Estimate how long a battery lasts from its capacity (mAh) and the device current draw (mA).
Calculated locally in your browser.
How do you calculate battery life?
Battery life (hours) = capacity (mAh) × efficiency ÷ current draw (mA), with capacity and draw in matching units. The efficiency factor (typically 80–90%) allows for heat, voltage conversion and discharge-curve losses, so real runtime is below the ideal. For example, a 4000 mAh battery powering a 500 mA load at 85% efficiency lasts 4000 × 0.85 ÷ 500 = 6.8 hours.
Understanding your result
No battery delivers its full rated capacity in practice — heat, voltage conversion and the discharge curve all reduce usable energy. The efficiency factor (typically 80–90%) accounts for this, which is why the estimated life is lower than the ideal figure.
Formula and method
Battery life (hours) = capacity (mAh) × efficiency ÷ current draw (mA). Capacity and draw must use the same units (mAh and mA) so they cancel correctly.
Worked example
A 4000 mAh battery powering a 500 mA load at 85% efficiency lasts 4000 × 0.85 ÷ 500 = 6.8 hours (about 6h 48m).
How to use this tool
- Enter the battery capacity in milliamp-hours (mAh).
- Enter the device current draw in milliamps (mA).
- Adjust the efficiency factor if needed (default 85%).
- Read the estimated battery life in hours and minutes.
Common mistakes to avoid
- Mixing units — entering Ah and A instead of mAh and mA.
- Using 100% efficiency, which overstates real runtime.
- Ignoring that current draw varies with what the device is doing.
About the Battery Life Calculator
The Battery Life Calculator estimates how long a battery will run a device from its capacity in mAh and the device current draw in mA. An efficiency factor allows for real-world losses, so the figure is closer to what you will actually see.
Who should use this tool
Makers, electronics hobbyists, drone and RC users, and anyone sizing a battery or power bank.
Benefits
- Converts mAh and mA into hours of runtime instantly.
- Adjustable efficiency factor for realistic estimates.
- Shows both the estimated and ideal (100%) runtime.
- Private — nothing leaves your browser.
Practical use cases
- Sizing a power bank for a phone or tablet.
- Estimating runtime for an Arduino or sensor project.
- Comparing battery options for a device.
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Frequently asked questions
How do I convert Ah to mAh?
Multiply amp-hours by 1000. For example, 2 Ah is 2000 mAh. Do the same for current: 0.5 A is 500 mA.
Why is the estimate lower than capacity ÷ draw?
The efficiency factor reduces the usable capacity to reflect heat, conversion and discharge losses, giving a more realistic runtime.
Does this work for power banks?
Yes, though power banks step voltage up to 5V, so use a lower efficiency (around 70–80%) and the device draw at 5V.