How to convert mAh to Wh
Multiply milliamp-hours by the voltage and divide by 1,000:
Wh = mAh × V ÷ 1,000
Example: a 5,000 mAh phone battery at 3.85 V holds 5,000 × 3.85 ÷ 1,000 = 19.3 Wh.
How to convert Wh to mAh
mAh = Wh × 1,000 ÷ V
Example: a 99 Wh laptop battery at 11.55 V is 99 × 1,000 ÷ 11.55 = 8,571 mAh. Larger batteries are usually rated in amp-hours; the Ah to Wh calculator handles those, and 1 Ah = 1,000 mAh.
Power bank mAh to Wh chart
Power banks are rated at the 3.7 V of their lithium-ion cells. Airline rules for spare lithium batteries (FAA and IATA): up to 100 Wh in carry-on only, 100–160 Wh with airline approval, over 160 Wh not allowed. Power banks are never allowed in checked bags.
| Power bank | Watt-hours | On a plane |
|---|---|---|
| 5,000 mAh | 18.5 Wh | Allowed in carry-on |
| 10,000 mAh | 37 Wh | Allowed in carry-on |
| 20,000 mAh | 74 Wh | Allowed in carry-on |
| 26,800 mAh | 99.2 Wh | Allowed in carry-on |
| 30,000 mAh | 111 Wh | Needs airline approval |
| 40,000 mAh | 148 Wh | Needs airline approval |
| 50,000 mAh | 185 Wh | Not allowed |
The largest power bank you can fly with without approval is about 27,027 mAh at 3.7 V. Many airlines also limit how many you carry and ban using them in flight, so check your airline before you travel. If the label already shows Wh, that figure is the one security staff go by.
Watt-hours of common devices
| Device | Typical capacity | Voltage | Watt-hours |
|---|---|---|---|
| Wireless earbuds case | 500 mAh | 3.7 V | 1.9 Wh |
| Smartwatch | 300 mAh | 3.85 V | 1.2 Wh |
| Smartphone | 5,000 mAh | 3.85 V | 19.3 Wh |
| Tablet | 8,000 mAh | 3.8 V | 30.4 Wh |
| Drone battery | 5,000 mAh | 15.4 V | 77 Wh |
| Laptop battery | 4,500 mAh | 11.6 V | 52 Wh |
Check the label of your own device for exact figures. mAh alone does not compare batteries of different voltages: a 5,000 mAh drone pack holds four times the energy of a 5,000 mAh phone battery, because it has four cells in series.
Why the 5 V USB output does not change the rating
A power bank’s mAh is the capacity of its cells at 3.7 V. When it charges a phone, a converter steps the voltage up to 5 V (or 9–20 V for fast charging), so fewer milliamp-hours come out of the USB port than the label says, even before losses. Watt-hours avoid that confusion, which is why airlines and laptop makers use them. To estimate how long a device runs on a battery, use the battery life calculator for mAh and mA, or the battery runtime calculator for Wh and watts.