Battery Capacity Calculator
Convert battery capacity between mAh and Wh at any voltage (3.7 V lithium-ion by default), see the amp-hour figure, and check whether a power bank clears the FAA's 100 Wh carry-on threshold before you fly.
Example: with Capacity value 20000 · Convert mAh → Wh · Nominal voltage (V) 3.7 → Converted capacity: 74.0 Wh.
- Amp-hours20.00 Ah
- Airline carry-on check74.0 Wh — allowed in carry-on without airline approval (FAA)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Wh = mAh × V ÷ 1,000. A 20,000 mAh power bank at the 3.7 V cell voltage is 74 Wh — under the FAA's 100 Wh carry-on threshold.
mAh is charge; Wh is energy
Milliamp-hours count charge, and charge only becomes energy when you multiply by voltage: Wh = mAh × V ÷ 1,000. That is why mAh comparisons mislead across different batteries. A 20,000 mAh power bank (3.7 V cells) holds 74 Wh; a 5,000 mAh drone pack at 14.8 V holds 74 Wh too — identical energy, wildly different mAh.
It also explains the classic power-bank disappointment: the 20,000 mAh rating is at 3.7 V, but the USB port delivers 5 V. 74 Wh ÷ 5 V is about 14,800 mAh before conversion losses, and closer to 12,000-13,000 mAh after — so a 4,000 mAh phone charges roughly three times, not five.
How it’s calculated
Wh = mAh × V ÷ 1,000; mAh = Wh ÷ V × 1,000; Ah = mAh ÷ 1,000. The default 3.7 V is the nominal lithium-ion cell voltage used on power bank labels. FAA/TSA rules: lithium batteries up to 100 Wh fly in carry-on freely; 101-160 Wh requires airline approval (max two spares); spare batteries are never allowed in checked bags.
Uses nominal voltage — the true average varies with chemistry and load, and usable energy at a USB port runs 10-20% below the cell rating after conversion losses.
Familiar batteries in watt-hours
| Battery | Rating | Energy |
|---|---|---|
| AA alkaline | 2,500 mAh at 1.5 V | ≈ 3.8 Wh |
| Phone battery | 4,000 mAh at 3.85 V | ≈ 15.4 Wh |
| Small power bank | 10,000 mAh at 3.7 V | 37 Wh |
| Large power bank | 20,000 mAh at 3.7 V | 74 Wh |
| Max carry-on laptop pack | ≈ 26,750 mAh at 3.7 V | ≈ 99 Wh (sold to duck the 100 Wh limit) |
| Car starter battery | 50 Ah at 12 V | 600 Wh |
Computed as mAh × V ÷ 1,000 from typical published ratings.
Common mistakes
- Using the 5 V USB voltage for a power bank — labels rate the 3.7 V cells, which is why a 20,000 mAh bank delivers only ~13,000 mAh at the port.
- Comparing batteries of different voltages by mAh; watt-hours is the honest measure of stored energy.
- Packing spare batteries in checked luggage — regulations require spares in carry-on, terminals protected.
- Slipping a factor of 1,000 between mAh and Ah: 20,000 mAh is 20 Ah.
Frequently asked questions
What is the mAh to Wh formula?
Wh = mAh × voltage ÷ 1,000. A 20,000 mAh power bank at 3.7 V is 74 Wh; going the other way, mAh = Wh ÷ voltage × 1,000.
Can I fly with my power bank?
Up to 100 Wh (about 27,000 mAh at 3.7 V) is allowed in carry-on without approval under FAA rules. 101-160 Wh needs airline approval, and spares must never go in checked bags.
Why does my 20,000 mAh bank only charge a 4,000 mAh phone three times?
The bank is rated at 3.7 V but charges your phone through a 5 V (or higher) converter, and conversion plus charging losses eat 20-35%. In energy terms: 74 Wh × ~0.65 ÷ 15 Wh per charge ≈ 3 charges.
Which matters when comparing batteries — mAh or Wh?
Wh, because it accounts for voltage. mAh only compares fairly between batteries of the same voltage, like two 3.7 V power banks.