Motor Capacity Calculator – kW, HP & Full-Load Amps

Estimate AC or DC shaft output or current from voltage, supply type, efficiency and power factor, with kW, mechanical horsepower, losses and an illustrative rating comparison.

Motor Capacity Formula

The motor capacity calculator estimates the mechanical output power of a DC or AC motor from steady-state electrical operating data. AC calculations assume sinusoidal quantities and balanced three-phase loading. In practice, motor capacity is usually expressed as horsepower (HP) or kilowatts (kW). The estimate depends on the measured voltage and current, plus how efficiently the motor converts electrical power into shaft power.

HP = (K \times V \times I \times eta \times PF) / (745.6999)

kW = (K \times V \times I \times eta \times PF) / (1000)

The phase constant depends on the motor type and the measurements used:

K = 1 for DC or single - phase AC; PF = 1 for DC

K = \sqrt(3) for three - phase AC using line - to - line voltage and line current

What the Formula Means

The calculation starts with real electrical input power, adjusts it for efficiency, and then converts the result into horsepower if needed. This makes the calculator especially useful when you know the motor’s operating voltage, running current, efficiency, and power factor but want a fast estimate of output capacity.

Pᵢₙ = K \times V \times I \times PF

Pₒᵤₜ = Pᵢₙ \times eta

HP = (Pₒᵤₜ) / (745.6999)

Variable Definitions

Symbol	Description	Typical Use
HP	Motor capacity in horsepower	Estimated shaft output power
kW	Motor capacity in kilowatts	Metric power rating
V	Voltage in volts	Use RMS voltage
I	Current in amperes	Use RMS running current
η	Efficiency as a decimal	Represents conversion from electrical input to mechanical output
PF	Power factor as a decimal	Accounts for the real portion of AC input power
K	Phase constant	DC, single-phase or balanced three-phase multiplier

Single-Phase and Three-Phase Forms

For clarity, the general equation can be written in its most common forms below.

HP 1φ = (V \times I \times eta \times PF) / (745.6999)

HP 3φ = (\sqrt(3) \times V LL \times I L \times eta \times PF) / (745.6999)

For three-phase motors, use line-to-line voltage and line current. Mixing line values with phase values will produce the wrong result.

How to Use the Calculator

Choose shaft output or current, then DC, single-phase AC or three-phase AC.
Enter supply voltage; use RMS line-to-line voltage for three-phase AC.
Enter current for output mode, or shaft output rating for current mode, with the matching units.
Enter efficiency as a percentage, and AC power factor from 0 to 1, for the same operating point.
Select Calculate. Expand Calculation details for conversions and the illustrative rating comparison.

If efficiency is given as a percentage, convert it to decimal form before using the formula:

eta = (eta %) / (100)

Rearranged Equations

The calculator provides output and current modes. The algebraic rearrangements below are reference equations; voltage, efficiency and power factor are entered values, not additional calculator solve modes.

Unknown	Rearranged Formula
Current	$I = (745.6999 \times HP) / (K \times V \times eta \times PF)$
Voltage	$V = (745.6999 \times HP) / (K \times I \times eta \times PF)$
Efficiency	$eta = (745.6999 \times HP) / (K \times V \times I \times PF)$
Power Factor	$PF = (745.6999 \times HP) / (K \times V \times I \times eta)$

Examples

Example 1: Single-phase motor

Given 220 V, 10 A, 85% efficiency, and 0.90 power factor:

HP = (1 \times 220 \times 10 \times 0.85 \times 0.90) / (745.6999) \approx 2.26

kW = 220 \times 10 \times 0.85 \times 0.90 / 1000 = 1.683

Example 2: Three-phase motor

Using the same electrical values but treating the motor as three-phase with line-to-line voltage and line current:

HP = (\sqrt(3) \times 220 \times 10 \times 0.85 \times 0.90) / (745.6999) \approx 3.91

kW = 0.7456999 \times 3.91 \approx 2.92

Practical Notes

Use running values, not startup values. Inrush current during motor starting is much higher than steady-state current and will overstate capacity.
Use RMS measurements. The formula assumes RMS voltage and current for AC operation.
Power factor matters. Two motors can draw similar current but produce different real power if their power factors differ.
Efficiency matters too. Higher efficiency means more of the input power becomes useful shaft power.
Nameplate output and calculated output may differ. Nameplate HP is a rated continuous output, while this calculation reflects the motor’s estimated output at the measured operating point.

Common Mistakes

Using three-phase phase voltage instead of line-to-line voltage.
Confusing the input percentage with the formula fraction: enter 90 in the efficiency (%) field; the calculator uses 0.90 internally.
Ignoring power factor and using apparent power as if it were real power.
Using no-load current, locked-rotor current, or transient measurements.
Assuming the result is exact even when efficiency or power factor are estimated rather than measured.

HP and kW Conversion

If you want to convert the result between horsepower and kilowatts, use the following relationship:

kW = 0.7456999 \times HP

HP = (kW) / (0.7456999)

This calculator estimates shaft output or current under the stated operating assumptions. The retained rating list is an illustrative comparison, not a universal standard or motor-selection recommendation. Starting torque, duty, thermal conditions, drive waveform, supply and manufacturer requirements need separate assessment.