🔋 Battery Life Calculator
Estimate roughly how many hours or days a battery will power your device based on its capacity and your device’s average current draw.
mAh
mA
%
What This Calculator Does
This tool gives a general estimate of how long a battery will power a device, based on the battery’s rated capacity, the device’s average current draw, and a usable-efficiency factor that accounts for real-world losses.
The Formula
Runtime in hours is calculated as (battery capacity in mAh × usable efficiency%) / average current draw in mA. The efficiency percentage matters because batteries rarely deliver their full rated capacity in practice — voltage regulation cutoffs, temperature, battery age, and converter losses typically leave only 80-90% of the rated capacity usable.
A Few Notes
- Average current draw varies a lot by usage pattern — a device idling draws far less than one under heavy load, so consider using a blended average.
- Battery capacity often degrades over time and with age, reducing real-world runtime below the rated spec.
- Provided for general planning and educational purposes, not a guaranteed runtime figure.
Last reviewed August 2026