Technically divide the Wh of the portable charger by the Wh of the device (e.g. phone) and multiply by about 0.9.
The issue is Wh is often not obvious on the device (e.g. phone) but some searching can find it. It is also in general very hard to know the Wh of an Anker portable charger, it is hard to search.
mAh and Wh are totally different because W(atts) is V(olts) times A(mps). So as the Volts inside the Portable charger is not necessarily the same as the Volts inside the device, if you simply divide mAh you'd be off usually. So when you only have mAh figures, use 2/3rds as a rough guide.
Here's an example where we know the Wh, from iphone:
- random product iphone 7 plus
- I searched for its specs, 1st hit was this
- I see it is a 11.1 Wh
Take say a Powercore 5000 Slim it is a 19.25Wh.
19.25/11.1*0.9 = 1.56
So you'd expect about 1.5 full recharges.
Here's an example from Android, where we may not know the Wh:
- Samsung S8
- quick search shows its battery is 3000mAh, we do not know its Wh in this case.
Say a Powercore 10000. Suppose we didn't know its Wh.
So then 10000 / 3000 * 2/3 = 2.2
So in this example I'd expect a little over 2 full recharges.