Most power bank manufacturers lie about how many times their power banks can charge certain devices. It turns out that you can actually calculate this yourself.

Apply the formula:
Power Bank capacity x Conversion Rate x Phone Battery health rate / Phone battery capacity.

Let’s take Merpower YN-050P 20000mAh power bank as a test example.
The iPhone X has a battery size of 2,716mAh.
20000 x 0.8 x 0.8 / 2716 = 4.7 charge times

When you use your phone while charging, there will be deviations in the data, and the number of times your power bank 20000mAh would reduce.

Theoretically, the capacity will reduce to 80% after 300 charging cycles. If so, a 20000mAh power bank may have a 16000mAh capacity. This is also relevant to the temperature when you’re using your power bank. And this point I believe you could find out on the manual when you brought the power bank.

More charging cycles the power bank has taken, its capacity will correspondingly reduce a little though you don’t need to care about that.