# Internal Resistance Calculator

URL: https://speedycalculator.com/calculators/physics/internal-resistance/
Last updated: 2026-10-30

> The internal resistance calculator finds the internal resistance of a battery or source from its EMF, the load resistance and the current, plus the terminal voltage and power lost inside.

## Inputs

- Electromotive force ε (V, mV, kV, µV, MV)
- Load resistance R (Ω, kΩ, MΩ, mΩ)
- Current I (A, mA, µA, kA)

## Results

- Internal resistance r
- Terminal voltage
- Power lost inside
- Efficiency

## Formula

```
r = ε/I − R,  V_terminal = I·R = ε − I·r
```

## Explanation

Real batteries have internal resistance, so their terminal voltage drops when they deliver current. A weak or cold battery has a higher internal resistance – which is why a car may not start on a frosty morning.

### How to use the Internal Resistance Calculator

- **Electromotive force ε**
- **Load resistance R**
- **Current I**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the EMF (open-circuit voltage), the load resistance and the measured current.

### Inputs explained

FieldWhat to enterElectromotive force εUnits: V, mV, kV, µV, MVLoad resistance RUnits: Ω, kΩ, MΩ, mΩCurrent IUnits: A, mA, µA, kA

### What each result means

- **Internal resistance r** = [Electromotive force ε]÷[Current I] - [Load resistance R]
- **Terminal voltage** = [Current I]×[Load resistance R]
- **Power lost inside** = [Current I]^2×([Electromotive force ε]÷[Current I] - [Load resistance R])
- **Efficiency** = [Load resistance R]÷([Electromotive force ε]÷[Current I])×100

### Example values

How the main result changes when **Electromotive force ε** changes and everything else stays at the example values above:

Electromotive force ε (V)Internal resistance r (Ω)6.3-0.959.45-0.42512.60.115.750.62518.91.1525.22.2

## Worked example

Inputs — Electromotive force ε: 12.6 V; Load resistance R: 2 Ω; Current I: 6 A.

Results — **Internal resistance r = 0.1 Ω**; Terminal voltage = 12 V; Power lost inside = 3.6 W; Efficiency = 95.2381 %.

## Key terms

- **EMF**: Electromotive force, the open-circuit voltage.

## FAQ

**How do I measure EMF?**
Measure the voltage with no load connected.

**Why does terminal voltage drop under load?**
Current through the internal resistance causes a voltage drop.

**What is maximum power transfer?**
When the load equals the internal resistance.

**Does internal resistance change?**
It rises with age and at low temperatures.


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)