Ohm's Law Calculator
Ohm's Law Calculator: Voltage, Current, Resistance and Power
An Ohm's law calculator solves the basic relationship between voltage, current and resistance in an electrical circuit. Pick the two values you already know, type them in with their units, and the tool works out the other two. It also shows power in watts, conductance and the energy used in one hour, so one entry gives you the whole picture of the circuit. It is useful for choosing a resistor for an LED, checking the load on a power supply, sizing a heater or simply finishing a homework problem.
Quick answer: a 12 V supply across a 6 Ω resistance drives a current of 2 A and the load uses 24 W.
The Ohm's Law Formula
Georg Ohm showed that for many materials the current is directly proportional to the voltage. The formula is usually written as V = I × R, where V is voltage in volts, I is current in amperes and R is resistance in ohms. Rearranged, it gives current as V ÷ R and resistance as V ÷ I.
Power links in through the second basic rule of circuits. Power in watts equals voltage times current. Substituting Ohm's law gives two more forms, which is why the calculator can start from power and one other value.
Formula Reference Table
Here is every rearrangement the calculator uses. Look up the quantity you want in the first column and pick the formula that matches the values you have.
| Find | From V and I or R | From P and another | From P and R or I |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (P) | P = V × I | P = I² × R | P = V² ÷ R |
Worked Examples
These examples were solved with the same formulas as the tool. Notice how the unit prefixes keep small and large numbers readable.
| You know | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| 12 V and 2 A | 12 V | 2 A | 6 Ω | 24 W |
| 5 V and 220 Ω | 5 V | 22.73 mA | 220 Ω | 113.6 mW |
| 20 mA and 330 Ω | 6.6 V | 20 mA | 330 Ω | 132 mW |
| 60 W and 120 V | 120 V | 500 mA | 240 Ω | 60 W |
| 100 W and 830 mA | 120.5 V | 830 mA | 145.2 Ω | 100 W |
| 250 mW and 1 kΩ | 15.81 V | 15.81 mA | 1 kΩ | 250 mW |
| 230 V and 46 Ω | 230 V | 5 A | 46 Ω | 1.15 kW |
| 3.3 V and 15 mA | 3.3 V | 15 mA | 220 Ω | 49.5 mW |
Units and Prefixes
Electronics rarely uses plain volts, amps and ohms. A sensor might draw a few microamps, an LED runs on tens of milliamps and a pull-up resistor can be tens of kilo-ohms. Each field in the calculator has a unit menu, so you can type the number exactly as it appears on the part or datasheet.
| Prefix | Symbol | Multiplier | Example |
|---|---|---|---|
| milli | m | 0.001 | 20 mA = 0.02 A |
| micro | µ | 0.000001 | 500 µA = 0.0005 A |
| kilo | k | 1,000 | 4.7 kΩ = 4,700 Ω |
| mega | M | 1,000,000 | 1 MΩ = 1,000,000 Ω |
| kilo (power) | kW | 1,000 W | 1.5 kW = 1,500 W |
Choosing a Resistor Power Rating
A resistor turns the power it handles into heat. If the rating is too low the part runs hot, drifts in value and can burn out. Calculate the power first, then pick a standard size at least double that figure. The tip under the results does this for you. Common ratings are 0.125 W, 0.25 W, 0.5 W, 1 W and 2 W for through-hole parts, with larger wire-wound types above that.
Ohm's Law and Everyday Devices
The same idea explains many household numbers. A 230 V kettle element of about 46 Ω draws 5 A and uses 1,150 W, while a 5 V USB port feeding a 1 A device delivers 5 W. If you know a device's rated power and your mains voltage you can find its current, which helps when you check whether a socket or an extension lead can carry the load.
Where Ohm's Law Does Not Apply
The law describes ohmic parts, where resistance stays steady as the voltage changes. Diodes, LEDs and transistors do not behave this way, because their current rises sharply once the voltage passes a threshold. You can still use the formulas on the resistor that sits in series with them. Resistance also changes with temperature, so the figures here are for a steady, normal operating state.
Related Tools
For converting other measurements, try the unit converter or the density calculator. The scientific calculator is handy when a circuit problem needs square roots or powers, and the binary calculator helps with digital logic.
How to Use This Ohm's Law Calculator
Choose the two values you know
Tap a mode such as Voltage and Current or Power and Resistance.
Enter the numbers
Type each value, pick its unit and fine tune it with the slider if you like.
Read the full result
See the missing values, plus power, conductance and energy for one hour.
Copy or start again
Copy the result to your notes, or press Reset to return to the starting example.