Power Bank Capacity Calculator - Real Charges and Wh

How many charges a power bank really gives

Why doesn't a 20,000 mAh power bank charge a 5,000 mAh phone four times? Because the two numbers are counted at different voltages and the energy passes through two converters on the way. This calculator turns both batteries into watt-hours, applies the efficiency you enter (or the measured average of about 2/3), and gives 2.56 charges for that pair, plus the FAA rule for taking the bank on a plane.

Dividing the labels

20,000 ÷ 5,000 = 4 charges
ignores that the phone cell sits at 3.85 V, the bank cells at 3.7 V
ignores the step up to 5 V and back down
says nothing about flying with it

Counting watt-hours

74 Wh in the bank, 19.25 Wh per phone charge
3.84 charges is the ceiling with no losses at all
2.56 at the measured average, 3.27 at 85%
74 Wh is under the 100 Wh carry-on limit

Six fields plus two optional ones

Power bank capacity and unit - mAh from the label, or Wh if the label prints it (airline-friendly banks usually do).
Power bank cell voltage - the voltage printed next to the mAh, nearly always 3.6 V or 3.7 V. Not needed for Wh.
Device battery and unit - the phone, tablet or earbud case capacity in mAh or Wh.
Device battery voltage - about 3.85 V for recent phones, 3.7 V for many other lithium-ion devices.
Overall efficiency - optional. Empty uses the measured average of about 2/3 and also shows the no-loss ceiling.
Device level when you plug in - optional. Topping up from 20% counts as 80% of a charge.
Read the result - charges, watt-hours delivered and lost, the 5 V figure, the FAA tier and a table of common bank sizes.

Where the missing milliamp-hours go

charges = (bank mAh × bank V ÷ 1000) × efficiency ÷ (device mAh × device V ÷ 1000 × share refilled)

Wikipedia's power bank article puts the first loss plainly: a 3.7 V rating read at a 5 V output is only 74% of the printed mAh, before any conversion loss. The same article cites measurements in which the average power bank moved about 2/3 of its cell energy into the devices it charged.

20,000 mAh into a 5,000 mAh phone. 20 × 3.7 = 74 Wh; 5 × 3.85 = 19.25 Wh. With no efficiency entered, 74 × 2/3 = 49.33 Wh reaches the phone, which is 2.56 charges, 2 full and 56% of the next. At 85% it becomes 3.27.

The 26,800 mAh bank Wikipedia describes. 99.16 Wh in the cells is 19,832 mAh at 5 V in theory; the measured output was 15,682 mAh, or 78.41 Wh, which is 79% of the cell energy at the port. Typed in as 79%, it gives 4.07 charges of the same phone before the phone's own charging loss.

Topping up instead of charging from empty. From 20%, one charge needs 15.4 Wh, so the 20,000 mAh bank at 85% gives 4.08 top-ups.

What the box promises and what the phone gets

Charges of a 5,000 mAh, 3.85 V phone from empty, all banks at 3.7 V. The middle column divides the labels; the last one is the calculator with the field left empty.

Power bank	Energy	Label ÷ label	At about 2/3
10,000 mAh	37 Wh	2	1.28
20,000 mAh	74 Wh	4	2.56
26,800 mAh	99.16 Wh	5.36	3.43
30,000 mAh	111 Wh	6	3.84
Gap between the columns	-	the label division overstates by 56% in every row

Flying with a power bank: the FAA tiers

The FAA treats power banks as spare lithium-ion batteries: carry-on bags only, never checked, and if a carry-on is checked at the gate the bank has to come out. The size tier depends on watt-hours, which is why the calculator reports it. The mAh column shows the same limits for 3.7 V cells.

Energy	At 3.7 V	FAA rule
up to 100 Wh	up to 27,027 mAh	carry-on
101-160 Wh	up to 43,243 mAh	airline approval, two at most
over 160 Wh	over 43,243 mAh	above the FAA limit for spares

Airlines can set stricter limits on how many power banks a passenger carries, so check with the carrier as well.

Asked before a trip

How many times will a 10000mAh power bank charge a phone?

For a 5,000 mAh phone, about 1.28 charges at the measured average and 1.92 at most. A smaller phone gets more: the calculator only needs its mAh and voltage.

Is a 26800mAh power bank allowed on a plane?

Yes, in carry-on. 26,800 mAh at 3.7 V is 99.16 Wh, just under the 100 Wh tier. A 30,000 mAh bank is 111 Wh and needs the airline's approval.

What efficiency should I enter for a good power bank?

If you have measured it with a USB meter, enter what the meter shows as a share of the label watt-hours; the 26,800 mAh example above gives 79% at the port. Otherwise leave it empty for the 2/3 average and read the ceiling next to it.

How many times will a 5000mAh power bank charge earbuds?

An earbud case of 500 mAh at 3.7 V holds 1.85 Wh; a 5,000 mAh bank at 80% delivers 14.8 Wh, so 8 full charges of the case.

Can a power bank charge a laptop?

If both support USB-C Power Delivery at the laptop's wattage. In energy terms a 99 Wh bank at 80% refills a 60 Wh laptop 1.32 times. Enter both in Wh.

Why is 20,000 mAh only 14,800 mAh at 5 V?

The energy stays 74 Wh, but at 5 V it takes fewer milliamp-hours to carry it: 74 ÷ 5 = 14,800 mAh, the same 74% ratio Wikipedia gives. Conversion losses come on top of that.

Related tools

Battery Charging Time Calculator

How long the bank itself takes to refill on a given USB charger - Open the calculator

Inductance Calculator

Coil inductance and turns, for the converter that steps the bank up to 5 V - Open the calculator

Battery Life Calculator

Hours of runtime from mAh, Ah or Wh and a load - Open the calculator

Electrical Power Calculator

Watts from volts and amps, for reading a USB meter - Open the calculator

Amps Calculator

Current from watts and voltage - Open the calculator

Capacitor Energy Calculator

Energy stored in a capacitor, set against battery figures - Open the calculator

Ready-made calculations

The most searched variants of this calculator. Each link opens it with the value already filled in, ready to calculate.