Begin with the source record's unit label. If it only says hp, keep the conversion unresolved until the definition is confirmed. The example below makes that choice visible and preserves the factor with the answer.
Three definitions, three conversion factors
The NIST power conversion table lists separate watt factors for mechanical horsepower, identified as 550 ft·lbf/s, metric horsepower, and electric horsepower. Using those printed values:
| Selected definition | Printed W per hp | One hp expressed in kW |
|---|---|---|
| Mechanical | 745.6999 | 0.7456999 |
| Metric | 735.4988 | 0.7354988 |
| Electric | 746 | 0.746 |
These calculations preserve the table's printed factor precision. Do not add extra digits from a different definition or call every displayed decimal an exact physical measurement. The small converter described here supports these three choices; other horsepower definitions require their own documented factors.
Convert through watts
For a supplied horsepower value h and selected factor F in W per hp:
Watts = h × F
Kilowatts = Watts / 1000
Two mechanical hp gives 2 × 745.6999 = 1491.3998 W, then 1.4913998 kW. Two metric hp gives 1470.9976 W, then 1.4709976 kW. The arithmetic is straightforward once the definition is explicit.
For the reverse direction, use the same selected factor:
Horsepower = supplied kW × 1000 / F
Thus 0.7456999 kW converts back to one mechanical hp under this factor. Changing the type to metric changes the meaning of the horsepower output, even though the kW input remains the same.
Preserve a record someone else can check
A useful plain-text result includes the original value, direction, horsepower definition, printed factor, watts, kilowatts or horsepower output, and an approximation note. For the two-hp example:
Input: 2 hp, mechanical (550 ft·lbf/s)
Factor used: 745.6999 W per mechanical hp, NIST printed value
Calculation: 2 × 745.6999 = 1491.3998 W
Output: 1.4913998 kW using that printed factor
The first screen should show an unselected definition field, input value, hp/kW direction controls, and the one-hp comparison above. Do not default to mechanical merely because it is listed first. Changing the selected definition must clear a previously calculated result until the reader calculates again.
Keep input errors separate from zero power
Use nonnegative finite dot-decimal entries with a leading digit, at most six fractional digits, and a maximum manually entered value of 1,000,000. Zero is valid and converts to zero. A blank input is missing, not zero. A generic hp type, negative number, unknown definition, comma-formatted number, or scientific notation is outside this bounded interface.
Reject invalid input with a specific text message and clear the old result. Input, type, or direction changes also invalidate the current record. Reset empties the value and record, clears the type selection, and restores the hp-to-kW direction.
Swap should reverse the units using the stored unrounded result and the same type. Preserve a computed value separately from manually typed text, so a seven-place result such as 0.7456999 is not truncated to fit the six-place typing rule. Label that entry as calculated. Editing it returns to ordinary input validation. If a swapped value exceeds the declared numeric range, disable Swap with a reason. Display formatting must not become a new rounded calculation input.
What the result does and does not tell you
Converting a supplied power value changes its unit. It does not determine an engine's displacement, a motor's electrical input, circuit requirements, or efficiency. For instance, 1.4913998 kW here is the same supplied two mechanical hp expressed differently; it is not an inferred mains-power requirement.
Keep any separate equipment-rating or efficiency information in its own source record. The helper has no fields or calculation for those tasks.
Copyable converter prompt
Create an original self-contained HTML/CSS/JavaScript hp/kW converter with readable source and an acceptance-test report. Require explicit mechanical, metric or electric horsepower; no generic-hp default. Use NIST printed W/hp factors 745.6999,735.4988,746 respectively. hp->W is h*F; W->kW divides by 1000; inverse kW->hp is k*1000/F with the same selected type.
Show type, original input, direction, printed factor, watts, output unit and a plain-text record. Identify printed-factor precision and any display rounding; retain unrounded internal calculation. Test mechanical 1->0.7456999 kW, mechanical 2->1.4913998, metric 1->0.7354988, electric 1->0.746, zero->zero and inverse round trips within documented numerical tolerance.
Manually typed inputs: finite nonnegative dot decimals, leading digit, up to six fractional digits, maximum 1000000. Blank is missing; ambiguous type, negative, unknown type, commas or scientific notation invalid. Input/type/direction changes clear the stale result. Native Convert/Swap/Reset and labeled controls must be keyboard usable. Reset clears input/result/type and restores hp->kW.
Swap uses the stored unrounded result, same type and reversed direction. Keep computed entries separately from manually typed text, label their provenance and preserve extra fractional places instead of truncating to six; editing switches back to typed validation. Disable Swap with a reason outside numeric scope. Never parse rounded result text for reverse calculations.
No horsepower-to-cc, efficiency, electrical-input, three-phase, circuit or equipment-rating inference. Check factors, inverse tolerance, invalid-after-valid state, keyboard focus, actual article width and 360 px. Mark unexecuted checks not_tested; no tested runtime, storage, export or host guarantee.
Keep the definition attached to the answer
Retain the source unit label and factor record in the native worksheet or helper source. If you prepare an HTML version for NOS, the supplied product documentation supports HTML saving and rendering; verify the type selection, inverse calculation, and Swap in the actual workspace before treating that version as a tested converter. Explore NOS with prepared HTML, keeping “mechanical,” “metric,” or “electric” beside the output so the number still means the same thing when reused.