Calculators
Ohm's Law Calculator
Solve voltage, current, resistance, and electrical power.
Ohm's Law Calculator tool
Loading tool…
Jump to a section
About This Tool
Ohm's law connects voltage, current, and resistance in a simple resistive circuit: voltage equals current multiplied by resistance. This calculator rearranges that relationship to solve for any of those three quantities and also calculates electrical power from voltage and current. It is useful for learning basic circuit relationships, checking hand calculations, and exploring how changing one quantity affects another. Calculations run locally in your browser.
How To Use It
- Choose whether you want to calculate voltage, current, resistance, or power.
- Enter the two requested known values using volts, amperes, and ohms as labeled.
- Read the calculated result and formula. You can copy the result for notes or another calculation.
- Treat the result as an ideal circuit calculation; verify component ratings and real-world electrical safety separately.
Examples
Find voltage
A current of 2 A through a 6 Ω resistance gives V = I × R = 2 × 6 = 12 V.
Find current
With 12 V across a 6 Ω resistance, I = V ÷ R = 12 ÷ 6 = 2 A.
Find resistance
If 12 V produces 2 A, R = V ÷ I = 12 ÷ 2 = 6 Ω.
Find power
At 12 V and 2 A, P = V × I = 24 W. This is the electrical power represented by those voltage and current values.
Useful Notes
Ohm's law formula
For an ideal ohmic resistance, V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Rearranging gives I = V / R and R = V / I.
Electrical power
Electrical power is P = V × I in watts. Combining this with Ohm's law can also produce P = I²R or P = V²/R when the same ideal resistive assumptions apply.
Zero and invalid inputs
Resistance must be greater than zero when it is used as a divisor, and current must be non-zero when calculating resistance. The calculator reports invalid or missing numeric inputs instead of producing an undefined result.
Ideal model and real circuits
Ohm's law in this form is a basic steady-state model. Real components can change resistance with temperature, have tolerances, or behave nonlinearly. AC circuits may require impedance and phase rather than resistance alone.
Electrical safety
A mathematically correct result does not establish that a circuit, component, wire, battery, power supply, or mains connection is safe. Use appropriate ratings, protection, procedures, and qualified guidance for hazardous electrical work.
FAQ
What are the units in Ohm's law?
Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (Ω), and electrical power in watts (W).
Can I use Ohm's law for AC circuits?
The simple V = IR form is directly suitable for purely resistive relationships. General AC circuits can involve impedance, reactance, RMS values, phase, and frequency, which this calculator does not model.
Why can't resistance be zero when calculating current?
The current formula divides voltage by resistance. Dividing by zero is undefined, and a real source and conductor also have physical limitations not represented by an ideal zero-resistance model.
Does this calculator tell me what resistor wattage to buy?
No. It calculates ideal electrical relationships. Selecting a component rating requires suitable safety margin, temperature and environment considerations, tolerances, and the component manufacturer's specifications.