Ohms Law Calculator
Calculate voltage, current, resistance, and power using Ohms Law
Flow of electric charge
Opposition to current flow
Rate of energy transfer
Voltage
12000 mV
Current
2000.0 mA
Resistance
6 Ω
Power
24000 mW
Ohm's Law Relationships
How it works
Ohm's law ties together the three basic quantities in an electrical circuit: voltage, current, and resistance. Voltage equals current times resistance, and rearranging it lets you find any one from the other two. Power follows from voltage times current.
Ohm's law & power
V = I · R P = V · I
- V
- voltage (volts)
- I
- current (amps)
- R
- resistance (ohms)
- P
- power (watts)
Worked example
- Current I = 2 A
- Resistance R = 6 Ω
- V = I × R = 2 × 6
- P = V × I = 12 × 2
V = 12 volts, drawing P = 24 watts.
Good to know
- Rearrange as needed: I = V ÷ R and R = V ÷ I.
- Power can also be written P = I²R or P = V² ÷ R.
- Ohm's law applies to resistive (ohmic) components; semiconductors and reactive parts behave differently.
Related Calculators
Frequently Asked Questions
What is Ohm's law?
Ohm's law states that voltage equals current times resistance: V = I × R. Given any two of the three quantities, you can find the third — I = V ÷ R and R = V ÷ I are the rearranged forms.
How do I calculate electrical power?
Power in watts is P = V × I, and combining it with Ohm's law gives the equivalent forms P = I²R and P = V²/R. A 12 V circuit drawing 2 A dissipates 24 W.
What units does Ohm's law use?
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). Watch for prefixes in real circuits — milliamps (mA) and kilohms (kΩ) are common, so convert before applying the formulas.
Does Ohm's law apply to every component?
No — it holds for ohmic components like resistors, where resistance stays constant. Diodes, LEDs, and transistors are non-ohmic, and even a bulb's filament changes resistance as it heats up. For those, use the component's characteristic curve instead.
How do I size a resistor for an LED?
Subtract the LED's forward voltage from the supply voltage and divide by the desired current: R = (Vsupply − Vf) ÷ I. For a 5 V supply, a 2 V LED, and 20 mA: R = 3 ÷ 0.02 = 150 Ω. Round up to the nearest standard value to stay under the current limit.