Battery Life Calculator — mAh and Current Draw
Enter capacity in mAh, current draw in mA and usable capacity percentage. Estimate battery runtime in hours and days for a constant-current load.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimated runtime is usable capacity (mAh) divided by current draw (mA). Capacity and load must use the same voltage basis; this is not a watt-hour conversion. Temperature, battery age, voltage behavior, and load can change actual runtime.
Worked example
calculated with the default values below
Battery capacity 2,000 mAh · Device current draw 100 mA · Usable capacity 85 % Battery life 17.00 hours · Usable capacity 1,700 mAh · Battery life 0.71 days
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
t = C × η ÷ 100 ÷ P
- C
- Battery capacity
- η
- Efficiency %
- P
- Load
- t
- Runtime, hours
Load units must match capacity: A vs Ah, or W vs Wh; η is entered in percent.
Frequently asked questions
Why does the battery last only 85 percent of its rated capacity?
The usable-capacity setting is a planning factor, not a fault. At 2,000 mAh and 85%, the estimate uses 1,700 mAh; a 100 mA load therefore gives about 17 hours. Capacity and current must refer to the same voltage basis, and age, temperature, voltage behavior, and load affect actual runtime.