Ohm's Law Calculator
Enter any two of voltage, current, resistance and power and this Ohm's law calculator instantly solves the other two using V=IR and P=VI.
V = 12 V I = 2 A R = 6 Ω P = 24 W
Working
- Known: V = 12 V, I = 2 A
- R = V ÷ I = 12 V ÷ 2 A = 6 Ω
- P = V × I = 12 V × 2 A = 24 W
This assumes an ohmic part — a resistor, or a wire at a steady temperature — whose resistance stays the same whatever the current. Diodes, LEDs, transistors and lamp filaments do not follow V = IR across their range, so for them an answer here holds at one operating point only.
Ohm’s law wheel — click a formula to fill the form with the two values it needs
What this Ohm's law calculator does
Enter any two of the four electrical quantities — voltage V in volts, current I in amps, resistance R in ohms, and power P in watts — and the calculator instantly derives the remaining two. Leave the fields you want to find blank, type the two you know, and the results update live as you type. It works for every valid pair, whether you know voltage and current, resistance and power, or any other combination.
The formulas it uses
The tool combines Ohm's law with the electrical power relationships. Ohm's law states V = I × R, so resistance is R = V ÷ I and current is I = V ÷ R. Power ties them together through P = V × I, which also gives P = I² × R and P = V² ÷ R. From any two known values the calculator picks the matching pair of equations and solves the rest. When you supply resistance and power it takes a square root, since V = √(P × R) and I = √(P ÷ R). If a combination would require dividing by zero or taking the root of a negative number, it tells you instead of returning a nonsense answer.
Where Ohm's law stops being true
Every answer assumes an ohmic part — a resistor, or a wire at a steady temperature — whose resistance stays the same whatever the current, and the page says so under the answer. Diodes, LEDs, transistors and lamp filaments do not follow V = IR across their range, so for them an answer holds at one operating point only. To see the arithmetic on a real circuit, the "Worked example" button fills in a resistor on a battery, with the current in milliamps and the power in milliwatts, and the working shows each step with those numbers.
Frequently asked questions
How many values do I need to enter?
Exactly two. Ohm's law links voltage, current, resistance, and power, so any two of them fully determine the other two. Enter fewer than two and there is nothing to solve; enter more than two and the inputs could conflict, so the tool asks you to clear the extras.
Why do I sometimes get an error instead of an answer?
Some combinations have no valid solution — for example a current of zero when solving for resistance would divide by zero, and a negative resistance or power under a square root has no real result. The calculator flags these instead of showing a wrong number.
Can I enter milliamps or kilohms, and see how the answer was worked out?
Yes. Each field has its own unit prefix, and the answer comes with its working — which formula was used and the arithmetic with your numbers.
Does Ohm’s law hold for every component?
No. Every answer assumes an ohmic part — a resistor, or a wire at a steady temperature — and the page says so under the answer: diodes, LEDs, transistors and lamp filaments do not follow V = IR across their range, so for them an answer holds at one operating point only.
Can Ohm's law calculate voltage drop?
Yes, across a resistance: the drop is V = I × R. Enter the current through the part and its resistance, and the voltage field is the drop across it — 20 mA through a 470 Ω resistor drops 9.4 V. For a wire, use the wire's own resistance, both conductors there and back.
What does Ohm's law say?
That the current through a conductor is proportional to the voltage across it: I = V ÷ R, where R stays the same whatever the current. Power follows from it: P = V × I = I² × R = V² ÷ R.
How do I pick the resistor for an LED?
Take the LED's forward voltage, from its datasheet, away from the supply voltage, and enter what is left as the voltage, with the current you want through the LED as the current. The resistance field is the resistor to use — round up to the next value you can buy — and the power field is the least it must be rated for. Only the resistor is solved here: the LED itself is not ohmic.
What are volts, amps, ohms and watts?
Four measures of one circuit. Voltage, in volts, is the push that drives charge round it; current, in amps, is how much charge flows past a point each second; resistance, in ohms, is how hard the part makes that flow; and power, in watts, is how fast energy is used — voltage times current. Any two of them fix the other two, which is why the page asks for exactly two.