Physics

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Internal Resistance Calculator.

Calculate source internal resistance from EMF, terminal voltage, and current.

On-device calculationNo signup
01

Set your values

Results update as you type.

Internal resistance: 0.25 Ω

Internal resistance

0.000000Ω
Voltage drop: 0.5 V

Voltage drop

0.000000V

Results update automatically as you type.

Use Cases

Battery Health Assessment

Determine the internal resistance of a battery to evaluate its condition. Higher internal resistance often indicates aging or damage.

Example: A battery with EMF 12V, terminal voltage 11.5V under 2A load has internal resistance of 0.25Ω.

Circuit Design and Analysis

Account for source internal resistance when designing circuits to predict voltage drops and power losses.

Example: Calculate internal resistance to ensure a power supply can deliver required current without excessive voltage drop.

Frequently Asked Questions

What is internal resistance?
Internal resistance is the opposition to current flow within a voltage source, such as a battery or generator. It causes the terminal voltage to drop below the EMF when current flows.
How do I use this calculator?
Enter the source EMF (in volts), the terminal voltage (in volts), and the load current (in amperes). The calculator will compute the internal resistance using the formula: internal resistance = (EMF - terminal voltage) / current.
What units should I use?
Use volts for EMF and terminal voltage, and amperes for current. The result will be in ohms. Ensure all values are positive and consistent.

Tips & Common Mistakes

Tips

  • Ensure the terminal voltage is measured under load, not open-circuit, for accurate results.
  • Use consistent units: volts for voltages and amperes for current.
  • If the load current is zero, internal resistance is undefined; ensure a non-zero current.
  • For real sources, internal resistance may vary with temperature and load; measure under typical operating conditions.

Common Mistakes to Avoid

  • Using open-circuit voltage as terminal voltage instead of measuring under load.
  • Forgetting to convert units (e.g., using millivolts or milliamps without conversion).
  • Entering the load current as the short-circuit current, which can lead to incorrect internal resistance.

Last updated: August 13, 2026