# Battery Life Calculator

URL: https://speedycalculator.com/calculators/everyday-life/battery-life/
Last updated: 2026-10-25

> The battery life calculator estimates how long a battery will last from its capacity in mAh or Wh and the device’s average current draw, with a derating factor for real-world efficiency.

## Inputs

- Battery capacity (mAh)
- Average current draw (mA)
- Efficiency (%)

## Results

- Estimated runtime
- In hours

## Formula

```
Runtime = capacity ÷ load current × efficiency
```

## Explanation

Real batteries rarely deliver their full rated capacity, especially at high currents or low temperatures.

### How to use the Battery Life Calculator

- **Battery capacity**
- **Average current draw**
- **Efficiency**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Divide capacity by current draw, then multiply by the efficiency factor.

### Tips

- Use the figures printed on your product or its manual when you have them.
- Round quantities up so you are never short.
- Measure twice before you buy anything cut to size.
- Everything runs in your browser – try different values to compare options.

### Common mistakes to avoid

- Mixing units (inches and centimetres, mAh and Ah).
- Ignoring efficiency losses and safety margins.
- Treating rules of thumb as exact.

### Inputs explained

FieldWhat to enterBattery capacityIn mAhAverage current drawIn mAEfficiencyIn %

### What each result means

- **Estimated runtime** = [Battery capacity]÷[Average current draw]×[Efficiency]÷100
- **In hours** = [Battery capacity]÷[Average current draw]×[Efficiency]÷100

## Worked example

Inputs — Battery capacity: 3,000 mAh; Average current draw: 150 mA; Efficiency: 85 %.

Results — **Estimated runtime = 17 h 0 min**; In hours = 17.

## FAQ

**How do I convert Wh to mAh?**
mAh = Wh × 1000 ÷ voltage.

**Why use efficiency?**
Converters, temperature and battery age reduce usable capacity.

**What formula does the Battery Life Calculator use?**
Runtime = capacity ÷ load current × efficiency.