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Ohms Law Calculator
Find voltage, current, resistance and power from any two values, and combine resistors in series or parallel.
Ohms law
Voltage: —
Current: —
Resistance: —
Power: —
Note: —
Resistors in series and parallel
Equivalent resistance: —
How to use the Ohms Law Calculator
- Fill in any two of voltage, current, resistance and power.
- Read the two values the tool works out for you in the result panel.
- Use the resistor helper to combine two resistances in series or in parallel.
Features
- Voltage, current, resistance and power from any two values
- Series and parallel resistor helpers
- Notes when a value makes the result undefined
- Runs locally, no uploads
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Frequently Asked Questions
Common questions about the free online ohms law calculator.
Fill in any two of voltage, current, resistance and power, and the tool works out the other
two values straight away.
Voltage equals current times resistance, and power equals voltage times current. Together
they cover every pair of known values.
Power is current squared times resistance, or voltage squared divided by resistance,
depending on which pair of values you know.
In series the resistances add, so the equivalent resistance is R1 plus R2 and the same
current flows through both of them.
In parallel the equivalent resistance is R1 times R2 divided by R1 plus R2, which is always
smaller than the smallest resistor.
Volts for voltage, amperes for current, ohms for resistance and watts for power, all in the
standard SI form used in physics.
A zero current or resistance makes some results undefined, so the tool says so instead of
returning a meaningless number.
Yes. Enter the voltage across it and the measured current, and the tool returns the
resistance that fits those two readings.
It works for DC and for AC only when you use the root mean square values, because reactance
is not part of these equations.
No, the Ohms law calculator runs entirely in your browser and never sends your values
anywhere.
Last updated: October 10, 2026