Last updated: October 2026
Power bank sizes are listed in mAh, a number that sounds precise but rarely translates directly into “how many charges.” This guide explains what mAh really means, why you get less than the label suggests, and which size fits your phone, earbuds, tablet or laptop.
What Does mAh Mean on a Power Bank?
mAh (milliamp-hours) measures how much charge the power bank’s internal battery cells can store. Bigger numbers mean more stored energy, but the figure is measured at the cells’ low internal voltage (about 3.7V), while USB devices are charged at 5V or higher. That conversion is the first reason you never get the full number out.
Why Do You Get Less Than the Rated Capacity?
Voltage conversion, heat and charging circuitry all lose energy. In practice, about 60–70% of the rated capacity reaches your device with a cable, and less with wireless charging, which loses extra energy as heat. That’s why a 10,000mAh bank won’t charge a 5,000mAh phone battery twice.
Which Power Bank Size Do You Need?
Match the size to your device and how long you’ll be away from an outlet. Phones need 5,000–10,000mAh, tablets 10,000–20,000mAh, and laptops 20,000mAh or more with a high-wattage USB-C port.
| Capacity | Typical use | Roughly |
|---|---|---|
| 5,000mAh | Evening top-up, earbuds | Partial to near-full phone charge |
| 10,000mAh | Full day out, commuting | About 1–2 phone charges |
| 20,000mAh | Weekend trips, tablets | About 3–4 phone charges |
| 20,000mAh+ (65W+) | Laptops | Partial to full laptop charge |
How Do You Estimate How Many Charges You’ll Get?
Multiply the power bank’s mAh by about 0.65 for wired charging (or about 0.5 for wireless), then divide by your phone’s battery capacity. The result is a realistic estimate of full charges.
Example: 10,000mAh × 0.65 = 6,500mAh usable. For a phone with a 3,500mAh battery, that’s roughly 1.8 charges wired, or about 1.4 charges wirelessly.
What Else Matters Besides Size?
- Output wattage: 20W+ USB-C PD fast-charges phones; laptops need 45–100W.
- Weight: 20,000mAh banks are heavy for pockets.
- Magnetic charging: convenient for iPhones, especially Qi2 models.
- Travel limits: keep power banks in carry-on luggage.
For iPhone users, magnetic packs between 5,000 and 10,000mAh are the most practical; see our picks for the best MagSafe power bank.
Frequently Asked Questions
Is 10,000mAh enough for a day?
For most smartphone users, yes. A 10,000mAh power bank usually provides about one to two full phone charges, enough to get through a long day of heavy use or travel.
Is 5,000mAh enough for an iPhone?
It’s enough for a substantial top-up, often taking an iPhone from low to most of the way full with a cable. With magnetic wireless charging, you’ll get a bit less because of heat losses.
What size power bank do I need for a laptop?
Look for at least 20,000mAh with a USB-C Power Delivery port rated for your laptop’s wattage, often 45W to 100W. Check your laptop charger’s wattage and match or exceed it.
Why does my power bank charge my phone fewer times than expected?
Because energy is lost converting from the battery’s internal voltage to USB voltage and as heat. Typically only 60–70% of the rated capacity reaches the device, less with wireless charging.
Is a bigger power bank always better?
No. Bigger banks are heavier, take longer to recharge and may exceed airline limits if very large. Choose the smallest size that covers your typical day or trip.
The Bottom Line
A 10,000mAh power bank suits most people, giving one to two phone charges in a pocketable size. Go smaller for light top-ups and larger for trips, tablets and laptops. iPhone users can skip the cable with our best MagSafe power bank picks.