Battery Life Calculator
Estimate how long a battery will run a device from its mAh rating and the current it draws, or how many times a power bank can charge your phone.
Batteries age, lose some energy in converters and shouldn't be run completely flat; 70% to 90% is realistic.
Battery life
16 h
- In days
- 0.67
- Usable capacity
- 2,400 mAh
A 3,000 mAh battery powering 150 mA lasts about 16 h.
How it's worked out
- Battery life
3,000 mAh × 80% ÷ 150 mA16 hours
- If you only know the device's power in watts, its current in mA is watts ÷ volts × 1,000.
The formulas
Runtime (h) = mAh × usable % ÷ mA · Energy (Wh) = mAh × V ÷ 1,000
Charges = power bank Wh × efficiency ÷ phone Wh
Examples
A 3,000 mAh battery powering 150 mA, with 80% usable, lasts 16 hours.
A 10,000 mAh power bank at 3.7 V stores 37 Wh. At 65% efficiency, 24.05 Wh reaches a phone with a 4,500 mAh, 3.85 V battery (17.3 Wh): about 1.4 full charges.
Questions people ask
How do I calculate battery life?
Battery life in hours = capacity (mAh) × the usable share ÷ current draw (mA). A 3,000 mAh battery at 150 mA with 80% usable lasts 16 hours.
How many times will a 10,000 mAh power bank charge my phone?
Usually about 1.4 times for a 4,500 mAh phone. The power bank's mAh is at 3.7 V, and about a third of the energy is lost in conversion and charging.
Why doesn't a 10,000 mAh power bank give two 5,000 mAh charges?
Its 37 Wh drops to about 24 Wh by the time it reaches the phone, because of the step up to 5 V USB, cable losses and heat inside the phone.
Can I take a power bank on a plane?
Most airlines allow power banks up to 100 Wh in hand luggage, never in checked bags. A 10,000 mAh bank is 37 Wh.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem