Battery Life Calculator
Enter a battery's capacity in mAh or Ah and the load your device draws to get its run time in hours, plus a derating factor for real-world losses.
Use this free battery life calculator to work out how long a battery lasts from its capacity and the load it drives. Enter the capacity in mAh or Ah, set the device load in mA or A (or as watts and volts), and read the run time in hours and days. As a battery run time calculator it divides capacity by load, then trims the result by a derating factor because no cell delivers its full rated mAh. Everything runs in your browser, nothing uploaded.
Battery & load
No battery delivers its full rated capacity. Around 85% is a sensible default; drop it for cold weather, high drain or an aged cell.
Estimated run time
8h 30m
8.5 hours · 0.35 days
- Ideal run time
- 10 h
- With derating
- 8.5 h
- Effective load
- 235.29 mA
- Actual load
- 200 mA
Run time is the battery capacity divided by the load, then trimmed by the derating factor. A 2000 mAh cell driving a steady 200 mA lasts about 10 hours on paper, or roughly 8.5 hours once you allow for real-world losses.
This assumes a constant current draw and a fixed capacity. Real drain varies with the device, and capacity falls with temperature, age and discharge rate, so treat the figure as a planning estimate rather than a guaranteed run time.
How it works
- 1
Enter the capacity
Type the battery capacity and switch between mAh and Ah to match what is printed on the cell or pack.
- 2
Set the load
Enter the current the device draws in mA or A, or switch to watts and volts and it works out the current for you.
- 3
Read the run time
See the run time as hours, a plain Xh Ym figure and days, with the derating factor applied.
Instant & 100% private — nothing is uploaded
Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.
Frequently asked questions
- How do I calculate battery life from mAh?
- Divide the battery capacity in mAh by the load in mA to get the run time in hours. A 2000 mAh battery driving a 200 mA load lasts about 10 hours on paper, before any losses.
- What is the derating factor for?
- A real battery never hands over its full rated capacity. Voltage sag, internal resistance and cutoff thresholds all cost you some, so the calculator multiplies the ideal run time by a derating factor, around 0.85 by default, for a more honest figure.
- Can I enter the load in watts instead of amps?
- Yes. Switch the load input to watts and volts and it converts power to current for you: current in mA is the watts divided by the battery voltage, times a thousand.
- How do I convert Ah to mAh?
- Multiply amp-hours by 1000. A 2 Ah cell is 2000 mAh. The capacity toggle does this for you, so you can enter whichever figure the label shows.
- Why does my battery last less than the calculator says?
- Cold temperatures, an aged cell, a high discharge rate and idle background drain all shorten real run time. Lower the derating factor to reflect a tired battery or a demanding load, and treat the result as a planning estimate.
- Does a higher discharge rate change the capacity?
- It can. Many batteries deliver less usable capacity when drained quickly, an effect described by Peukert's law. This calculator uses a flat derating factor rather than modelling that curve, so pull the factor down for high-drain devices.
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