Introduction
Our Battery Capacity Calculator estimates how long a battery will last when it powers a device with a known current draw. Enter the battery's capacity and the device's consumption, and the calculator returns the expected runtime in minutes, hours, days, weeks, months, and years.
Battery Capacity Basics
Battery capacity is usually rated in amp-hours (Ah) or milliamp-hours (mAh). It represents how much charge a battery can deliver over time: a 2000 mAh battery can theoretically supply 2000 mA for one hour, or 1000 mA for two hours, and so on. Device consumption is the current the device draws while operating, in amps (A) or milliamps (mA).
Battery Life Formula
Once both values are expressed in the same unit (mAh and mA), the runtime is simply capacity divided by consumption:
Battery Life (hours) = Battery Capacity (mAh) / Device Consumption (mA)
For example, a 2000 mAh battery powering a device that draws 250 mA lasts approximately 8 hours.
Unit Conversions
- 1 Ah = 1000 mAh
- 1 A = 1000 mA
The calculator converts Ah to mAh and A to mA automatically before dividing, so you can mix units freely.
Real-World Factors
This formula gives a theoretical maximum. Real battery life is usually shorter because of factors such as the discharge curve at the actual load (Peukert effect on lead-acid batteries), temperature, battery age and internal resistance, and standby/idle current that isn't captured by a single consumption figure. Use the result as an estimate, not a guarantee.
Using the Calculator
Enter the battery capacity, choose mAh or Ah, enter the device's current consumption, choose mA or A, and press "Calculate". The calculator displays the estimated runtime broken down into multiple time units.
