Ohm's Law Calculator
Instantly calculate Voltage, Current, Resistance, and Power. Just enter exactly two known values to find the others.
An Ohm’s Law calculator determines voltage, current, or resistance using the formula V = I × R. Calculate voltage by multiplying current (amps) by resistance (ohms), calculate current by dividing voltage by resistance, and calculate resistance by dividing voltage by current.
Ohm's law is fundamental to electrical engineering and physics. It describes the relationship between voltage, current, and resistance in an electrical circuit. This calculator extends the basic V = I × R formula to include Joule's Law of electrical power (P = V × I), allowing you to solve for any missing variables.
Voltage (V)
Current (I)
Resistance (R)
Power (P)
How to Use the Ohm's Law Calculator
Follow these simple steps to solve for the missing variables in your electrical circuit.
Identify Known Variables
You must know exactly two of the four variables: Voltage (V), Current (I), Resistance (R), or Power (P).
Enter Your Data
Type the two known values into their respective input fields. Leave the other two fields entirely blank.
Calculate
Click the "Calculate" button. The system will automatically run the appropriate formulas.
Read the Results
The calculator will display the complete set of all four parameters for your circuit.
The Core Formulas
Ohm's Law
- V = Voltage (Volts)
- I = Current (Amperes)
- R = Resistance (Ohms)
Joule's Law (Power)
- P = Power (Watts)
- V = Voltage (Volts)
- I = Current (Amperes)
Ohm's Law & Power Formulas
This table shows the mathematical formulas used by the calculator to solve for any missing parameter.
| To Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| 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 |
Frequently Asked Questions
What is the difference between Voltage, Current, and Resistance?
Think of electricity like water flowing through a pipe connected to a battery:
- Voltage (V) is the water pressure pushing the water through the pipe. A battery or power supply provides this initial push.
- Current (I) is the actual volume of water flowing past a point, measured in amperes.
- Resistance (R) is the pipe's thickness or any obstruction (like a physical component or load) that restricts the water flow, measured in ohms.
Why is Power (Watts) included in Ohm's Law calculations?
While the strict equation for Ohm's Law only covers voltage, current, and resistance, Joule's Law of electrical power (P = V × I) is almost always taught alongside it. Combining the two in this ohm's law calculator allows you to calculate the physical work or heat generated by the circuit in watts. This is crucial for safely sizing wires, fuses, and any connected load.
Does Ohm's Law apply to AC (Alternating Current) circuits?
Yes, but with caveats. The standard formula perfectly applies to DC (Direct Current) circuits driven by a battery and purely resistive AC setups (like heaters or simple light bulbs acting as the load). However, if the AC system contains inductive or capacitive components (like motors or transformers), you must factor in "Impedance" (Z) rather than simple DC resistance, altering the math slightly.
Why did I get an error saying I need exactly two values?
Mathematically, every equation built into this ohm's law calculator requires exactly two known variables to confidently solve for the remaining two. Whether you are using a multimeter to measure voltage and current, or you know the wattage and resistance, you only need those two inputs. If you provide three or four variables, they may contradict each other mathematically. If you provide only one, there isn't enough data to perform a calculation.
Can I calculate resistance if I only have Power and Voltage?
Yes. By rearranging the power formula P = V² / R, you can find resistance using R = V² / P. This is particularly useful for determining the internal ohm rating of a component like a heating element or lightbulb where only the wattage and operating voltage are provided on the label, and you don't have a multimeter handy to test it directly.
Does the physical length of a wire affect Ohm's Law?
Directly, yes. Every physical conductor acts as a small component with its own resistance. Longer wires or thinner wires have higher ohm values. In a circuit, this is known as "Voltage Drop." When performing precise calculations, you must factor in the resistance of the wiring itself—especially between the battery and the load—if the distances are significant.
What units should I use for input?
This ohm's law calculator uses standard base units: Volts (V) for voltage, Amperes (A) for current, Ohms (Ω) for resistance, and Watts (W) for power. If your multimeter gives you readings in milli-units (like 500mA) or kilo-units (like 1.2kΩ), you should convert them to base units first (0.5A or 1200Ω) before entering them into the equation to ensure accurate results.
How does power relate to resistance at a constant voltage?
At a constant voltage, power is inversely proportional to resistance. This means if you decrease the ohms of a circuit, the current in amperes goes up, and the power consumed by the load (and heat generated) will increase. This is why a short circuit (extremely low resistance across the battery terminals) causes a massive surge in watts, leading to blown fuses or fire.