are in one BTU per hour?

One international-table BTU per hour equals approximately 0.2930711 watts. Therefore 10,000 BTU per hour is about 2.93 kW of heating or cooling capacity.

Does capacity in kW equal electricity use?

No. The displayed kW is thermal heating or cooling capacity. Electrical input depends on equipment efficiency, operating conditions, cycling and controls.

Can I size several connected rooms together?

You can enter the combined floor dimensions only when the spaces are genuinely open and air can circulate freely. Separate or closed rooms normally need a zone-by-zone load calculation.

What if the cooling area is larger than 2,500 square feet?

The room-air-conditioner table ends at 2,500 square feet. Larger or multi-zone spaces need a proper HVAC load calculation, so the cooling mode stops rather than extrapolating beyond the published table.

Sources and scope

Calculeter is independent. Results are planning estimates and do not replace a Manual J, BS EN 12831, CIBSE, MCS or other locally required professional load calculation.