Power bank calculator: how many charges you get, and if it can fly

mAh is not energy, so we convert to watt-hours. A power bank’s mAh is counted at its battery cells’ voltage, about 3.7 V, and a phone battery’s mAh at about 3.85 V, so the two can’t be compared directly. We turn both into watt-hours (Wh = mAh × volts ÷ 1,000). A 20,000 mAh bank holds 20,000 × 3.7 ÷ 1,000 = 74 Wh. An iPhone 17 Pro battery of 4,252 mAh holds 4,252 × 3.85 ÷ 1,000 = 16.4 Wh. Tablets and laptops are listed in Wh by their makers, so we use that figure as it is.

Not all of that energy reaches your phone. Charging loses some to voltage conversion and heat. We assume 65% gets in over a cable and 50% over magnetic wireless charging. These are assumptions, not lab results: expect about 60 to 70% over a cable and roughly half over magnetic wireless, and a bit less with a cheap cable, a thick case or a phone you use while it charges. Charges = the bank’s Wh × that share ÷ the device’s Wh.

Two ways to count a charge. The main number is a full charge from 0 to 100%, which is how most people ask the question. We also show top-ups from 20% to 100%, which is how phones are really used and gives about a quarter more. A bank that gives 0.8 of a full charge or more can take a phone from 20% back to full.

Where the battery sizes come from. Phone batteries are the published capacities in BeastCompare’s phone database, each linked to its phone page. The Nintendo Switch 2 battery comes from Nintendo, and the iPad Air and MacBook Air energy from Apple’s tech specs. Pick “Custom” to type your own device’s mAh. Laptops also need real USB-C power output: about 30 W or more from the power bank to charge while you use them. That is a rule of thumb: with less, a laptop may charge very slowly, or not at all, while you work. Only phones charge wirelessly, so for tablets, laptops and consoles we use the cable figure.

The plane rule (FAA, for US flights). Spare lithium batteries, which includes power banks, must go in your carry-on bag, never in checked luggage. Up to 100 Wh (about 27,000 mAh) is allowed with no approval. From 100 to 160 Wh (about 27,000 to 43,000 mAh) you need the airline’s approval and can carry two at most. Over 160 Wh is not allowed on passenger planes. Other countries are similar but not identical, and some airlines now limit using a power bank during the flight, so check your airline before you fly. The Wh printed on the bank is the number that counts.