Ohm's Law Calculator — Voltage, Current, Resistance & Power
Ohm's Law (V = I × R) relates voltage, current, and resistance in any electrical circuit. Enter any two known values and this calculator instantly solves for the third — plus computes the power dissipated in watts using P = V × I.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the voltage (V) across a conductor equals the current (I) through it multiplied by its resistance (R): V = I × R. It is one of the most fundamental relationships in electrical engineering and physics, describing how voltage, current, and resistance interact in a circuit.
How do I calculate voltage using Ohm's Law?
To calculate voltage, multiply current by resistance: V = I × R. For example, if 2 amperes flow through a 6-ohm resistor, the voltage across it is 2 × 6 = 12 volts.
How do I calculate current using Ohm's Law?
To calculate current, divide voltage by resistance: I = V / R. For example, a 9-volt battery connected to a 450-ohm resistor produces a current of 9 / 450 = 0.02 amperes (20 milliamperes).
How do I calculate resistance using Ohm's Law?
To calculate resistance, divide voltage by current: R = V / I. For example, if 12 volts produces a current of 2 amperes, the resistance is 12 / 2 = 6 ohms.
What is the power formula in Ohm's Law?
Power (in watts) is calculated as P = V × I. It can also be expressed as P = I² × R or P = V² / R. For example, a circuit with 12 V and 2 A dissipates 12 × 2 = 24 watts of power.
What units are used in Ohm's Law?
Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). Sub-units are common in electronics: milliamperes (mA = 0.001 A), kilohms (kΩ = 1000 Ω), milliwatts (mW), and kilowatts (kW).
Does Ohm's Law apply to all materials?
Ohm's Law applies to ohmic materials where resistance stays constant regardless of voltage and current, such as most metals at constant temperature. It does not directly apply to non-ohmic devices like diodes, transistors, and light bulbs whose resistance changes with current or temperature.
How do I calculate the resistor for an LED using Ohm's Law?
Subtract the LED's forward voltage from the supply voltage to get the voltage drop across the resistor, then divide by the desired current: R = (Vsupply − Vforward) / I. For a 5 V supply, 2 V LED, and 20 mA current: R = (5 − 2) / 0.02 = 150 Ω. Use the nearest standard resistor value.
Ohm's Law Formula
Ohm's Law states that the voltage across a conductor is directly proportional to the current through it, with resistance as the constant of proportionality. The three fundamental forms of the formula are:
Voltage (V)
V = I × R
Example: 2 A through 6 Ω → 2 × 6 = 12 V
Current (I)
I = V / R
Example: 9 V across 450 Ω → 9 / 450 = 0.02 A (20 mA)
Resistance (R)
R = V / I
Example: 12 V with 2 A → 12 / 2 = 6 Ω
Power Formula
Power (measured in watts) describes how much energy a circuit consumes per second. It can be derived from any two of the three Ohm's Law variables:
P = V × I
Example: 12 V × 2 A = 24 W
P = I² × R
Example: (2 A)² × 6 Ω = 24 W
P = V² / R
Example: (12 V)² / 6 Ω = 24 W
How to Use This Calculator
- Select what you want to solve for — choose Voltage, Current, or Resistance using the mode buttons at the top.
- Enter the two known values— fill in the two input fields. Units are shown next to each field (V, A, or Ω).
- Read the result instantly — the calculator shows all four values: Voltage, Current, Resistance, and Power.
- Copy the result — click the Copy button to copy all four values to your clipboard.
Common Applications
LED Circuits
Use Ohm's Law to calculate the current-limiting resistor for an LED. For a 5 V supply, an LED forward voltage of 2 V, and desired current of 20 mA: R = (5 V − 2 V) / 0.02 A = 150 Ω.
Motor Control
Determine the stall current or running current of a DC motor. Knowing the motor's winding resistance and supply voltage, you can calculate the maximum current draw and size your fuse accordingly.
Heating Elements
Calculate the power output of a resistive heating element. A 10 Ω element on a 120 V mains supply draws 12 A and dissipates 1440 W (1.44 kW) of heat.
Voltage Dividers
Design voltage dividers for sensor circuits and analog-to-digital converter inputs by applying Ohm's Law across each resistor in the chain.
Quick Reference Table
Common voltage and resistance combinations with calculated current and power:
| Voltage (V) | Resistance (Ω) | Current (A) | Power (W) |
|---|---|---|---|
| 5 | 100 | 0.05 | 0.25 |
| 9 | 470 | 0.0191 | 0.172 |
| 12 | 1000 | 0.012 | 0.144 |
| 24 | 220 | 0.109 | 2.618 |
| 120 | 60 | 2 | 240 |
| 240 | 10 | 24 | 5760 |
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