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Ohm's Law Calculator

Solve V, I, R, or P from any two known values

Voltage

12.000 V

V

12.000 V

P

24.000 W

Voltage

12.000 V

Voltage

12.000 V

Current

2.000 A

Resistance

6.000 Ω

Power

24.000 W

Example Calculations

1Find voltage: 2A through 6Ω

Inputs

Current2 A
Resistance6 Ω

Result

Voltage12.000 V
Power24.000 W

V = I × R = 2 × 6 = 12V. P = V × I = 12 × 2 = 24W.

2Find resistance: 5V, 20mA LED

Inputs

Voltage5 V
Current0.02 A

Result

Resistance250.000 Ω
Power100.000 mW

R = V / I = 5 / 0.02 = 250Ω. P = 5 × 0.02 = 0.1W = 100mW.

Frequently Asked Questions

Q

What is Ohm's law?

Ohm's law states V = I × R: voltage equals current times resistance. Combined with P = V × I, you can solve for any electrical quantity from two knowns.

  • V = I × R (voltage = current × resistance)
  • I = V / R (current = voltage / resistance)
  • R = V / I (resistance = voltage / current)
  • P = V × I = I²R = V²/R (power formulas)
Known ValuesFormula for VFormula for IFormula for RFormula for P
V and I——V/IV×I
V and R—V/R—V²/R
I and RI×R——I²×R
P and IP/I—P/I²—
Q

How do I calculate resistance from voltage and current?

Divide voltage by current: R = V / I. For example, 12V across a component drawing 2A means R = 12/2 = 6Ω.

  • R = V / I is the most common form
  • 12V / 2A = 6 ohms
  • If current is in milliamps, convert: 20mA = 0.02A
  • Power check: P = 12 × 2 = 24W
Q

What is electrical power and how is it calculated?

Power (watts) = voltage × current. A 12V supply delivering 2A provides 24W. Power also equals I²R or V²/R.

  • P = V × I (basic formula)
  • P = I² × R (useful when V unknown)
  • P = V² / R (useful when I unknown)
  • 1 watt = 1 joule per second
Q

When does Ohm's law not apply?

Ohm's law applies to linear (ohmic) devices like resistors. Non-linear components like diodes, transistors, and LEDs don't follow V=IR directly—they have voltage-current curves instead.

  • Resistors: always ohmic
  • Diodes: non-linear V-I curve
  • LEDs: need forward voltage concept
  • Capacitors/inductors: impedance varies with frequency

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Last Updated: Mar 20, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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