Power Bank Size Calculator

Power bank capacity is measured in milliampere-hours (mAh). But the rated mAh on the label is not what your device actually receives.

Why you get less than the rated capacity: Power banks store energy at 3.7V (lithium battery voltage). Your phone charges at 5V via USB. Stepping 3.7V up to 5V loses energy on the way, typically 10 to 15% of it. Add heat loss and self-discharge and a good bank lands around 85 to 90%, a budget or ageing one nearer 75%, and a bank fast-charging a phone in a warm room can drop to 65%.

The core formula: Usable mAh = Rated mAh × Efficiency Full Charges = Usable mAh ÷ Device Battery mAh

Choosing the right power bank size:

• 5,000 mAh: Ultra-compact. Fits in a pocket. Gives roughly 1 full phone charge. Good for day trips.
• 10,000 mAh: The everyday carry sweet spot. Provides 2–3 phone charges. Fits in a bag easily.
• 20,000 mAh: Travel size, 4 or 5 charges, still comfortably under the airline limit.
• 26,800 mAh: The largest that boards a plane without asking. Not a coincidence, see below.
• 30,000 mAh and up: Camping and group use, and past the airline limit. Fine in a car, a problem at the gate.

Rough device battery sizes:

• Compact phone: 3,000 to 3,500 mAh
• Standard phone: 4,000 to 5,000 mAh
• Large or “Pro Max” phone: 4,500 to 5,500 mAh
• Tablet: 7,000 to 11,000 mAh
• Wireless earbud case: 300 to 700 mAh
• E-reader: 1,500 to 2,000 mAh

Your exact figure is in the device’s technical specifications, usually printed as a mAh rating alongside the battery.

Fast charging is not free

USB Power Delivery and Quick Charge move energy faster, and they also run the bank and the phone hotter. Heat is loss. A bank pushing 45W into a phone in a warm room delivers noticeably fewer real charges than the same bank trickling in overnight, which is why the efficiency dropdown here goes down to 65%.

The airline limit is the real ceiling on size

Lithium batteries are capped at 100 watt-hours for cabin baggage without airline approval, and they are not allowed in checked bags at all. Convert with Wh = mAh × 3.7 / 1000, so a 26,800 mAh bank is 99.2 Wh and a 30,000 mAh bank is 111 Wh and needs permission. That is precisely why 26,800 mAh is such a common capacity: it is the largest bank that boards a plane without a conversation.

Which size to buy

10,000 mAh is the size most people should own. It clears airport security without a thought, fits a jacket pocket, and covers a long day plus a delayed flight. Go to 20,000 only if you are charging a tablet or covering several days off-grid, and accept that it weighs about twice as much for capacity you will usually carry home unused.