Formula
Georg Ohm's relation for a constant resistance:
volts = amps × ohms
Current is volts divided by ohms, and resistance is volts divided by amps. Power on this page is volts times amps. The electricity-cost and battery pages already start from watts, so wattage is not its own calculator.
Assumptions
- Resistance does not change with temperature or current.
- The current is direct, or you are using RMS values for a single sine wave.
- One component is in the circuit. A two-resistor divider has its own page.
Worked example
2 amps through 10 ohms is 20 V.
Input definitions
- Voltage
- Volts across the component. Zero is allowed when it is an input.
- Current
- Amps through the component.
- Resistance
- Ohms. It must be greater than zero when you are solving for current.
Limitations
Real supplies have internal resistance, and many components are not ohmic. This page does not size a wire or say a circuit is safe.
Questions
Where is the watt result?
It is the Power row. Watts equal volts times amps once the missing value is known.
Can resistance be zero?
Only when you are solving for voltage, which is then zero. Solving for current divides by resistance, so zero ohms is rejected.