Ohm’s Law Calculator
Solve any two of voltage, current, resistance and power to get the other two, with the full power triangle, unit prefixes and a check against common component ratings.
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Four quantities, two equations
Ohm's law states that voltage equals current times resistance. Joule's law states that power equals voltage times current. Between them, knowing any two of voltage, current, resistance and power determines the other two. That is why the classic wheel diagram has twelve formulas: they are all rearrangements of those two relationships, and remembering which rearrangement applies is the only difficulty.
Where the arithmetic goes wrong
Almost every mistake in practice is a unit prefix rather than an equation. Milliamps entered as amps produce an answer a thousand times too large, and a kilohm entered as an ohm does the same in the other direction. The second common error is using peak voltage where RMS is required, which matters the moment mains power is involved. This calculator takes an explicit unit for each field and shows the base unit value it used, so a misplaced prefix is visible rather than silent.
Power is the part that burns
The result people most often need is dissipation. A quarter watt resistor in a circuit dissipating half a watt will discolour, drift and eventually fail, and the calculation that would have caught it takes ten seconds. The tool reports the power alongside the nearest standard resistor ratings, so you can see immediately whether the part you have in the drawer is adequate or whether you need the next size up.
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Frequently Asked Questions
Which two values do I need to enter?
Any two. Voltage and current, voltage and resistance, current and resistance, or power with any one of the others. The calculator picks the correct rearrangement and fills in the remaining two.
Does this work for AC circuits?
For resistive loads, yes, using RMS values throughout. For circuits with capacitance or inductance the relationship involves impedance and a phase angle, so plain Ohm’s law under states the current. Use RMS, not peak, or your answers will be high by a factor of about 1.41.
Why does the tool suggest a resistor power rating?
Because dissipation is the calculation people skip. A resistor run at its rated power will get very hot; the usual engineering practice is to pick a rating at least twice the calculated dissipation, which is what the suggestion reflects.
Can I enter values in milliamps or kilohms?
Yes, each field has its own unit selector covering micro, milli, kilo and mega prefixes. The base unit value used in the calculation is displayed so you can confirm the prefix was applied as you intended.
What if I enter three values that disagree?
The first two you filled in are used and the tool tells you that the third was recomputed rather than trusted. Clearing a field releases it back to being calculated.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Fill in any two of the four values and the remaining two are calculated automatically.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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