Enter value in W:
Formula: hp = W × 0.0013410220888438076
In many fields—automotive, mechanical engineering, pump and compressor selection, and performance testing—power is quoted in either watts (W) or horsepower (hp). Watts provide an absolute, base‐SI measure of power, while horsepower remains a popular historic unit, especially in engine ratings and industrial equipment. Converting accurately between W and hp enables direct comparison of electric motor output, engine performance, and mechanical drive capability.
A watt represents one joule of energy per second:
1 W = 1 J/s
It is the fundamental SI unit for power, used universally in electrical, mechanical, and thermal contexts.
James Watt coined the term “horsepower” in the late 18th century to compare steam engine output with draft horses. Several definitions exist:
Different industries and regions adopted the version best fitting local unit systems and round‐off practices.
To convert watts into horsepower, divide by the chosen horsepower factor:
hp = W ÷ Factor
Common factors:
| hp Type | Definition | 1 hp → W | W → 1 hp |
|---|---|---|---|
| Mechanical | 550 ft·lb/s | 745.699872 W | ÷ 745.699872 |
| Metric | 75 kgf·m/s | 735.49875 W | ÷ 735.49875 |
| Electrical | Motor convention | 746 W | ÷ 746 |
Confirm the power in watts (from nameplate, dynamometer, or test bench). Ensure you have the continuous output rating.
Select mechanical, metric, or electrical hp based on application context (e.g., vehicle specs use metric hp).
Divide the watt value by the chosen factor. Example:
hp = 10 000 W ÷ 745.699872 ≈ 13.41 hp (mechanical)
Round to two decimal places for clarity: e.g., 13.41 hp.
A motor rated at 5 kW. Convert to mechanical horsepower:
5 000 W ÷ 745.699872 = 6.710 hp
Rounded: 6.71 hp
Though watts are SI‐standard, horsepower persists in:
Compare an internal combustion engine output (hp) with an electric motor (kW): convert kW to hp for one‐to‐one comparison.
Pump curves often specify required shaft horsepower. When sizing an electric driver rated in kW, convert motor kW to hp to match pump requirements accurately.
Dynamometers measure torque and rpm to compute power in watts; convert to horsepower for standardized reporting.
If A2 contains watts:
=A2 / 745.699872 // mechanical hp
=A2 / 735.49875 // metric hp
=A2 / 746 // electrical hp
Identical syntax applies. Use named ranges like =W / FACTOR_MECH for clarity.
function wattsToHp(watts, type='mechanical') {
const factors = {
mechanical: 745.699872,
metric: 735.49875,
electrical:746
};
return watts / factors[type];
}
// Usage:
console.log(wattsToHp(5000)); // ≈6.71 hp (mechanical)
console.log(wattsToHp(5000,'metric')); // ≈6.80 hp (metric)
def watts_to_hp(w, hp_type='mechanical'):
factors = {
'mechanical': 745.699872,
'metric': 735.49875,
'electrical': 746
}
return w / factors[hp_type]
print(watts_to_hp(10000)) # ≈13.41
print(watts_to_hp(10000,'metric')) # ≈13.60
For engineering accuracy:
Nameplate kW often represents electrical input. To find actual shaft output in hp, multiply by motor efficiency:
Wshaft = Winput × Efficiency → hp = Wshaft ÷ Factor
VFDs modulate motor speed. While power (W) varies, converting in real time to hp helps assess torque capability at different speeds.
Display dual axes: one in kW/W and the other in hp. Enable unit toggles so users can switch views seamlessly.
Avoid these errors:
Mechanical hp is common in the U.S.; metric hp is used in Europe and Asia. Electrical hp fits motor catalogs and ANSI standards. Always check your equipment’s nameplate for the specified standard.
Yes—multiply hp by the same factor:
W = hp × Factor
No—the unit definitions are fixed. However, motor efficiency and available power may vary with ambient conditions.
Converting watts to horsepower bridges SI precision with traditional power metrics. To ensure accuracy:
Whether sizing pumps, comparing engine outputs, or reporting motor test data, reliable W ↔ hp conversion is essential for clear communication, proper equipment selection, and consistent performance evaluation across industries.
When testing internal combustion engines, two distinct horsepower metrics appear:
IHP is obtained by integrating the pressure–volume (P–V) diagram of one engine cycle:
IHP = (PLAN) / 33,000
Where P = mean effective pressure (psi), L = stroke length (ft), A = piston area (in²), N = rpm, and 33,000 is ft·lb/min per hp. Converting IHP from ft·lb/s to watts then to hp allows comparison with BHP readings.
BSFC links fuel consumption to power output, measured in lb/hp·hr or g/kWh. It’s defined as:
BSFC = Fuel flow rate ÷ BHP
Converting power output to watts and then to hp ensures consistent BSFC units when fuel flow is in g/s or kg/h.
Engine and motor performance curves plot torque (Nm) versus speed (rpm) alongside power (kW or hp). The fundamental relationship is:
Power (W) = Torque (Nm) × Angular speed (rad/s)
Converting watts to hp clarifies peak torque locations in traditional engine tuning tables.
A motor at 3,000 rpm delivering 50 Nm:
ω = 2π × (3,000/60) ≈ 314 rad/s → P = 50 × 314 ≈ 15,700 W → ≈21.05 hp
Engine horsepower decreases at altitude due to lower air density. Manufacturers publish derating curves—often as % of sea-level hp versus altitude.
For a sea-level BHP of 100 hp, at 2,000 m where density is 80%:
Derated BHP = 100 hp × 0.80 = 80 hp → in watts ≈ 80 × 745.7 = 59,656 W
Converting engine output from watts to hp enables calculation of specific fuel consumption in kW·hr per liter (kW·h/L) or mpg·hp.
An engine producing 50 hp (≈37,285 W) consuming 10 L/h of gasoline (≈2.64 gal/h) at 38 MJ/L:
In hybrid and electric vehicles, powertrain maps show combined efficiency vs. speed and torque. Converting motor output to hp aligns with legacy ICE metrics in OEM calibration tools.
Overlaying kW contours onto hp axes helps engineers verify that electric motor performance meets or exceeds equivalent ICE benchmarks.
Certification cycles (e.g., EPA, Euro 6) measure emissions at defined engine loads (in hp). Dynamometer control systems convert commanded hp setpoints into corresponding wattage to drive test dynos accurately.
Adjustable‐speed drives control motor output in kW but often present user interfaces in hp. Converting between W and hp in real‐time ensures correct torque and speed profiling.
Industrial PLC code might include:
// Pseudocode
setpoint_hp = user_input;
setpoint_w = setpoint_hp * 745.7;
drive.setPower(setpoint_w);
Bearings and gears have load ratings in hp or kW. Converting actual power draw from W to hp enables fatigue and bearing life calculations using S–N curves specified in hp-loading terms.
L10 hours ≈ (C/P)³ where C = dynamic load capacity, P = equivalent hp converted to W for consistency, then back to hp.
A pump requires 20 hp at best‐efficiency point (BEP). An electric motor rated in kW must be converted:
20 hp × 745.7 W/hp = 14,914 W → 14.9 kW motor selected, with margin for 85% efficiency → 17.5 kW required input.
Modern engine test benches log torque and rpm to compute power in W, then convert to hp for database compatibility with historical test runs.
Test software exports CSV with columns: RPM, Torque (Nm), Power (W), Power (hp). Conversion uses:
hp = W / 745.699872;By extending W ↔ hp conversion into efficiency mapping, emission testing, altitude derating, and advanced control loops, engineers unlock cross‐discipline insights and ensure continuity between SAE/ISO horsepower conventions and SI‐based power measurements. To implement these in your workflows: