Ohm’s Law Calculator
Enter any two of voltage, current, resistance and power — the calculator works out the other two and shows the exact formula it used.
Fill in any two boxes and leave the rest blank. The other two are worked out for you.
Ohm’s Law describes how three basic properties of an electrical circuit hold together: the voltage across a component (V, in volts), the current flowing through it (I, in amperes) and its resistance (R, in ohms). The core relationship is V = I × R, and once you add electrical power (P, in watts) with P = V × I, any two of the four values are enough to pin down the remaining two. This calculator does exactly that: type in the two figures you know, choose their units, and it fills in the rest — and tells you which formula it applied. Everything is worked out in your browser, so nothing you enter leaves your device.
How the four quantities connect
Voltage, current, resistance and power are tied together by two well-known relationships and their rearrangements. From Ohm’s Law itself you get voltage, current or resistance whenever you know the other two:
V = I × R • I = V ÷ R • R = V ÷ I
Bringing power into the picture (P = V × I) and substituting Ohm’s Law gives the rest of what electricians and hobbyists call the "Ohm’s Law wheel":
P = V × I = I² × R = V² ÷ R
That is why entering any pair works. Give it voltage and resistance and it uses I = V ÷ R for the current and P = V² ÷ R for the power. Give it current and power instead and it turns to V = P ÷ I and R = P ÷ I². All twelve arrangements are covered, so you never have to rearrange the algebra by hand.
Entering values and reading the result
Fill in exactly two of the four boxes and leave the others blank. Each field has its own unit menu, so you can work in the prefixes that real components use — millivolts, milliamps and microamps, kilohms and megohms, milliwatts or kilowatts — and the tool converts everything to base units before calculating. The moment two boxes hold a value, the two results appear and the quantities it solved for are highlighted, alongside a line naming the formula used.
Results are scaled to whatever reads most naturally: a current of 0.12 A is shown as 120 mA, and a resistance of 4,700 Ω as 4.7 kΩ, rather than as long strings of zeros or decimals. Because the maths assumes real, working components, all entered values must be positive; if you enter a third value the tool asks you to keep it to two so the answer is never over-determined.
What it covers, and what it does not
This calculator applies to direct-current (DC) and purely resistive circuits — the everyday case of a battery or supply driving resistors, LEDs with a series resistor, heating elements and similar loads. It treats each value as a simple magnitude, which is exactly what you want for sizing a current-limiting resistor, checking whether a resistor’s wattage rating is high enough, or finding the current a supply will push through a known load.
It is not built for alternating-current analysis, where impedance, reactance and power factor replace plain resistance, so the power figure here is real power into a resistive load rather than apparent or reactive power. It is also a maths helper, not a safety tool: it will happily compute large currents or voltages that would be dangerous in practice, so always respect component ratings and wiring limits. For the ordinary DC arithmetic it is designed for, it gives an exact answer every time, entirely on your own machine.
Frequently Asked Questions
What is Ohm’s Law in simple terms?
Ohm’s Law states that the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. Rearranged, it also gives I = V ÷ R and R = V ÷ I, so knowing any two of voltage, current and resistance reveals the third.
How many values do I need to enter?
Exactly two of the four — any pair of voltage, current, resistance and power. Two values are always enough to determine the other two, so the calculator asks you to fill in two boxes and leave the rest blank.
Can I enter milliamps, kilohms and other prefixes?
Yes. Every field has a unit menu covering the common prefixes — microamps and milliamps, millivolts and kilovolts, kilohms and megohms, milliwatts and kilowatts. Your entries are converted to base units for the calculation, and the answers are scaled back to whichever prefix reads most clearly.
Does it work for AC circuits?
It is designed for DC and purely resistive circuits. AC analysis replaces resistance with impedance and involves reactance and power factor, which this tool does not model, so the power it reports is real power into a resistive load rather than apparent or reactive power.
Is anything I type sent to a server?
No. The unit conversions and every formula run in your browser with JavaScript, and nothing is uploaded. Closing the tab clears the values, so you can work through circuit figures privately.