How to use the power bank calculator
- Enter the power bank capacity. Type the mAh printed on the bank, or tap a preset such as 10,000 or 20,000.
- Enter your phone's battery size. Use the mAh from your phone's spec sheet, for example 4,500, or pick the closest preset.
- Choose an efficiency. Leave it on 75% for a realistic everyday figure, or pick 85% for a quality USB-C PD power bank.
- Read the charges and flight check. The big number is how many full phone charges you get. The badge shows whether the bank is within the 100 Wh carry-on limit.
How many times can a power bank charge a phone?
A power bank never delivers its full rated capacity to your phone. The mAh figure on the label is measured at the cell voltage, usually 3.7 V, while the USB port outputs 5 V or more, and some energy is lost as heat during the conversion. This calculator converts both batteries to watt-hours, applies a realistic efficiency and tells you how many full charges to expect.
Quick answers for popular sizes
For a 4,500 mAh phone at 75% efficiency, a 10,000 mAh bank gives about 1.6 charges, a 20,000 mAh bank about 3.2 charges and a 30,000 mAh bank about 4.8 charges. Change the phone and efficiency above to see your own numbers.
mAh to Wh, and why airlines use Wh
Watt-hours measure energy regardless of voltage, so airlines use them for lithium battery limits. Multiply mAh by the voltage and divide by 1,000: a 20,000 mAh bank at 3.7 V is 74 Wh. A 100 Wh limit works out to roughly 27,000 mAh at 3.7 V. Rules differ by airline and country and have tightened recently, so always check your carrier. Read the power bank capacity guide for the details.
Frequently asked questions
For a 4,500 mAh phone at 75% efficiency, a 10,000 mAh power bank gives about 1.6 full charges. A smaller 3,500 mAh phone gets about 2 charges, and a 5,500 mAh phone about 1.3.
Capacity is rated at the internal cell voltage, about 3.7 V, but the output is 5 V or higher, and conversion and cable losses waste some energy. A 10,000 mAh bank is about 7,400 mAh at 5 V before losses and roughly 4,800 to 6,300 mAh after them.
Multiply the mAh by the voltage and divide by 1,000. A 10,000 mAh bank at 3.7 V is 37 Wh, and a 20,000 mAh bank is 74 Wh.
At about 74 Wh it is under the common 100 Wh limit, so it is normally allowed in carry-on baggage. Power banks must never go in checked luggage, and many airlines now restrict in-flight use or the number you can carry, so check your airline.
Up to 100 Wh is allowed in carry-on without approval, roughly 27,000 mAh at 3.7 V. Between 100 and 160 Wh you need airline approval, and above 160 Wh is not allowed on passenger aircraft.
Divide the bank's watt-hours by the charger's watts times about 0.85. A 37 Wh bank on an 18 W charger takes about 2.4 hours, and slightly longer because charging slows near full.