Ohm's Law Calculator
Calculate voltage, current, resistance and power using Ohm's Law (V = IR). Enter any two values to find the other two instantly.
How to use this tool
- Enter voltage (v), current (i) and resistance (r) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your voltage and the full breakdown beneath it.
Ohm's Law states V = I × R: voltage equals current times resistance. Power is P = V × I = I²R = V²/R. Enter any two of V, I, R and the calculator finds the rest.
Formula
Ohm's Law: V = I × R
Derived forms: I = V / R | R = V / I
Power: P = V × I = I2R = V2 / R
Where V = voltage (V), I = current (A), R = resistance (Ω).
How it works
The calculator accepts any two of the three Ohm's Law quantities and solves for the third, then computes power as P = V × I. It detects which value is zero (the unknown) and applies the appropriate rearrangement. Ohm's Law is strictly valid for linear, resistive elements at constant temperature; it does not apply to non-linear components such as diodes, transistors, or devices whose resistance varies with current or voltage.
Worked example
- Known values: Current I = 2 A, Resistance R = 3 Ω; Voltage V is unknown (entered as 0).
- Apply V = I × R = 2 × 3 = 6 V.
- Power P = V × I = 6 × 2 = 12 W.
Voltage: 6.0 V; Current: 2.0 A; Resistance: 3.0 Ω; Power: 12.0 W.
Common mistakes to avoid
- Confusing voltage (V) with power (W) -- plugging wattage into the voltage field gives a completely wrong resistance result.
- Using milliamps (mA) instead of amps (A) without converting first, inflating calculated resistance by a factor of 1000.
- Forgetting that Ohm's Law assumes a linear (resistive) load -- applying it directly to LEDs or diodes, which are non-linear, yields unreliable results.
Key terms
- Voltage (V)
- The electrical potential difference between two points, measured in volts. It is the 'pressure' that drives current through a circuit.
- Current (I)
- The flow of electric charge through a conductor, measured in amperes (A). Defined as charge per unit time.
- Resistance (R)
- The opposition a material offers to the flow of current, measured in ohms (Ω). Higher resistance means less current for a given voltage.
- Power (P)
- The rate at which electrical energy is converted to another form (heat, light, etc.), measured in watts (W). P = V × I.
- Ohm's Law
- The linear relationship V = IR, stating that voltage across a resistor equals the current through it multiplied by its resistance. Formulated by Georg Simon Ohm in 1827.
Frequently asked questions
- What units are used?
- Volts (V) for voltage, Amperes (A) for current, Ohms (Ω) for resistance, and Watts (W) for power. Keep units consistent.
- Does Ohm's Law apply to AC circuits?
- For pure resistors, yes. For capacitors and inductors you need impedance Z (complex resistance), and V = IZ replaces V = IR.