Quick answer
This Ohm’s law calculator finds voltage, current or resistance using V = IR. Enter any two of the three values and it calculates the third, plus the electrical power P = VI, showing the formula and substitution.
Voltage, current and resistance calculator
How to Use the Ohm’s Law Calculator
- Choose voltage, current or resistance.
- Enter the other two values in volts (V), amperes (A) or ohms (Ω).
- Press Calculate to see the answer and the power used.
Tip: Convert prefixes first: 250 mA = 0.25 A, and 4.7 kΩ = 4,700 Ω.
Ohm’s Law Formulas
| To find | Formula |
|---|---|
| Voltage | V = IR |
| Current | I = V ÷ R |
| Resistance | R = V ÷ I |
| Power | P = VI = I²R = V² ÷ R |
Ohm’s law holds for ohmic conductors at a constant temperature. Learn the full story, including when it fails, in our Ohm’s law guide.
Worked Example
Worked example: a car headlight
Question: A 12 V car battery drives a current of 4.5 A through a headlight bulb. What is the bulb’s resistance and power?
- Choose “Resistance (R)”.
- R = V ÷ I = 12 ÷ 4.5 ≈ 2.67 Ω.
- P = VI = 12 × 4.5 = 54 W.
Answer: The bulb has a resistance of about 2.67 Ω and uses 54 W.
Safety Note
Common mistake: Mains electricity is dangerous. Use these calculations for learning and low-voltage projects, and leave mains wiring to qualified electricians.
Learn more: study Coulomb’s law with formula, units and solved numericals.
Frequently Asked Questions
What is Ohm’s law?
Ohm’s law states that the current through a conductor is proportional to the voltage across it, as long as temperature stays constant: V = IR.
How do I calculate power from voltage and current?
Multiply them: P = VI. For example, 230 V × 2 A = 460 W.
Does Ohm’s law work for every component?
No. Filament lamps, diodes and many other components are non-ohmic, so their resistance changes with current or voltage.