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01Power bank calculator
A power bank will charge your phone about 0.65 times its mAh divided by your phone's battery mAh. So a 10,000 mAh bank gives a typical iPhone around 1.7 full charges over a cable, not the 2.6 the raw numbers suggest.
The calculator assumes about two-thirds of the bank's rated capacity reaches your phone over a cable, and a little under half when you charge wirelessly. Those are typical real-world figures, not best cases. A very good bank and a cool phone can beat them. An old bank in a hot car won't.
02Why you never get the full mAh
The mAh on the box describes the cells inside the bank at their own voltage, about 3.6 to 3.7 volts. Getting that energy into your phone costs some of it along the way:
- Voltage conversion. The bank boosts 3.7 V up to 5 V, 9 V or more for USB output. Every conversion turns some energy into heat.
- The phone's own charging circuit. Your phone steps the voltage back down to charge its battery, losing a bit more.
- Cables and connections. Small losses, bigger with long, thin or worn cables.
- Heat and age. Batteries deliver less when they're hot, cold or a few years old.
Full charges ≈ bank mAh × 0.65 ÷ phone mAhExample: 10,000 × 0.65 ÷ 3,700 ≈ 1.8 charges for an iPhone 17.
Look at the small print on the back of your bank. Many list a rated capacity at 5 V alongside the headline cell capacity. That lower rated figure is a more honest guide to what comes out of the USB port. For the units themselves, see what mAh means and mAh vs Wh.
03Charges by phone and power bank size
Here's the same math for popular current phones, using each maker's published or labeled battery size and the 0.66 cable factor the calculator uses.
| Phone | Battery | 5,000 mAh bank | 10,000 mAh bank | 20,000 mAh bank |
|---|---|---|---|---|
| iPhone Air | 3,149 mAh | 1.0 | 2.1 | 4.2 |
| iPhone 17 | 3,692 mAh | 0.9 | 1.8 | 3.6 |
| iPhone 17 Pro | 4,252 mAh | 0.8 | 1.6 | 3.1 |
| iPhone 17 Pro Max | 5,088 mAh | 0.6 | 1.3 | 2.6 |
| Galaxy S26 | 4,300 mAh | 0.8 | 1.5 | 3.1 |
| Galaxy S26 Ultra | 5,000 mAh | 0.7 | 1.3 | 2.6 |
| Pixel 10 | 4,970 mAh | 0.7 | 1.3 | 2.7 |
Notice the pattern: a 5,000 mAh pocket bank is roughly one full charge for a small iPhone and about two-thirds of a charge for a big Android. If you want at least one full refill of a Pro Max or Ultra, start at 10,000 mAh.
04Wireless vs cable: how much you lose
Magnetic and wireless power banks are lovely to use: snap it on, keep scrolling. But wireless charging loses more energy as heat than a cable, and some of that warmth ends up in your phone. Switch the calculator to "Wireless / magnetic" and the number of charges drops by roughly a quarter.
Cable
- About two-thirds of capacity reaches the phone
- Faster, with a USB-C PD bank
- Phone stays cooler
Magnetic / wireless
- A little under half reaches the phone
- No cable to snag
- Slower, and runs warmer
Most magnetic banks also have a USB-C port. Use the magnet when convenience matters and the cable when you need every percent, like on a long travel day. Our magnetic vs regular power bank comparison goes further.
05Get more charges from the bank you have
Banks wear out like phone batteries do, so an older one will fall short of the calculator's numbers. See how long power banks last for the signs it's time to retire one.
06Which size should you buy?
Match the bank to how long you're away from an outlet, not to the biggest number you can find. Bigger banks are heavier, slower to recharge, and above about 27,000 mAh they cross the 100 Wh airline line. Check the rules for flying with a power bank before you buy for travel.
| Bank size | Good for | Typical full charges |
|---|---|---|
| 5,000 mAh | A night out, a pocket backup | About 1 for a small iPhone, less for big phones |
| 10,000 mAh | Daily carry, commutes, weekend trips | About 1.3 to 2 |
| 20,000 mAh | Long travel days, festivals, sharing | About 2.5 to 4 |
From pocket-size magnetic banks to 20,000 mAh all-dayers. All under 100 Wh, so carry-on friendly.
Questions people ask
How many times will a 10,000 mAh power bank charge my phone?
About 1.5 to 2 times for most phones over a cable. Smaller iPhones get closer to 2; big-battery phones like a Pro Max or Galaxy Ultra get about 1.3. Wireless charging lowers that by roughly a quarter.
How many times will a 20,000 mAh power bank charge an iPhone?
Roughly 3 to 4 full charges for an iPhone 17 or iPhone Air over a cable, and about 2.6 for an iPhone 17 Pro Max. Results drop if you use the phone heavily while it charges or charge wirelessly.
Why doesn't my power bank give the full mAh?
The mAh rating describes the cells at about 3.7 volts. Converting that to USB voltage and then back down inside your phone loses energy as heat. Typically around two-thirds of the rated capacity actually reaches the phone through a cable.
Does a power bank charge a phone slower than a wall charger?
Not necessarily. A power bank with USB-C Power Delivery at 20W or more can charge an iPhone about as fast as a 20W wall charger. Older USB-A banks and magnetic banks are usually slower.
Is a 5,000 mAh power bank enough?
For one evening or an emergency top-up, yes. It gives about one full charge to a smaller iPhone and around 60 to 80 percent to most larger phones. For a full day away from outlets, go for 10,000 mAh.