Loading Calculator...
Please wait a moment
Please wait a moment
V = I × R
Voltage equals current times resistance
V = P / I
Voltage equals power divided by current
Voltage (V), also called electromotive force or potential difference, is the electrical pressure that drives current through a circuit. Measured in volts, it represents the energy per unit charge available to move electrons from one point to another. Using Ohm's Law, voltage equals current multiplied by resistance (V = I x R), and from the power equation, voltage equals power divided by current (V = P / I). Voltage exists between two points and is always measured as a difference -- a single wire does not have a voltage on its own, only a voltage relative to another reference point such as ground.
If you know current (I) and resistance (R), use V = I x R. If you know power (P) and current (I), use V = P / I. Select the formula based on the values you have available.
Current must be in amperes (not milliamps), resistance in ohms (not kilohms), and power in watts (not milliwatts). Convert before calculating: 500 mA = 0.5 A, 2.2 kilohm = 2,200 ohm.
Multiply or divide as required. For example, 2A flowing through a 6 ohm resistor produces V = 2 x 6 = 12V. A 100W device drawing 0.833A operates at V = 100 / 0.833 = 120V.
Set your multimeter to DC or AC volts as appropriate, connect the probes across the two points in question, and compare the reading to your calculation. The reading should match within component tolerances.
Long cable runs lose voltage due to wire resistance. Calculating the voltage drop (V = I x R_wire) ensures your load receives adequate voltage, which is critical for motors, lighting, and sensitive electronics.
Choosing the correct supply voltage is the first step in any circuit design. Too little voltage and the circuit will not function; too much can damage components and create safety hazards.
LEDs require a specific forward voltage. Calculating the remaining voltage across the current-limiting resistor (V_supply minus V_LED) determines the resistor value needed for safe operation.
Common voltage standards used in residential, industrial, and electronic applications around the world.
| Application | Voltage | Type | Regions / Notes |
|---|---|---|---|
| USB Standard | 5 V | DC | Universal standard for USB devices |
| Automotive | 12 V | DC | Cars, motorcycles, small boats |
| USB-C PD (max) | 20 V | DC | USB Power Delivery high-voltage mode |
| Heavy Trucks | 24 V | DC | Trucks, buses, heavy equipment |
| PoE (Standard) | 48 V | DC | Power over Ethernet for network devices |
| Japan Mains | 100 V | AC | Japan (50/60 Hz) |
| North America Mains | 120 V | AC | US, Canada, Mexico (60 Hz) |
| Europe / Asia Mains | 230 V | AC | EU, UK, Australia, most of Asia (50 Hz) |
| NA Split-Phase | 240 V | AC | US dryers, ovens, EV chargers |
| Industrial 3-Phase | 480 V | AC | US industrial motors, heavy machinery |
Voltage is the electrical pressure (potential difference) that pushes charge through a circuit, measured in volts. Current is the actual flow of charge, measured in amperes. A common analogy is water in a pipe: voltage is the water pressure, and current is the flow rate. You need voltage to create current, but they are distinct quantities.
Yes, but only for purely resistive loads (heaters, incandescent bulbs). For circuits with capacitors or inductors, replace R with impedance (Z), which accounts for the phase angle between voltage and current. Use RMS values for both voltage and current in AC calculations.
Voltage drop is the reduction in voltage along a wire due to the wire's own resistance. Long runs of undersized wire can drop enough voltage that devices at the end malfunction. The NEC recommends no more than 3% voltage drop for branch circuits and 5% for feeders and branch circuits combined.
Historical choices during electrification led to different standards. Higher voltage (230V) allows the same power delivery with half the current, enabling thinner wires and lower losses. Lower voltage (120V) was considered safer for residential use. Both work effectively with proper infrastructure design.
Use a multimeter rated for the voltage category (CAT III or CAT IV for mains). Hold probes by the insulated handles, keep one hand in your pocket to avoid completing a circuit through your body, and never exceed the meter's rated voltage. For voltages above 600V, use differential probes or voltage dividers designed for the purpose.