How many times will my power bank charge my phone?

Four lines of arithmetic, all shown under the result:

1. Bank energy (Wh) = cell mAh ÷ 1000 × the voltage printed beside it.
2. Out of the bank = bank energy × its conversion efficiency.
3. One charge (Wh) = your battery's Wh × the share you fill (100 % for a full charge, 60 % for 20 to 80 %).
4. Charges = out of the bank ÷ one charge.

The efficiency is the number most labels hide, but many print a rated capacity at 5 V next to the cell capacity. The ratio of the two is the maker's own figure for what comes out. If your label has no such line, pick “Not stated”: you get the ceiling no bank of that size can beat.

One loss stays out of every answer: the charging circuit inside your phone or tablet. The maker spec pages used here don't state it, so the calculator can't count it. It can only lower the real number, which is why each result says “at most”.

The bank's INPUT line says how fast it can take energy in. Multiply each volts-and-amps pair and take the highest: that is its input rating. The cell watt-hours divided by that rating is the floor, the time it would take if every watt went into the cells at full rate, start to finish. The real refill is slower.

• Look for the rated-capacity line before anything else. It already includes the converter’s loss, so you don’t have to guess at it.
• Read the INPUT line for refill time and the OUTPUT lines for your device. They are different numbers, and mixing them up gives nonsense.
• If your device’s maker states the battery in watt-hours, as both tablet spec pages here do, use that figure as it is: it saves the voltage step.
• Count top-ups, not full charges, if you never run the phone flat. A 20 % to 80 % top-up uses 60 % of the battery.
• A charger above the bank’s input rating does not refill it faster. One below the rating does slow it down.
• Same name, different label: one maker’s India page for its 20,000 mAh bank prints no rated-capacity line, while the global page does. Read the label on the model you own.