Power banks are sold on capacity figures, and the relationship between that number and how many times you can charge a phone is considerably less direct than it appears.
Several factors reduce the usable output, and they're consistent enough to estimate.
What the number means
Capacity is generally stated in milliamp-hours, which describes charge rather than energy.
Energy is charge multiplied by voltage. Lithium cells typically operate around 3.7 volts nominal, so a stated capacity at that voltage corresponds to a particular energy content.
Your phone's battery is also rated at cell voltage. But the transfer between them doesn't happen at cell voltage.
The conversion losses
Charging over USB requires converting from the pack's cell voltage up to 5 volts or higher, then the receiving device converts back down to its own cell voltage.
Each conversion loses energy as heat. The combined losses are substantial.
Typical real-world efficiency for a power bank charging a phone lands somewhere in the region of 60 to 80 percent depending on the quality of the electronics, the charging rate and the temperature.
Which means a pack rated at a given capacity delivers meaningfully less to the device than a simple comparison of the two capacity figures would suggest.
A rough estimate: divide the stated capacity by your phone's battery capacity, then multiply by around 0.7. That gets considerably closer than the naive calculation.
The other reductions
Self-discharge. A pack loses charge while sitting unused, at a low rate that accumulates over months.
Ageing. Capacity declines with cycles and with calendar time. A pack several years old holds meaningfully less than when new.
Temperature. Cold reduces available capacity substantially. A pack that performs adequately indoors may deliver considerably less outdoors in winter.
Fast charging. Higher rates generate more heat and are less efficient. Charging slowly delivers more total energy from the same pack.
Overstated ratings
Beyond legitimate losses, capacity ratings from some manufacturers are simply inaccurate.
Independent testing of power banks has repeatedly found products delivering well below their stated capacity, sometimes dramatically so.
Extremely cheap products with implausibly high stated capacities are the usual offenders, and physics provides a check: energy density of lithium cells has practical limits, so a very high capacity in a very small light package is not possible.
Weight is a useful heuristic. Cells have mass, and a pack claiming large capacity while being notably light is claiming something that cannot be true.
Reading a specification properly
What to look for beyond the headline number.
Energy in watt-hours. More informative than milliamp-hours because it accounts for voltage. Required on packaging for air travel purposes and frequently in small print.
Output power in watts. Determines charging speed. A high-capacity pack with low output power charges slowly regardless of its capacity.
Supported charging protocols. Fast charging requires both devices to support a common protocol. A pack without the right protocol charges at the basic rate however capable it is.
Pass-through charging. Whether it can charge a device while itself being charged. Useful and not universal.
Certification marks. Genuine safety certification matters for a device containing substantial stored energy.
The air travel constraint
Worth knowing because it catches people out.
Lithium batteries in checked baggage are prohibited by most airlines. Power banks must travel in the cabin.
There are limits on capacity, expressed in watt-hours, with packs above a threshold generally requiring airline approval and those above a higher threshold prohibited entirely.
The specific limits vary by carrier and jurisdiction. A pack without a clearly marked watt-hour rating may be refused, which is a practical reason to prefer products that state it.
Looking after them
Same principles as any lithium battery.
Avoid heat, which is the main determinant of ageing.
Don't store fully charged or fully flat for long periods. Around half charge is the recommendation for extended storage.
Charge occasionally if unused, since a pack left flat for a long period can drop into a state from which it won't recover.
That last point accounts for a great many power banks that stopped working. They were bought, used twice, and left in a drawer for two years.
Choosing one
For most people, moderate capacity from a reputable manufacturer beats maximum capacity from an unknown one.
Consider what you actually need. A pack sized for two phone charges is considerably lighter and cheaper than one sized for six, and most people carrying the larger one never use most of it.
And check the output power against what your devices can accept. A laptop requires substantially more than a phone, and a pack that can't deliver it will charge slowly or not at all.
Charging a laptop
A specific case where the numbers matter more than for phones.
Laptops require considerably more energy and higher power delivery than phones. A pack that charges a phone several times may not charge a laptop once, and may not charge it at all if the output power is insufficient.
The requirement is both capacity and output wattage, and the second is the one people miss. A laptop expecting a certain power level may charge extremely slowly from a lower-output pack, or may drain while in use rather than charging.
Checking your laptop's power requirement and matching it against the pack's rated output, rather than only comparing capacity figures, is the step that prevents disappointment.