Every electrical quantity you will ever work with, voltage, current, resistance, power, and more, reduces to a small set of standard units. Knowing these units and their SI prefixes cold is what lets you read any datasheet or schematic without hesitation.
Electrical units of measure express standard quantities, along with their prefixes, whenever a value is too small or too large to write comfortably as a base unit.
The standard electrical units of measure for voltage, current, and resistance are the Volt, Ampere, and Ohm respectively. These are based on the International System of Units, the SI System, with most other electrical units derived directly from SI base units.
Because electrical quantities span such an enormous range, from a fraction of a picofarad to millions of watts, engineers rely on multiples and submultiples of these base units rather than writing long strings of zeros. This guide covers all 10 standard electrical units, the full table of SI prefixes from pico to tera, and a live calculator to convert between them instantly.
The 10 Standard Electrical Units of Measure
| Parameter | Unit | Symbol | Formula |
|---|---|---|---|
| Voltage | Volt | V or E | V = I × R |
| Current | Ampere | I or i | I = V ÷ R |
| Resistance | Ohm | R or Ω | R = V ÷ I |
| Conductance | Siemen | G | G = 1 ÷ R (reciprocal of resistance) |
| Capacitance | Farad | C | C = Q ÷ V |
| Charge | Coulomb | Q | Q = C × V |
| Inductance | Henry | L or H | VL = −L(di/dt) |
| Power | Watt | W | P = V × I, or I² × R |
| Impedance | Ohm | Z | Z² = R² + X² |
| Frequency | Hertz | Hz | ƒ = 1 ÷ T |
Notice how many of these formulas are just rearrangements of Ohm's law. Once V = I × R is second nature, resistance, current, conductance, and power all fall out of the same relationship almost automatically.
10 Standard Electrical Unit Calculators
Enter values into any calculator below to get an instant result, using the exact formula from the table above.
Voltage: V = I × R
Current: I = V ÷ R
Resistance: R = V ÷ I
Conductance: G = 1 ÷ R
Capacitance: C = Q ÷ V
Charge: Q = C × V
Induced Voltage: VL = −L × (dI/dt)
Power: P = V × I
Impedance: Z = √(R² + X²)
Frequency: ƒ = 1 ÷ T
SI Prefixes: Multiples and Submultiples
Resistance alone can range from under 0.01 ohm to over 1,000,000 ohms in real circuits. SI prefixes exist purely to avoid writing out that many zeros every time.
| Prefix | Symbol | Multiplier | Power of Ten |
|---|---|---|---|
| Tera | T | 1,000,000,000,000 | 10¹² |
| Giga | G | 1,000,000,000 | 10⁹ |
| Mega | M | 1,000,000 | 10⁶ |
| kilo | k | 1,000 | 10³ |
| none | none | 1 | 10⁰ |
| centi | c | 1/100 | 10⁻² |
| milli | m | 1/1,000 | 10⁻³ |
| micro | µ | 1/1,000,000 | 10⁻⁶ |
| nano | n | 1/1,000,000,000 | 10⁻⁹ |
| pico | p | 1/1,000,000,000,000 | 10⁻¹² |
A few common examples: 1 kV equals 1,000 volts, 1 mA equals one thousandth of an ampere, 47 kΩ equals 47,000 ohms, 100 µF equals 100 millionths of a farad, and 1 MHz equals one million hertz.
🧮 Interactive SI Prefix Converter
Enter a value, pick its current prefix, then pick the prefix you want to convert it into.
Watch: SI Unit Prefixes in Electrical Engineering
This video covers exactly how SI prefixes are used in electrical engineering, complementing the calculator above.
Video: "SI Unit Prefixes in Electrical Engineering", via YouTube.
Other Common Electrical Units
Watt Hour (Wh)
The amount of electrical energy consumed over time, commonly seen as kWh (1,000 watt hours) or MWh (1,000,000 watt hours) on utility bills.
Decibel (dB)
A logarithmic unit representing gain or loss in voltage, current, or power, calculated as 20log₁₀(Vout/Vin) for voltage ratios. Zero dB means input equals output.
Phase Angle (θ)
The difference in degrees between a voltage waveform and a current waveform sharing the same periodic time, described as leading or lagging.
Angular Frequency (ω)
A rotational unit used in AC circuits, equal to 2πƒ, measured in radians per second. A full cycle equals 360 degrees, or 2π radians.
Time Constant (τ)
The time it takes for a first order system's output to reach roughly 63.7% of its maximum or minimum value after a step input, a measure of reaction speed.
FAQs on Electrical Units of Measure
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What we learn today
- Ten standard electrical units, Volt, Ampere, Ohm, Siemen, Farad, Coulomb, Henry, Watt, Ohm (impedance), and Hertz, cover nearly every electrical quantity.
- SI prefixes from pico to tera let engineers express both tiny and enormous quantities without writing long strings of zeros.
- Converting between prefixes is just a matter of shifting the decimal point by the difference in their powers of ten.
- Additional units like the watt hour, decibel, phase angle, angular frequency, and time constant cover energy, gain, and AC timing relationships.
- Nearly every core electrical formula traces back to Ohm's law in one rearranged form or another.