Power Bank Calculator — How Many Charges Will You Really Get?

 

Quick answer: a 10,000mAh power bank charges a typical 5,100mAh phone about 1.4 times — not twice. The mAh printed on the box is measured at a different voltage, and roughly 30% of the energy is lost in conversion. Use the free calculator below to get the honest number for your phone and your power bank.

Your phone Enter my phone's battery size manually Samsung Galaxy S25 — 4,000 mAh Google Pixel 9a — 5,100 mAh Apple iPhone 16 — 3,561 mAh Apple iPhone 15 — 3,877 mAh Typical Android phone — 5,000 mAh Phone battery capacity (mAh)

Find yours in Settings > Battery, or on the maker's spec page.

Power bank size

5,000 mAh — pocket size 10,000 mAh — most popular 20,000 mAh — travel size 26,800 mAh — flight maximum Custom size

Real-world efficiency: 70%

Energy is lost when the power bank converts its voltage up to USB level and again inside your phone. 70% is a fair real-world average; cheap no-name power banks are often closer to 60%.

1.4

full charges for your phone (real-world estimate)

7,000

Usable mAh

37.0

Watt-hours (Wh)

37%

Left after 1 charge

Carry-on friendly — under the 100Wh airline limit

Why don't you get the full capacity?

The mAh printed on a power bank is measured at its internal battery voltage (3.7V). Your phone charges at 5V or higher, so the power bank must convert the voltage up — and roughly 30% of the energy is lost as heat in that conversion and in your phone's own charging circuit. That is physics, not a scam — but it means the honest way to compare power banks is by usable capacity, which is exactly what this calculator shows.

• Quick rule: real charges ≈ power bank mAh × 0.7 ÷ your phone's mAh.
• Flights: airlines allow power banks up to 100Wh in carry-on (about 26,800mAh). Bigger ones need airline approval or are banned.
• Buying tip: for most phones, 10,000mAh is the sweet spot — about 1.5–2 real charges in a pocketable size.

Which power bank size should you buy?

5,000mAh — an emergency top-up, good for about one partial charge. 10,000mAh — the everyday sweet spot; see our hands-on comparison of the Anker Nano 10K vs UGREEN Nexode 20K. 20,000mAh — travel and multi-device days; compare the Baseus Blade 20000mAh vs UGREEN Nexode 20000mAh. Prefer something magnetic and pocketable? Read TORRAS MiniMag vs Anker MagGo. Hiking light? Our roundup of the 5 best ultralight power banks for hiking covers the smallest serious options.

Frequently asked questions

How many times will a 10,000mAh power bank charge my phone?
For a typical 5,000mAh phone, about 1.4 full charges in real-world use — not the 2 charges the box implies — because roughly 30% of the energy is lost in voltage conversion.

Why is a power bank's usable capacity less than its rated capacity?
Rated capacity is measured at the internal 3.7V. Charging happens at 5V or more, and converting up loses energy as heat; the phone's own charging circuit loses a little more. A 70% real-world efficiency is a fair planning number.

Can I take a 20,000mAh power bank on a plane?
Yes. A 20,000mAh power bank is about 74Wh, under the 100Wh carry-on limit that applies to almost all airlines. Power banks must go in your carry-on bag, never in checked luggage.

What size power bank do I need for a weekend trip?
One phone for two days: 10,000mAh is enough for most people. Two phones, or a phone plus earbuds and a watch, or long days away from outlets: choose 20,000mAh.

By Tech Gear Hub — budget tech deals & honest reviews. Calculator figures are estimates for guidance; your results vary with cable quality, temperature and battery age. Last updated: November 10, 2026.