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mAh to Wh Calculator With the FAA carry-on check

Convert a battery’s mAh rating to watt-hours, or watt-hours back to mAh, then see where it falls against the FAA’s limits for spare lithium-ion batteries.

Works in both directions
Any voltage, with 3.7 V filled in
Checks the FAA’s 100 Wh and 160 Wh bands

Built by Marcus Reed, laptop and desktop repair technician

mAh ↔ Wh calculatorWh = mAh × V ÷ 1000

Convert

Capacity (mAh)As printed on the battery or power bank

Voltage (V)The voltage printed next to the mAh rating

Example

Watt-hours

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Use the voltage printed with the rating. A mAh figure means nothing on its own: the same mAh at a higher voltage stores more energy. Airline limits apply to the Wh figure, and airlines can set stricter rules than the FAA.

V × Ah= Wh, the whole formula
74 Whin a 20,000 mAh pack at 3.7 V
100 WhFAA limit for spare lithium-ion batteries
160 Whupper band, with airline approval

Quick answer

To convert mAh to Wh, multiply the mAh rating by the battery’s voltage and divide by 1,000. A 20,000 mAh power bank rated at 3.7 V holds 74 Wh. Going the other way, mAh equals Wh times 1,000 divided by the voltage. The calculator above does both directions and checks the result against the FAA’s limits for spare lithium-ion batteries.

01 / 05

The formula

Watt-hours = milliamp-hours × volts ÷ 1,000. The FAA states the same relationship on its lithium battery page: multiply the battery voltage by the amp-hours to get Wh. Since 1,000 mAh is 1 Ah, dividing by 1,000 handles the unit change.

mAh at 3.7 V	Wh	FAA band
5,000 mAh	18.5 Wh	100 Wh or less
10,000 mAh	37 Wh	100 Wh or less
20,000 mAh	74 Wh	100 Wh or less
26,800 mAh	99.16 Wh	100 Wh or less
30,000 mAh	111 Wh	101 to 160 Wh
40,000 mAh	148 Wh	101 to 160 Wh

The table assumes 3.7 V. Use the voltage on your own label: a battery listed at a higher voltage holds more energy for the same mAh.

02 / 05

The most common mistake: using 3.7 V for every battery

3.7 V is a fair figure for a single lithium-ion cell, which is what phones and most power banks use inside. Laptop, camera and drone packs wire several cells in series, so their labels show a higher voltage, and converting them at 3.7 V gives a number that is far too low.

Label says	Right answer	Wrong answer at 3.7 V
Laptop pack, 11.4 V, 4,000 mAh	45.6 Wh	14.8 Wh
Laptop pack, 15.2 V, 3,500 mAh	53.2 Wh	12.95 Wh
Camera battery, 7.2 V, 2,000 mAh	14.4 Wh	7.4 Wh
Power bank, 3.7 V, 20,000 mAh	74 Wh	74 Wh

Two rules cover almost every label. Use the voltage printed next to the mAh figure on the same label, not one from the charger. And if the label already shows Wh, use that number as it is; for air travel, the Wh on the label is the one that counts.

03 / 05

How many times will a power bank charge my phone?

Fewer times than the big mAh number suggests. The headline figure on a power bank is the capacity of its cells at about 3.7 V, but the energy has to be raised to 5 V or more for the USB port, and some is lost as heat on the way. Many labels print a second figure, often called rated capacity, measured at the USB output. That one is closer to what actually reaches the phone.

Find the rated capacity on the power bank label and the output voltage it was measured at, for example 12,000 mAh at 5 V. Convert it: 12,000 × 5 ÷ 1,000 = 60 Wh that can leave the bank.
Convert the phone battery to Wh. A 4,500 mAh phone battery at 3.87 V holds about 17.4 Wh. If you can’t find the phone’s battery voltage, 3.7 V gives a rough figure.
Divide: 60 ÷ 17.4 is about 3.4 full charges. Expect a little less in practice, because the phone also loses some energy as heat while it charges.

If your power bank only lists the cell figure, the same steps with 74 Wh would give about 4.2 charges, but that ignores the conversion to 5 V, so the real number will be lower. The rated capacity is the better guide whenever the label has one.

Power bank cells also wear like any lithium battery, so an older bank gives fewer charges than it did new. On r/anker, an owner described a 24,000 mAh bank that began shutting off with 47% still showing after about 122 cycles. If a bank starts dying early with charge left on its display, it is likely worn out.

04 / 05

What the FAA allows for spare batteries

The FAA’s PackSafe page on lithium batteries sets these rules for spare (uninstalled) lithium-ion batteries and power banks:

Spares and power banks must go in carry-on bags only. If a bag is checked at the gate, take the spares out and keep them with you.
Lithium-ion batteries are limited to 100 Wh each.
With airline approval, a passenger may carry up to two spare larger batteries of 101 to 160 Wh.
Battery terminals must be protected from short circuit, and damaged or recalled batteries must not be carried.

The FAA also notes that many airlines set stricter limits on how many power banks each passenger may bring, whatever their Wh rating, so check with the airline before you fly. Batteries installed in a laptop or phone fall under the FAA’s separate entry for portable electronic devices.

05 / 05

From Wh to runtime

Watt-hours also tell you how long a laptop battery lasts: divide the usable Wh by the watts the laptop draws. The battery life calculator does that, and accounts for a worn battery’s lower capacity. A battery that has swollen or bulges the case is a different matter: stop using it and take it to a recycler or repair shop.

FAQ

Questions people ask

How many Wh is a 20,000 mAh power bank?

At 3.7 V, 20,000 mAh is 74 Wh: 20,000 times 3.7, divided by 1,000. Check the voltage printed on your own power bank, because the result changes with it.

Can I take a 30,000 mAh power bank on a plane?

At 3.7 V, 30,000 mAh works out to 111 Wh. That is above the FAA’s 100 Wh limit and inside the 101 to 160 Wh band, which the FAA allows with airline approval, up to two spares per person. Airlines can set stricter limits, so check with yours.

Why do I need the voltage?

mAh measures charge, not energy. Energy in watt-hours is voltage times amp-hours, which is the formula the FAA gives for working out Wh. Two batteries with the same mAh store different energy if their voltages differ.

Are laptop batteries rated in Wh?

Most are, on the battery label. If yours only lists mAh and volts, convert it here, then use the battery life calculator to turn Wh into hours of runtime.

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