Calculating your estimated battery life is straightforward once you know the Battery Life formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Battery Life Calculator.
What is Battery Life?
The Battery Life calculation tells you your estimated battery life from a few simple inputs. The figure you are solving for here is the estimated battery life.
The Battery Life formula
The core formula is:
Estimated battery life = Battery capacity ÷ Device current draw × Efficiency factor ÷ 100Here is what each input means:
- Battery capacity — a value measured in mAh. Example: 3,000 mAh.
- Device current draw — a value measured in mA. Example: 200 mA.
- Efficiency factor — a percentage, such as an annual rate. Example: 7%.
How to calculate it step by step
- Write down the battery capacity (for example, 3,000 mAh).
- Write down the device current draw (for example, 200 mA).
- Write down the efficiency factor (for example, 7%).
- Apply the formula above to get your estimated battery life.
- Double-check the result with the Battery Life Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Battery capacity | 3,000 mAh |
| Device current draw | 200 mA |
| Efficiency factor | 7% |
| Estimated battery life | 10.50 |
With battery capacity of 3,000 mAh, device current draw of 200 mA and efficiency factor of 7%, the estimated battery life works out to 10.50.
Example 2
With battery capacity of 6,000 mAh, device current draw of 200 mA and efficiency factor of 7%, the estimated battery life works out to 21.00.
| Result | Value |
|---|---|
| Estimated battery life | 21.00 |
Example 3
With battery capacity of 1,500 mAh, device current draw of 200 mA and efficiency factor of 7%, the estimated battery life works out to 5.25.
| Result | Value |
|---|---|
| Estimated battery life | 5.25 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Battery Life Calculator does it instantly, for free, with the formula and a worked example built in.
Related calculators
Continue exploring electrical calculators with these tools: Wire Resistance Calculator, Watts to Amps Calculator, kVA to kW Calculator, kWh to Joules Calculator, Resistance from Power Calculator.