Ohm's Law Calculator
Calculate voltage, current, or resistance using Ohm's Law. Enter any two values to solve for the third, plus power output.
V
A
Ω
Voltage
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Current —
Resistance —
Power —
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A
Ω
Voltage (V)
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Power (W) —
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V
A
Ω
Voltage (V)
—
Current (A) —
Resistance (Ω) —
Power (W) —
Power (kW) —
Energy in 1 hr (Wh) —
Wire Resistance (Ω) —
Wire Voltage Drop (V) —
How to Use This Calculator
- Select what you want to solve for (Voltage, Current, or Resistance).
- Enter the two known values.
- Click Calculate to see the missing value and power output.
Formula
V = I × R | I = V ÷ R | R = V ÷ I
Power: P = V × I = I² × R = V² ÷ R
Example
Example: V = 12 V, R = 4 Ω → I = 3 A, P = 36 W.
Frequently Asked Questions
- Ohm's Law states that Voltage (V) = Current (I) × Resistance (R). It describes the linear relationship between voltage, current, and resistance in a conductor at constant temperature. Georg Ohm published this relationship in 1827. The law holds for most resistive materials (ohmic conductors) but does not apply to non-linear devices like diodes, LEDs, or transistors. The three forms are: V = I × R (solve for voltage), I = V ÷ R (solve for current), and R = V ÷ I (solve for resistance). Enter any two values to find the third. The calculator also computes power automatically.
- To find current, use I = V ÷ R. Divide the voltage (in Volts) by the resistance (in Ohms) to get current in Amperes. For example, a 12 V battery connected to a 4 Ω resistor draws I = 12 ÷ 4 = 3 A of current. In this calculator, select "Current (I)" from the Solve For dropdown, enter the voltage and resistance values, and the current is calculated instantly. Common mistake: if resistance is 0 (a short circuit), current would be infinite — real circuits have some minimum resistance in the wiring.
- To find resistance, use R = V ÷ I. Divide voltage (Volts) by current (Amperes) to get resistance in Ohms (Ω). For example, if a component operates at 9 V and draws 0.5 A of current, its resistance is R = 9 ÷ 0.5 = 18 Ω. Select "Resistance (R)" in the Solve For dropdown and enter the known voltage and current values. This is useful for finding the resistance of an unknown component in a circuit, or for verifying that a resistor has the correct value before installation.
- This calculator uses SI electrical units throughout. Voltage is measured in Volts (V). Current is measured in Amperes (A), sometimes called amps. Resistance is measured in Ohms (Ω). Power is measured in Watts (W). For practical electronics, common prefixes apply: milliamps (mA) = 0.001 A, kilohms (kΩ) = 1,000 Ω, megaohms (MΩ) = 1,000,000 Ω, milliwatts (mW) = 0.001 W, kilowatts (kW) = 1,000 W. Convert to base units before entering values.
- Electrical power is calculated from any two of the three Ohm's Law variables: P = V × I (voltage × current), P = I² × R (current squared × resistance), or P = V² ÷ R (voltage squared ÷ resistance). All three formulas give the same result for any consistent set of values. For example, V = 12 V, I = 3 A, R = 4 Ω: P = 12 × 3 = 36 W. Check: P = 3² × 4 = 36 W. Check: P = 12² ÷ 4 = 36 W. The calculator computes and displays power automatically for any solved combination.