Voltage Calculator
Calculate electrical voltage from current and resistance, or from power and current. Also shows power dissipation for any circuit.
A
Ω
W
Voltage
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Power —
Current —
Extended More scenarios, charts & detailed breakdown ▾
A
Ω
Voltage (V)
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Power (W) —
Professional Full parameters & maximum detail ▾
A
Ω
W
Voltage (V)
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Power (W) —
Current (A) —
RMS Voltage (V) —
Peak Voltage (V) —
Energy/hour (Wh) —
Energy/day (kWh) —
Impedance Voltage (V) —
How to Use This Calculator
- Select whether to solve from current + resistance or power + current.
- Enter the two known values.
- Results show voltage and power.
Formula
V = I × R (current × resistance)
V = P ÷ I (power ÷ current)
Power: P = V × I = V² ÷ R = I² × R
Example
Example: I = 2 A, R = 10 Ω → V = 20 V, P = 40 W.
Frequently Asked Questions
- Use V = I × R (Ohm's Law). Multiply current in amperes by resistance in ohms to get voltage in volts. For example, a current of 2 A flowing through a 10 Ω resistor produces a voltage of V = 2 × 10 = 20 V across that resistor. This is the most common form of the voltage formula in circuit analysis. Select "From I & R" in this calculator, enter current and resistance, and the voltage (plus power) is calculated instantly. This method applies to any resistive element in a DC circuit.
- Use V = P ÷ I, where P is power in watts and I is current in amps. For example, a 40 W device drawing 2 A operates at V = 40 ÷ 2 = 20 V. Alternatively, if you know power and resistance but not current, use V = √(P × R). For example, a 100 W heater with 10 Ω resistance: V = √(100 × 10) = √1000 = 31.6 V. Select the appropriate input method in the "From P & I" or "From P & R" tabs in this calculator.
- Voltage drop is the reduction in electrical potential along a conductor due to its resistance. Every wire has some resistance, and current flowing through it produces a voltage drop V = I × R. For example, a 100-foot copper wire run (200 ft total round trip) carrying 15 A may drop several volts, reducing the available voltage at the load. The NEC recommends keeping voltage drop below 3% on branch circuits and 5% total. Voltage drop causes lights to dim, motors to run hotter, and electronics to malfunction. Use larger wire gauge for long runs to minimize resistance.
- DC (direct current) voltage is constant in magnitude and direction — like the output of a battery (1.5 V, 12 V). AC (alternating current) voltage reverses polarity periodically at a set frequency — 60 Hz in North America, 50 Hz in Europe. AC voltage is described by its RMS (root mean square) value: 120 V AC RMS means the peak voltage is actually 120 × √2 = 169.7 V. Most household power is AC. Electronic devices convert AC to DC internally. This calculator uses DC or RMS AC values and is applicable to both, as long as you use consistent RMS values for AC calculations.
- In the United States and Canada, standard household voltage is 120 V AC at 60 Hz for outlets, and 240 V AC (two-phase) for high-power appliances like electric dryers, ranges, and EV chargers. In most of Europe, the standard is 230 V AC at 50 Hz. The UK uses 230 V AC at 50 Hz. Australia uses 230 V AC at 50 Hz. Japan uses 100 V AC at 50 or 60 Hz (depending on region). Always verify local voltage standards before plugging in imported electrical equipment, as using the wrong voltage can damage devices or create safety hazards.