Ohms Law and Power Calculator
Enter any two of voltage, current, resistance and power and get the other two, with the rearrangement shown and a resistor power rating suggested.
Enter any two values. The other two follow.
The twelve formulas are two relationships
Every wheel chart in every electronics textbook is V = IR and P = VI rearranged. Four quantities with two independent degrees of freedom means any two determine the rest — there is nothing else to learn.
- Ohm's law is not a law of nature. It is an observation about a class of materials. Diodes, transistors and filament lamps are deliberately non-ohmic — a lamp's resistance climbs several-fold as it heats — so applying a single resistance to them gives a number that means nothing.
- Derate the power rating. A resistor's rating assumes free air at 25°C, and yours is probably in a warm box. The convention is to run at half the rating or less.
- Zero resistance is a short, not a big current. Real sources have internal resistance and real wires have some too, so the calculation returning infinity is the arithmetic telling you the model has stopped applying.
How to use
- Enter exactly two of the four values.
- Read the other two, with the formula used shown beneath.
- Check the suggested resistor power rating before ordering parts.
- Enter three values only if you want them checked against each other.
Frequently asked questions
Why only two values?
Because two is all it takes. Voltage, current, resistance and power are four quantities with two independent degrees of freedom, so any two determine the rest. Entering three lets you contradict yourself, which is why the tool asks for exactly two rather than quietly picking which of your numbers to believe.
Do I need to memorise twelve formulas?
No. Every wheel chart in every electronics textbook is V = IR and P = VI rearranged. There are two relationships and nothing else to learn, which is worth knowing because the wheel makes it look like twelve separate facts.
Is Ohm's law actually a law?
Not in the sense that gravity is. It is an empirical observation about a class of materials that happen to have constant resistance over a useful range. A great many components are deliberately not ohmic — diodes, transistors, and filament lamps whose resistance climbs several-fold as they heat — so applying a single resistance to them produces a number that means nothing.
Why does the tool refuse zero resistance?
Because zero ohms across a voltage source is a short circuit rather than an arithmetic problem. The formula returns infinite current, which is the model telling you it has stopped applying — real sources have internal resistance and real wire has some too, and what actually happens is decided by those.
What power rating should I use?
At least twice the calculated dissipation. A resistor's rating assumes free air at 25°C and yours is probably in a warm enclosure, so the convention is to run at half the rating or below. Run one at its nominal rating and it gets hot enough to discolour the board and drift in value.
What if I need more than 10 W?
Then you are past standard through-hole parts and into wirewound resistors with heatsinking, or a different approach entirely. Dissipating serious power in a resistor is usually a sign the circuit should be doing something else — a switching regulator instead of a linear one, for instance.
Does this work for AC?
For resistive loads at mains frequency, near enough, using RMS values. It does not model reactance, so anything with significant inductance or capacitance — motors, transformers, switched supplies — needs impedance rather than resistance and a power factor the calculation here does not carry.
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