How many mAh your power bank needs for the charges you want

How the power bank size is calculated

The right power bank holds the energy of every charge you want plus the energy lost while charging. The calculator works in watt-hours (Wh), because the mAh of a phone and the mAh of a power bank are rated at different voltages and can't be added together.

1. Energy of one charge: mAh × V ÷ 1000. A 5,000 mAh phone battery at 3.85 V holds 19.25 Wh. Laptop batteries are already rated in Wh, on the bottom of the computer or in the spec sheet.
2. Energy into the devices: energy of one charge × number of charges, added up for all your devices.
3. Energy from the power bank: some is lost in the cells, in the circuit that steps 3.7 V up to USB voltage, and in the phone as it charges. Anker puts the loss at 30% to 45% and estimates charges with a 0.65 factor, so the calculator starts at 65%. You can change it in the advanced options.
4. Rated mAh: Wh from the power bank ÷ 3.7 V × 1000. The mAh printed on the box refer to the lithium cells inside, at 3.7 V.
5. Size: the smallest of 5,000, 10,000, 20,000 and 26,800 mAh that is enough. If even the largest falls short, the calculator splits the need across several identical power banks.

The phone's voltage is printed on the battery or listed in the spec sheet. If you can't find it, leave 3.85 V: at 3.7 V the energy drops by about 4%.

Worked example: two charges of a 5,000 mAh phone

These are the values the calculator opens with: a phone with a 5,000 mAh battery at 3.85 V, two full charges.

• One charge: 5,000 × 3.85 ÷ 1000 = 19.25 Wh
• Two charges: 38.5 Wh
• From the power bank at 65%: 38.5 ÷ 0.65 = 59.2 Wh
• At 3.7 V: 59.2 ÷ 3.7 × 1000 = 16,009 mAh
• Size to buy: 20,000 mAh, that is 74 Wh, with 25% headroom

A 10,000 mAh power bank (37 Wh) gives this phone about 1.25 charges, not two. The 20,000 mAh pack stays under 100 Wh, so it flies in your carry-on without airline approval.

Which power bank for a phone or a laptop

With 65% of the energy reaching the batteries and mAh rated at 3.7 V:

What you charge	From the bank	mAh at 3.7 V	To buy
5,000 mAh phone, once	29.6 Wh	8,005	10,000 mAh
5,000 mAh phone, twice	59.2 Wh	16,009	20,000 mAh
5,000 mAh phone, 3 times	88.8 Wh	24,013	26,800 mAh
50 Wh laptop, once	76.9 Wh	20,791	26,800 mAh
70 Wh laptop, once	107.7 Wh	29,107	2 × 20,000 mAh

A laptop also needs a power bank with a USB-C Power Delivery output at the wattage the computer accepts, printed on its charger. The mAh alone don't tell you that.

Sizes on sale and the 100 Wh airline limit

Power banks come in a handful of sizes, and the Wh follow from the mAh at 3.7 V:

• 5,000 mAh = 18.5 Wh
• 10,000 mAh = 37 Wh
• 20,000 mAh = 74 Wh
• 26,800 mAh = 99.2 Wh, the largest size that stays under 100 Wh

According to IATA, power banks travel in carry-on baggage only. Up to 100 Wh you don't need the airline's approval, with a limit of 20 spare batteries per person; over 100 Wh up to 160 Wh you need approval and can bring 2 at most; above 160 Wh they can't go as baggage. That's why, when your need goes over 100 Wh, the calculator suggests two or more power banks under the limit instead of one big one. Airlines can set stricter rules, for example on using power banks on board, so check the rules for your flight.

Common mistakes

• Adding up phone mAh and power bank mAh. They're at different voltages: the math only works in Wh.
• Counting on 100% of the capacity. In the Stiftung Warentest test, the 20,000 mAh power banks delivered between 14,760 and 17,680 mAh, before the losses inside the phone.
• Buying a power bank too big for the flight. Over 100 Wh you need the airline's approval: check the Wh printed on the pack, not just the mAh.
• Forgetting the output wattage. A laptop only charges if the power bank reaches the wattage the computer asks for.
• Packing it in checked luggage. A power bank always goes in your carry-on, however small.