Approximately 30-40% capacity of all batteries is lost in circuit heat and voltage conversions. Thus the battery is able to supply 60%-70% power to other devices.
Formula: (Capacity of the battery *0.6~0.7) / Capacity of your device = Number of possible charges
Eg: how many charges 5200mAh battery could add to iphone 6(1810mAh)? --- 1.7-2 times
the available discharging capacity is related to the discharge current, our portable charger can discharge 80% of cell capacity under the rated maximum output current, so you can get your answer by this calculation, if your phone's cell is 1800mA (6.67Wh), and a 5200mAh battery (18.7Wh), with 80% discharged capacity is 14.9Wh, 14.9Wh/6.67Wh=2.24 times. so one 5200mAh battery can charge around 2.5 times of full charge for iPhone 6 (because during the charging cycle, the charging current won't stay at maximum current)