Battery Life Calculator

How to use this calculator

1. Enter the battery capacity in mAh.
2. Enter the load current in mA.
3. Set the usable capacity % (70–85% is realistic).
4. Click Calculate for the estimated runtime.

The formula

Capacity and load must use matching units (mAh with mA, or Ah with A). The usable capacity factor accounts for the fact that you rarely get the full rated capacity in practice — converter losses, high discharge rates and not fully draining the cell all reduce it. A value of 70–85% is a realistic starting point; use 100% only for a theoretical maximum.

Example

A 2000 mAh battery driving a 150 mA load at 85% usable capacity lasts about (2000 ÷ 150) × 0.85 ≈ 11.3 hours.

Frequently asked questions

How do I calculate battery life?

Runtime ≈ (capacity ÷ load current) × usable fraction. With matching units (mAh and mA, or Ah and A).

Why can't I use the full rated capacity?

Converter losses, high discharge rates and not fully draining the cell reduce the usable capacity — 70–85% is realistic.

How long does a 2000 mAh battery last at 150 mA?

About 11 hours assuming 85% usable capacity: (2000 ÷ 150) × 0.85 ≈ 11.3 h.