Ohm's Law Calculator
Calculate voltage, current, resistance using V = IR
Ohm's Law Calculator tool
Results
Voltage (V)
0.00
Voltage (mV)
0.00
Voltage (kV)
0.00
Power (P = V × I)
0.00 W
Ohm's Law Calculator: key facts
- What it does
- Calculate voltage, current, resistance using V = IR
- Category
- Science Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
About the Ohm's Law Calculator
Ohm's Law is the fundamental relationship in electrical circuits: voltage equals current times resistance (V = IR). The Ohm's Law Calculator solves for any of these three variables when you know the other two, supporting multiple unit systems for voltage (V, mV, kV), current (A, mA, kA), and resistance (Ω, kΩ, MΩ). It also calculates power automatically using P = VI.
Students solve electronics homework problems, electricians design and troubleshoot circuits, and engineers calculate component values for PCB design. The calculator handles unit conversions automatically and displays results in multiple units simultaneously for easy comparison.
Because calculations run entirely in your browser using JavaScript, you can solve circuit problems without any data leaving your device. The tool clearly explains the relationship between voltage, current, resistance, and power in the FAQ section.
How to use it
- Select what you want to calculate from the dropdown: voltage, current, or resistance.
- Fill in the two electrical quantities you know, such as the supply voltage and the resistance.
- Match each input to its unit — volts, millivolts, or kilovolts; amps or milliamps; ohms, kilohms, or megohms.
- Click Calculate to solve for the unknown variable using V = IR.
- View the result plus the calculated power (P = VI) displayed in multiple units for convenience.
Ohm's law, plus the power it dissipates
Voltage, current, and resistance are related by V = IR, and the tool rearranges that for whichever quantity you are missing. Alongside the answer it also reports power, calculated as voltage times current, because in practice the power figure is what tells you whether a component will survive.
That second calculation is the one people forget. A resistor chosen correctly for resistance can still fail if it cannot dissipate the heat, so knowing that a circuit draws a quarter of a watt versus two watts determines which physical part you need. Power ratings are why a correctly-valued resistor sometimes burns out.
All three of the standard rearrangements are available, and the underlying relationship is linear — double the voltage across a fixed resistance and the current doubles, while the power quadruples, since it depends on both.
- The defaults — 12 volts across 6 ohms — give 2 amperes of current and 24 watts of power.
- A standard LED wanting 20 mA from a 5 V supply with a 2 V forward drop needs a 150 Ω resistor dissipating just 60 mW.
- Solving for resistance: 240 V drawing 10 A implies 24 Ω and 2.4 kW — roughly a domestic kettle.
Why use this version
- Circuit calculations run locally in your browser—no data is sent to any server, and the tool works offline for secure, private electrical design work.
- Solves for any of the three variables (voltage, current, resistance) when you know the other two, and automatically calculates power as a bonus.
- Handles electrical unit conversions automatically, working with volts/millivolts/kilovolts, amps/milliamps/kiloamps, and ohms/kiloohms/megaohms.
- Displays voltage, current, resistance, and power in multiple units at once, covering the conventions used in different regions and industries.
Where Ohm's law does not hold
Ohm's law describes ohmic conductors — components whose resistance stays constant regardless of the voltage applied. Plenty of real components are not ohmic. A diode conducts almost nothing below its forward voltage and then conducts heavily; a filament lamp's resistance rises substantially as it heats; thermistors are designed to change resistance with temperature. Applying a single resistance value to any of those gives a figure valid only at one operating point.
The relationship as used here also assumes direct current. Alternating-current circuits containing capacitance or inductance need impedance rather than plain resistance, which is frequency-dependent and involves a phase relationship between voltage and current. Power in AC circuits additionally depends on the power factor, so P = VI overstates real power wherever voltage and current are out of phase.
A safety note: mains voltages are dangerous and the arithmetic being straightforward does not make the work so. Anything involving household wiring should be left to a qualified electrician.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, expressed as V = IR, where V is voltage, I is current, and R is resistance. This fundamental relationship underlies all DC circuit analysis.
Which voltage, current, and resistance units are supported?
The calculator supports volts, millivolts, and kilovolts for voltage; amperes, milliamperes, and kiloamperes for current; and ohms, kilohms, and megohms for resistance. Power is calculated and displayed in watts.
Can I calculate power as well?
Yes, the calculator automatically calculates power (P = VI) along with voltage, current, and resistance. Power tells you how much energy the circuit consumes or delivers, measured in watts.
What are the practical applications of Ohm's Law?
Ohm's Law is used extensively in electronics for circuit design, troubleshooting, and analysis. It helps calculate component values like resistors, determine current flow through branches, and ensure safe operating conditions by preventing excessive current.
What is the relationship between power and Ohm's Law?
Power can be calculated using P = VI (power equals voltage times current). Using Ohm's Law to substitute V = IR, you can also express power as P = I²R or P = V²/R. These alternative forms are useful when you know different combinations of variables.
