Power Bank Capacity (2026): What mAh Means, Clearly Explained in 5 Essential Facts
Why does a 10,000mAh power bank not give you three full phone charges? Here is what mAh really means and how to calculate how many charges you will get.
Power bank capacity is printed in big numbers on the box, usually in mAh, but that number does not tell you how many times you can charge your phone. A 10,000 mAh power bank will not give you three full charges of a phone with a 3,300 mAh battery, even though the math seems to say it should.
This guide explains what mAh actually measures, why you lose part of the rated capacity, how to convert to watt-hours, and how to work out the real number of charges you will get. It also covers airline rules, output wattage and how to keep a power bank healthy for years.
mAh (milliamp-hours) measures electric charge stored at the battery’s internal voltage, usually about 3.7 volts. Because your phone charges at 5 volts or more, and some energy is lost as heat, you typically get around 60 to 70% of the printed power bank capacity into your device. A 10,000 mAh bank is good for roughly 1.5 to 2 full charges of most phones. For airline travel, the number that matters is watt-hours (Wh), and anything up to 100 Wh is allowed in carry-on bags.
What Is mAh?
mAh stands for milliamp-hours. It is a unit of electric charge. One milliamp-hour means a battery can supply one milliamp of current for one hour. A 10,000 mAh battery could, in theory, supply 10,000 milliamps (10 amps) for one hour, or 1 amp for 10 hours.
The important detail is voltage. mAh on its own does not describe energy, because energy also depends on voltage. Two batteries with the same mAh rating can hold very different amounts of energy if their voltages differ.
Almost every power bank uses lithium-ion or lithium-polymer cells with a nominal voltage of about 3.6 to 3.7 volts. The mAh printed on the box is measured at that cell voltage, not at the 5, 9 or 20 volts your phone or laptop actually receives.
Why You Do Not Get the Full Power Bank Capacity
Three things eat into the rated power bank capacity before any of it reaches your phone.
- Voltage conversion. The bank must boost its 3.7-volt cells up to 5 volts or higher for USB. Boosting voltage lowers the available current, so the mAh at 5 volts is smaller than the mAh at 3.7 volts.
- Heat losses. The boost circuit, the cable and your phone’s own charging circuit all turn some energy into heat.
- Phone charging overhead. Your phone is running, its screen may be on, and its battery management system also uses some energy while charging.
Put together, a good power bank typically delivers around 60 to 70% of its printed capacity to the device. Many brands print a second figure called rated capacity on the label, measured at 5 volts. That number is always lower than the headline mAh and is a more honest guide.
Wireless charging adds even more loss. Magnetic and Qi wireless charging lose more energy as heat than a cable, so a wireless power bank delivers fewer charges from the same power bank capacity.
How Many Charges Will You Get?
The quick rule of thumb is simple: multiply the power bank’s mAh by about 0.65, then divide by your phone’s battery size in mAh.
| Power bank capacity | Usable (approx. 65%) | Phone with 3,500 mAh battery | Phone with 5,000 mAh battery |
|---|---|---|---|
| 5,000 mAh | About 3,250 mAh | About 0.9 charges | About 0.6 charges |
| 10,000 mAh | About 6,500 mAh | About 1.8 charges | About 1.3 charges |
| 20,000 mAh | About 13,000 mAh | About 3.7 charges | About 2.6 charges |
| 26,800 mAh | About 17,400 mAh | About 5 charges | About 3.5 charges |
These numbers are estimates. Phone batteries also use slightly different voltages, often 3.85 volts or more in newer models, so the most accurate way to compare is with watt-hours.
mAh vs Wh (Watt-Hours)
Watt-hours measure energy, which is what you actually care about. To convert, use this formula:
Wh = mAh x volts / 1000
So a 10,000 mAh power bank at 3.7 volts holds 37 Wh. A 20,000 mAh bank holds 74 Wh. A phone with a 4,000 mAh battery at 3.85 volts holds about 15.4 Wh.
Now the math lines up. A 37 Wh bank delivering 65% gives about 24 Wh. Divide by 15.4 Wh and you get roughly 1.5 charges for that phone. That is the real answer to how far your power bank capacity will go.
Watt-hours are also the only sensible way to compare power banks with laptops. Laptop batteries are almost always rated in watt-hours, and many thin laptops sit somewhere between 40 and 100 Wh. A 74 Wh power bank (20,000 mAh) delivering about 65% gives roughly 48 Wh, which is less than one full charge for many laptops.
| Printed capacity | Watt-hours at 3.7V | Allowed in carry-on? |
|---|---|---|
| 5,000 mAh | 18.5 Wh | Yes |
| 10,000 mAh | 37 Wh | Yes |
| 20,000 mAh | 74 Wh | Yes |
| 25,600 mAh | 94.7 Wh | Yes, just under the limit |
| 27,000 mAh | 99.9 Wh | Yes, right at the limit |
| 40,000 mAh | 148 Wh | Only with airline approval |
How to Read a Power Bank Label
The small print on the back of a power bank tells you more than the big number on the front. Here is what each line usually means.
- Capacity (for example 10000mAh 3.7V 37Wh): the cell capacity and its energy in watt-hours. This is the headline power bank capacity.
- Rated capacity (for example 6000mAh 5V/3A): what the bank can deliver at 5 volts after conversion losses. Use this for realistic charge estimates.
- Input: how the bank is recharged, such as 5V/3A, 9V/2.22A or 20W. Higher input means faster refills.
- Output: what each port can deliver, such as 5V/3A, 9V/2.22A or 20V/3.25A (65W).
- Certification marks: look for marks like UL, CE or FCC from a known testing body. Missing or odd marks are a warning sign.
Be wary of listings that promise huge capacity in a tiny, light body. Lithium cells have a fairly fixed energy density, so a 30,000 mAh claim in a pocket-sized bank is very likely inflated.
What Power Bank Capacity Do You Need?
Bigger is not always better. Higher power bank capacity means more weight, more bulk and longer recharge times. Match the size to how you use it.
- 5,000 mAh: about one phone top-up. Pocketable, often magnetic. Best for a day out when your phone just needs a boost.
- 10,000 mAh: one to two phone charges. The most popular size for daily carry and travel.
- 20,000 mAh: three or more phone charges, or a tablet charge. Good for weekend trips and long flights.
- 25,000 mAh and up: for laptops, multiple devices and camping. Heavy, but often the only way to charge a laptop away from an outlet.
If you mainly charge an iPhone, our best power banks for iPhone guide covers magnetic and cable options across these sizes.
Flying With a Power Bank
Airline rules use watt-hours, not mAh, which is another reason to understand power bank capacity in Wh. Under US FAA rules, which match most international airline rules:
- Power banks must go in your carry-on bag, never in checked luggage.
- Up to 100 Wh is allowed without airline approval. That covers nearly all phone power banks up to about 27,000 mAh.
- 100 to 160 Wh needs airline approval, and you are usually limited to two.
- Over 160 Wh is not allowed on passenger planes.
Some airlines add their own rules, such as banning power bank use during the flight or requiring the bank to stay visible. Check your airline before you fly. A power bank without a clear capacity label can be refused at security.
Other Specs That Matter
Power bank capacity is only one part of the picture. These specs decide how fast and how conveniently that capacity reaches your devices.
Output wattage
Watts decide charging speed. 20W is enough to fast-charge most phones. Tablets like 30W or more, and laptops usually need 45W to 100W. Some large banks offer up to 140W or more using USB Power Delivery 3.1.
Charging standards
USB Power Delivery (USB PD) is the universal fast-charging standard for USB-C, used by iPhone, iPad, Pixel and most laptops. Samsung phones also use PPS, a feature of USB PD, for their fastest wired speeds. The USB Implementers Forum maintains the USB PD spec. Our USB-C vs Thunderbolt vs USB4 guide explains how cables affect wattage.
Input speed
A large power bank can take many hours to refill. Look for a USB-C input of 30W or more on 20,000 mAh banks, and pair it with a capable wall charger. See our best USB-C chargers for matching bricks.
Ports and pass-through
Count the ports you need at once. Pass-through charging lets the bank charge your phone while it is itself plugged in, which is handy overnight in a hotel.
Wireless and magnetic charging
Qi2 magnetic power banks snap onto compatible phones and charge without a cable. They are convenient, but they lose more energy as heat, so expect fewer charges from the same capacity.
How to Keep Your Power Bank Healthy
Lithium batteries wear out slowly with use and age. A few habits help a power bank hold its capacity for longer.
- Avoid heat. Do not leave a power bank in a hot car or in direct sun. Heat is the biggest enemy of lithium cells.
- Store it partly charged. For long storage, around half charged is ideal. Top it up every few months, because batteries slowly self-discharge.
- Avoid running it to zero every time. Partial cycles are easier on the cells than full discharges.
- Use good cables and chargers. Cheap uncertified gear can deliver unstable power.
- Retire swollen banks. If the case bulges, gets very hot or smells odd, stop using it and recycle it at a battery drop-off point.
Most lithium-ion power banks are designed for several hundred full charge cycles before their capacity drops noticeably. Banks with lithium iron phosphate (LiFePO4) cells last for many more cycles but are heavier for the same capacity.
Common Mistakes to Avoid
- Expecting the printed mAh to reach your phone. Plan on about two thirds of it.
- Comparing banks by mAh alone. Output wattage and charging standard decide how fast you charge.
- Buying a huge bank for daily carry. A 20,000 mAh bank can weigh close to a pound, so it often stays at home.
- Using a slow wall charger to refill it. A big bank on a 5W cube can take a whole day to recharge.
- Packing it in checked luggage. Power banks belong in your carry-on.
Frequently Asked Questions
What does mAh mean on a power bank?
mAh means milliamp-hours, a measure of stored electric charge at the battery’s internal voltage of about 3.7 volts. It is the headline power bank capacity number, but you get only around 60 to 70% of it into your device.
Is a higher mAh always better?
More mAh means more charges, but also more weight and a longer recharge. Pick the smallest power bank capacity that covers a typical day for you.
Why does my 10,000 mAh power bank only charge my phone about twice?
Because of voltage conversion and heat. The 10,000 mAh is measured at 3.7 volts. After boosting to USB voltage and losing some energy as heat, about 6,000 to 7,000 mAh reaches the phone.
Can a power bank charge a laptop?
Yes, if it has a USB-C PD port with enough wattage, usually 45W or more. Capacity of 20,000 mAh or more is best, and you may still get less than one full charge.
Does cold weather reduce power bank capacity?
Yes, temporarily. Lithium batteries deliver less energy in the cold, so a bank can seem weaker on a ski trip. Capacity returns once the bank warms up. Keep it in an inside pocket in winter.
How do I find a power bank’s watt-hours?
Check the label. Most reputable banks print Wh. If not, multiply the mAh by 3.7 and divide by 1000.
The bottom line
mAh tells you how much charge a power bank stores at its internal voltage, not how much reaches your phone. Expect about 60 to 70% of the printed power bank capacity after conversion and heat losses.
For real comparisons and airline rules, use watt-hours. A 10,000 mAh bank suits daily carry, 20,000 mAh suits travel and tablets, and laptop users should look at 20,000 mAh or more with high-watt USB-C PD output.
