kW to BTU Calculator

How the kW to BTU/hr conversion works

A kilowatt (kW) and a BTU per hour (BTU/hr) are both units of power — the rate at which energy is transferred or converted. The two are linked by a fixed, defined factor:

1 kW = 3,412.142 BTU/hr

Enter a power figure in kilowatts and the calculator multiplies it by 3,412.142 to give the BTU/hr equivalent, rounded to the nearest whole number. It also shows horsepower (kW × 1.34102) and watts (kW × 1,000). The conversion is exact — there is no estimation or approximation in the factor itself.

The exact conversion factor

The BTU (British Thermal Unit) used here is the international-table BTU, defined as exactly 1,055.05585262 joules. One kilowatt is exactly 1,000 watts, and one watt is one joule per second. Dividing 3,600 seconds per hour by the joule value of one BTU gives the conversion factor: 3,600 ÷ 1.05505585262 ≈ 3,412.142 BTU per kWh, equivalently 3,412.142 BTU/hr per kW. This is the internationally agreed figure used in HVAC engineering references.

What kilowatts and BTU/hr measure

Both units express the same physical quantity — power, or energy per unit time — in different measurement systems. The kilowatt is the SI unit used throughout European and UK engineering. BTU/hr is still widely used in North American HVAC practice and persists on many air-conditioning product spec sheets regardless of where the equipment is sold, because much of the global air-conditioning manufacturing supply chain uses US engineering conventions.

For heating output: a 3.5 kW electric radiator delivers 3,500 W of heat; the same output expressed in BTU/hr is 3.5 × 3,412.142 ≈ 11,942 BTU/hr. A domestic gas boiler rated at 18 kW delivers about 61,419 BTU/hr.

Worked examples

• 1 kW: 1 × 3,412.142 = 3,412 BTU/hr (1.341 hp, 1,000 W)
• 2.5 kW: 2.5 × 3,412.142 = 8,530 BTU/hr (3.353 hp, 2,500 W)
• 3.5 kW: 3.5 × 3,412.142 = 11,942 BTU/hr (4.694 hp, 3,500 W)
• 5 kW: 5 × 3,412.142 = 17,061 BTU/hr (6.705 hp, 5,000 W)
• 10 kW: 10 × 3,412.142 = 34,121 BTU/hr (13.410 hp, 10,000 W)

Where this conversion appears in UK construction and HVAC practice

UK boiler, heat-pump, and radiator specifications are published in kilowatts — the kW rating is the primary figure on a UK product's data sheet and on MCS (Microgeneration Certification Scheme) documentation for heat pumps. Air-conditioning units, particularly split systems and portable units imported from or specified by manufacturers using North American conventions, are frequently rated in BTU/hr on their packaging, remote-control displays, and technical data sheets.

The practical need for this conversion arises in several situations:

• Selecting an air-conditioning unit. A room heat-load calculated in kW using a heat-loss method can be matched against AC units rated in BTU/hr. A 3.5 kW load corresponds to approximately 12,000 BTU/hr, often labelled as a 12,000 BTU unit or a 1-tonne unit in US convention.
• Cross-referencing imported equipment specifications. Control panels, dataloggers, and process equipment from North America or the Far East often display or record power in BTU/hr rather than kW.
• Commercial HVAC plant sizing. Chiller and fan-coil-unit catalogues from some manufacturers list capacities in BTU/hr; converting to kW lines the figure up with UK electrical supply calculations and MCS documentation.
• Underfloor heating and heat-pump output. Heat-pump COP and output figures are quoted in kW on UK installers' design tools; a client's imported radiant panel specification may state the required BTU/hr.

Horsepower — the third output

The calculator also shows mechanical horsepower (kW × 1.34102). One mechanical horsepower is defined as 745.69987 W, giving the factor 1,000 ÷ 745.69987 ≈ 1.34102 hp/kW. Horsepower appears on pump, compressor, and motor data sheets — typically from older UK documents or from North American manufacturers — where it serves the same role as the kW rating on equivalent European specifications.

The tonne of refrigeration relationship

In commercial cooling, capacity is sometimes expressed in tonnes of refrigeration (TR). One tonne of refrigeration is 3,517 W = 3.517 kW = 12,000 BTU/hr — the heat required to melt one short ton of ice in 24 hours. The BTU/hr output of this calculator lines up directly with that scale: a unit rated at 24,000 BTU/hr is a 2-tonne (2 TR) unit, equivalent to 7.034 kW.

How precise are the results?

The factor 3,412.142 BTU/hr per kW is the standard rounded value used in engineering references; the underlying defined figure is 3,412.1414799… BTU/hr/kW. The calculator displays BTU/hr to the nearest whole number, which is appropriate given that the accuracy of the underlying kW measurement in any real installation is typically no better than ±1–2% of nameplate. Horsepower is shown to three decimal places to allow the figure to be compared directly against nameplate ratings. For engineering calculations requiring tighter precision, the full factor should be carried through.

Using this alongside other BuildMetricLab tools

The Celsius to Fahrenheit calculator handles the temperature-unit conversion that frequently accompanies HVAC work. The room volume calculator provides the volume input needed before a heat-loss calculation can determine the kW figure to convert. The pipe insulation and pipe length calculators support the distribution side of a heating system design. All BuildMetricLab tools run entirely in the browser — no sign-up, no data sent anywhere, and the formula is shown on-page so the maths can be audited.