Watt Converter
Convert one power value across SI units, horsepower, and commonly used engineering power units with synchronized live results.
Power inputs
Edit any power field. Every other field is recalculated from the same watt-equivalent model.
Accepted format: non-negative decimal numbers using a period as the decimal separator; standard comma thousands separators are allowed.
Live results
The watt-equivalent value used for every conversion.
target value = watts ÷ target factor
Complete conversion table
Equivalent values share one physical power quantity; only the numerical scale and unit definition change.
| Unit | Symbol | Equivalent value | Watts per unit |
|---|
Factors are applied through a watt base: source value × source factor = watts, then watts ÷ target factor = target value.
How to use the Watt Converter
What this calculator does
This calculator expresses one power quantity in several units at the same time. Power is the rate at which energy is transferred or work is done. The watt is the SI derived unit of power, and one watt equals one joule per second. The converter normalizes every entry to watts, then derives milliwatts, kilowatts, horsepower, BTU per hour, and the selected unit in the “Other power units” control. It is a unit-conversion tool, not an electrical sizing calculator: it does not infer power from voltage and current, estimate electricity cost, or determine whether equipment is safe for a circuit.
When to use it
Use the converter when comparing an appliance rating stated in watts with a generator or motor rating stated in kilowatts or horsepower; translating a heating or cooling specification from BTU per hour to watts; checking engineering documentation that mixes SI prefixes; or preparing a consistent power-unit table for a report, quotation, or spreadsheet model. The NIST definition of the watt provides the underlying relationship between watts, joules, and seconds.
How to calculate
- The calculator opens with a ready-to-use demonstration of 1,200 watts. Its equivalent fields, result cards, table, and validated Excel workbook are already populated.
- Replace any one of the displayed values. You may type in Milliwatts, Watts, Kilowatts, Mechanical horsepower, or Other power value. The field you edit becomes the active source.
- For a nonstandard target, choose a definition in Other power unit. Changing the selection converts the current watt-equivalent value without changing the physical power represented.
- Read Power in watts as the normalized base value, then review Power in kilowatts, Mechanical horsepower, BTU per hour, and the Complete conversion table.
- Select Download Excel to create a fresh workbook from the current validated controls. Select Reset to clear the demonstration and all calculated content. After Reset, Excel export remains unavailable until you enter a complete valid value again.
Input guide
Milliwatts is a required source field when you choose to edit it. Enter a non-negative decimal in mW; for example, 500. One milliwatt is 0.001 watt, so increasing this input increases every result proportionally. Do not enter a percent sign, unit letters, scientific notation, or a decimal comma such as “1,5”.
Watts accepts a non-negative decimal in W; for example, 1,200. It is the most direct source because the model uses watts as its canonical base. A zero value is valid and produces zero in every unit. A negative value is rejected because this converter represents a non-negative power magnitude rather than signed power flow.
Kilowatts accepts a non-negative decimal in kW; for example, 1.2. One kilowatt equals 1,000 watts. Higher values scale every equivalent upward. A common mistake is multiplying by 1,000 when converting watts to kilowatts; the correct direction is to divide watts by 1,000.
Mechanical horsepower accepts a non-negative decimal in mechanical hp; for example, 1.609226507514. This implementation uses 745.6998715822702 watts per mechanical horsepower. Do not assume that metric horsepower or electrical horsepower is identical; those definitions are separately available in the other-unit selector.
Other power value is paired with Other power unit. The value is required only when it is the field you are editing. Choose among SI prefixes, metric or electrical horsepower, BTU/h, foot-pound force per second, thermochemical calorie per second, and erg per second. For example, select Megawatts (MW) and enter 0.0012. Changing the unit selection recalculates the displayed other-unit value from the current watts; it does not reinterpret the old number under a new definition.
Output guide
Active source identifies the field currently driving the model. Normalized power chooses a readable SI-scaled presentation while preserving the exact watt model. Workbook reports whether the current state has passed the in-page Open XML package validation. Power in watts is the canonical conversion base. Power in kilowatts, Mechanical horsepower, and BTU per hour are direct equivalent conversions, not recommendations or estimates. Zero means zero power; a large or small value simply reflects the selected unit scale.
The Conversion rule states the exact identity used by the model: target value equals watts divided by the target factor. The Complete conversion table lists each Unit, its Symbol, the current Equivalent value, and Watts per unit. “Watts per unit” is the factor multiplied by a source value to obtain watts. The table and workbook use the same canonical model, so changing any input updates both. Very large or very small equivalents may use scientific notation to remain readable.
Worked example
The startup example uses 1,200 W. Milliwatts are calculated as 1,200 ÷ 0.001 = 1,200,000 mW. Kilowatts are 1,200 ÷ 1,000 = 1.2 kW. Mechanical horsepower is 1,200 ÷ 745.6998715822702 = 1.609227 hp at the displayed precision. BTU per hour is 1,200 ÷ 0.2930710701722222 = 4,094.570 BTU/h. These values match the first-open controls, result cards, conversion table, and workbook checkpoints.
Learn more
The BIPM SI Brochure is the authoritative description of the International System of Units and identifies the watt as J/s. For practical presentation, the NIST guidance on expressing SI quantities explains why prefixes are often chosen so numerical values remain easy to read.
How the conversion model works
Every supported unit has a factor stated as watts per one unit. The model first multiplies the active source value by its factor to obtain watts. It then divides that watt value by each target factor. This two-step approach makes conversions consistent even when neither the source nor the target is a watt-based SI prefix. For example, converting mechanical horsepower to BTU/h does not need a separate horsepower-to-BTU table: both definitions meet at the same watt base.
Interpretation and common mistakes
Equivalent power values can look dramatically different because the units use different scales. A million milliwatts and one kilowatt describe the same power. Comparing raw numbers without their unit symbols can therefore be misleading. Keep the unit attached to every value, distinguish power from energy, and avoid treating watts and watt-hours as interchangeable. Watts measure a rate; watt-hours measure accumulated energy over time.
Horsepower definitions also require care. Mechanical horsepower, metric horsepower, and electrical horsepower have different factors, so a specification should identify which definition applies. For formal engineering work, consult the relevant equipment standard or contract and use the published unit definition. NIST's conversion-factor guidance explains how published factors should be applied and reported.