Ohm's Law Calculator (V = IR)

GCSE PhysicsAQAEdexcelOCRFoundation & Higher

Find potential difference, current or resistance using V = I × R. Enter milliamps or kilohms if the question uses them and the solver converts to SI units.

Quick answer

Potential difference = current × resistance. Use current in amps and resistance in ohms to get volts. For example, 0.3 A through a 40 Ω resistor gives V = 0.3 × 40 = 12 V. To find resistance, use R = V ÷ I.

By Mehmood Kakar · Updated

V = I × R
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How to use V = IR

Potential difference, current and resistance are linked by one equation. It is probably the most used equation in GCSE electricity:

potential difference = current × resistance

  • V is potential difference in volts (V)
  • I is current in amperes (A)
  • R is resistance in ohms (Ω)

A 40 Ω resistor with 0.3 A flowing through it has a potential difference of 0.3 × 40 = 12 V across it.

Resistance tells you how hard it is for current to flow. For a fixed resistor at constant temperature, current is directly proportional to potential difference, so the I–V graph is a straight line through the origin. That is what makes the resistor an ohmic conductor. Filament lamps and diodes don't behave this way, but V = IR still gives their resistance at any single point on the graph.

Watch the prefixes. Resistances are often given in kilohms (kΩ), so multiply by 1000. Currents are often in milliamps (mA), so divide by 1000.

Common mistakes that cost marks

  1. Not converting kΩ. 4.5 kΩ is 4500 Ω. Multiply by 1000.
  2. Not converting mA. 300 mA is 0.3 A. Divide by 1000.
  3. Rearranging the wrong way. R = V ÷ I, not I ÷ V. A resistance of 0.025 Ω for a 12 V, 0.3 A circuit means you divided the wrong way.
  4. Using the wrong p.d. In a series circuit, use the p.d. across that component, not the supply p.d., unless it is the only component.
  5. Writing the unit as "ohm" with no symbol. Write Ω, or the word ohms. Both are fine; a missing unit loses the mark.

Practice questions

Try each one on paper first, then reveal the worked answer.

Q1A current of 0.3 A flows through a 40 Ω resistor. Calculate the potential difference across it.[2 marks]
  1. V = I × R
  2. V = 0.3 × 40
  3. V = 12 V
Q2A 12 V battery drives a current of 300 mA through a lamp. Calculate the resistance of the lamp.[3 marks]
  1. Rearrange: R = V ÷ I
  2. Convert: I = 300 ÷ 1000 = 0.3 A
  3. R = 12 ÷ 0.3
  4. R = 40 Ω
Q3A potential difference of 9.0 V is applied across a 4.5 kΩ resistor. Calculate the current.[3 marks]
  1. Rearrange: I = V ÷ R
  2. Convert: R = 4.5 × 1000 = 4500 Ω
  3. I = 9.0 ÷ 4500
  4. I = 0.0020 A (2.0 mA)

Frequently asked questions

What is the equation for Ohm's law?

Potential difference = current × resistance (V = IR). Potential difference is in volts (V), current in amps (A) and resistance in ohms (Ω).

How do I calculate resistance?

Divide potential difference by current: R = V ÷ I. For example, 12 V ÷ 0.3 A = 40 Ω.

What is an ohmic conductor?

A conductor where current is directly proportional to potential difference at constant temperature. Its resistance stays the same, so its I–V graph is a straight line through the origin.

Does V = IR work for a filament lamp?

Yes, at any single point. A filament lamp's resistance rises as it heats up, so it isn't ohmic, but V ÷ I still gives its resistance at that particular current.