Fill in any two values. The other two are calculated from the two you entered last.
| Quantity | Value | Source |
|---|---|---|
| Voltage | ||
| Current | ||
| Resistance | ||
| Power |
About this tool
This Ohm's law calculator solves the four basic electrical quantities: voltage, current, resistance and power. Type any two of them and it calculates the other two, with unit prefixes from microamps and milliwatts up to kilovolts and megohms. It shows which formulas it used, and a formula wheel below lists every combination. All the arithmetic happens in your browser.
How to use the Ohm's law calculator
- Enter two known values, for example the supply voltage and the resistor value.
- Pick the unit next to each box. Choose mA for milliamps, kΩ for kilohms, and so on. Calculated values are written back into their boxes in whatever unit is selected there.
- Read the results. They update as you type. The table shows each value with an automatic prefix and marks which ones you entered and which were calculated. The box underneath shows the formulas with the numbers substituted.
- Change any value to recalculate. The tool always works from the two boxes you edited most recently. Clear empties everything so you can start again.
Ohm's law and the power formula
Ohm's law says that the current through a resistance is proportional to the voltage across it: V = I × R. Electrical power is the rate of energy transfer, P = V × I. In SI units the ohm is one volt per ampere and the watt is one joule per second, which is also one volt-ampere for DC (NIST Guide to the SI, chapter 4). Substituting one equation into the other produces the twelve forms in the wheel:
| To find | From V and I | From V and R | From V and P | From I and R | From I and P | From R and P |
|---|---|---|---|---|---|---|
| Voltage V (volts) | given | given | given | V = I × R | V = P ÷ I | V = √(P × R) |
| Current I (amps) | given | I = V ÷ R | I = P ÷ V | given | given | I = √(P ÷ R) |
| Resistance R (ohms) | R = V ÷ I | given | R = V² ÷ P | given | R = P ÷ I² | given |
| Power P (watts) | P = V × I | P = V² ÷ R | given | P = I² × R | given | given |
The calculator picks the right row for the pair you entered. Converting every value to base units first (volts, amps, ohms, watts) keeps the formulas simple, and the prefixes are applied only on input and output. The SI prefixes used here are micro (µ, 10⁻⁶), milli (m, 10⁻³), kilo (k, 10³) and mega (M, 10⁶).
Worked example
The calculator opens with a 12 V supply across a 470 Ω resistor. Those are the two known values, so it uses the V and R column of the wheel:
I = V ÷ R = 12 ÷ 470 = 0.0255319 A = 25.53 mA P = V² ÷ R = 144 ÷ 470 = 0.306383 W = 306.4 mW
That 306.4 mW is more than a common 1/4 W (250 mW) resistor is rated for, so this circuit needs at least a 1/2 W part. It is a good illustration of why the power check matters as much as the resistance value.
A second everyday case is sizing a series resistor for an LED. Suppose a 5 V supply, an LED whose datasheet gives a forward voltage of 2 V, and a target current of 20 mA. The resistor has to drop the difference, 3 V. Enter 3 V and 20 mA and the tool returns R = 150 Ω and P = 60 mW.
Common mistakes
- Mixing up mA and A. 25 mA is 0.025 A. Typing 25 with the unit left on A gives a current a thousand times too high. Always check the unit box, or type the value in base units.
- Ignoring the resistor's power rating. A resistor with the right ohm value can still overheat. Compare the calculated power with the part's rating and leave some headroom, because ratings assume particular ambient conditions and decrease at higher temperatures.
- Using the supply voltage instead of the voltage across the part. In a circuit with several components, use only the voltage across the component you are solving for, as in the LED example.
- Applying it to non-ohmic parts. LEDs, diodes, lamps and motors do not have a fixed resistance. The formulas still hold at a single operating point, but the "resistance" changes with conditions.
- Using P = V × I for AC loads with a power factor below 1. For AC, V × I gives apparent power in volt-amperes. Real power in watts also depends on the power factor.
Use cases
- Choosing resistor values and wattage ratings for electronics projects.
- Working out how much current a heater, bulb or other resistive load draws from its wattage and voltage.
- Checking homework and exam problems in physics and electronics courses.
- Finding an unknown resistance from a measured voltage and current.
For the voltage lost along long cable runs, use the Voltage Drop Calculator, which applies the same V = I × R relationship to the resistance of copper and aluminum wire. The Unit Converter handles other unit conversions, and the Percentage Calculator is useful for tolerance and efficiency figures.
Frequently asked questions
Using I = P ÷ V, 1500 W ÷ 120 V = 12.5 A. Enter 1500 W and 120 V in the calculator to see the same result, along with the heater element resistance of 9.6 ohms when hot. Real current varies a little with the actual outlet voltage.
No. Ohm's law relates voltage, current and resistance with V = I × R. The power formula P = V × I is a separate relationship. Combining the two is what lets you find any two of the four quantities from the other two, which is what this calculator and the formula wheel do.
For purely resistive AC loads such as heaters and incandescent lamps, yes, using RMS voltage and current. With motors, transformers or capacitors, the opposition to current is impedance, which includes reactance, and real power also depends on the power factor. This calculator treats every value as resistive.
k means thousand and M means million, so 4.7 kΩ is 4,700 ohms and 1 MΩ is 1,000,000 ohms. Select kΩ or MΩ next to the resistance box and type the number as printed, and the calculator converts it for you.
Every resistor turns electrical energy into heat at a rate of P = I² × R. If the power exceeds what the part can shed to its surroundings, its temperature rises until it fails or changes value. Calculate the power here and choose a part with a higher rating.
With zero resistance, V = I × R would require infinite current for any nonzero voltage, which is a short circuit. In practice wiring, the source and fuses limit the current. The calculator asks for values greater than zero because the formulas divide by resistance.