Battery Life Calculator

Estimate how long a battery will last from its capacity in mAh and the device’s average current draw, with a real-world derating factor. Calculates in your browser.

Estimated runtime

-

Total hours-

Show the math

Runtime is capacity divided by load, derated for real-world losses:

hours = (capacity ÷ load) × efficiency

The efficiency factor (often 0.7–0.9) covers losses from heat, voltage sag, and the regulator. All math runs in your browser.

What this does

A battery life calculator estimates how long a battery will last from its capacity in mAh and the device’s average current draw, with a real-world derating factor.

How to use it

  1. Enter the battery capacity in mAh.
  2. Enter the average current draw.
  3. Set a derating factor.
  4. Read the estimated runtime.

How it works

Real batteries lose effective capacity to heat, voltage cutoff, age, and peak draws, so a 0.7–0.85 efficiency factor keeps the estimate realistic.

runtime ≈ capacity ÷ load × efficiency

Understanding your result

The derating factor is what makes this realistic, because a battery never delivers its full rated capacity in practice, it loses to heat, the voltage cutoff, age, and current spikes. Treat the result as an upper-middle estimate and design margin in, especially for devices with brief high-current bursts, which drain a battery faster than their average draw suggests.

Example

A 2,000 mAh battery driving 100 mA lasts roughly 16–17 hours.

Sources & methodology

Last updated .

Frequently asked questions

How is battery life estimated?

Runtime ≈ capacity (mAh) ÷ load (mA), then reduced by an efficiency factor to allow for real-world losses.

Why apply a derating factor?

Real batteries lose effective capacity to heat, voltage cutoff, age, and peak draws, so the ideal figure is optimistic. A 0.7–0.85 factor is a common allowance.

Is my data stored?

No. Everything is calculated locally in your browser.