Every circuit you'll ever touch — from a phone charger to a house's wiring — obeys one relationship between voltage, current, and resistance. Ohm's law ties the three together so cleanly that if you know any two, the third falls out of a single division or multiplication. This calculator does that arithmetic for you and throws in power for free.
How to Use the Ohm's Law Calculator
Pick which quantity you want to solve for — current, voltage, or resistance — from the selector, then enter the other two values. If you're solving for current, enter voltage in volts and resistance in ohms. If you're solving for voltage, enter current in amps and resistance in ohms. If you're solving for resistance, enter voltage in volts and current in amps. The calculator returns all three quantities plus power, so you always see the full picture of the circuit, not just the one number you asked for.
A Note on Real Circuits
This calculator assumes a simple, single-resistance circuit. Real circuits often have resistors in series or parallel, which combine differently — series resistances simply add together, while parallel resistances combine by the reciprocal formula 1/R_total = 1/R₁ + 1/R₂ + .... Once you've reduced a circuit to a single equivalent resistance using those rules, Ohm's law applies exactly as shown here.
The Ohm's Law Formula
Ohm's law states that voltage equals current times resistance:
V = I × R
Rearranged, current equals voltage divided by resistance (I = V/R), and resistance equals voltage divided by current (R = V/I). Say you have a 12-volt battery connected to a 100-ohm resistor. Current works out to 12 ÷ 100, or 0.12 amps. Power, which is voltage times current, comes to 12 × 0.12, or 1.44 watts. That's the exact example this calculator uses by default, so you can check your own numbers against a known answer.
Working Backward From a Known Component
Electronics hobbyists often use Ohm's law in reverse: they know the resistor value printed on the component (or decoded from its color bands) and the supply voltage, and they need to find the current an LED or sensor will actually receive. Plug the supply voltage and the resistor's rated ohms into this calculator, solving for current, and you'll see immediately whether that current is safe for the component — most standard LEDs, for example, want somewhere between 10 and 20 milliamps (0.01 to 0.02 amps), and too much current burns them out.
Why Resistance Limits Current
Resistance is a material's opposition to the flow of electric charge. A thick copper wire has very low resistance and lets current flow almost freely; a thin nichrome wire (the stuff inside a toaster's heating element) has much higher resistance and turns that resistance into heat. For a fixed voltage, doubling the resistance halves the current — that's why a dimmer switch, which is really just a variable resistor, lowers the brightness of a bulb by choking down the current reaching it.
Power: The Fourth Quantity
Power is how fast electrical energy is being converted into another form — light, heat, motion. It's calculated as P = V × I, and it's the number that determines your electricity bill and whether a component will overheat. A 1.44-watt draw is trivial; a phone charger might handle 10-20 watts, while an electric kettle can pull over 1,500 watts. If you know power and voltage, you can also back into current with I = P/V, which is how appliance labels list "amp draw" from a wattage rating.
