Battery Life Calculator

Work out how long a battery lasts, and how many full charges a power bank really delivers once conversion losses are counted.

Battery capacity (mAh)

Battery voltage (V)Phone cells and most power banks: 3.6–3.85 V

Device power draw (W)Idle phone ~0.5–1 W, video streaming ~2–3 W, laptop 10–60 W

Conversion efficiency (%)Power banks typically lose 10–35% to heat and voltage conversion

Your phone's battery (mAh, for power-bank charges)

Estimated runtime–

–Energy stored

–Usable after losses

–Full phone charges

How the maths works

mAh alone does not tell you how much energy a battery holds, because it ignores voltage. Energy in watt-hours is mAh × V ÷ 1000. A 10,000 mAh power bank at 3.7 V stores 37 Wh.

When a power bank charges a phone it boosts its cell voltage to 5 V or more for the USB cable, and the phone converts it back down. Each step turns some energy into heat, so only part of the 37 Wh reaches your phone. That is why a 10,000 mAh bank does not give two full charges to a 5,000 mAh phone.

Runtime is simply usable watt-hours divided by the device's draw in watts. Real devices vary their draw constantly, so treat the result as a planning estimate.