Power Bank Real Capacity Calculator

Why a 20,000 mAh power bank does not charge a 5,000 mAh phone four times: the energy in the cells, what the boost converter keeps on the way to 5 V, what the phone’s own charger keeps on the way back down to 3.85 V, and the honest number of charges left over — with the IATA watt-hour figure for flying.

Last updated 24 Sep 2026

Power bank real capacity

Cell mAh → watt-hours → honest charges

Power bank capacity on the case (mAh)The big number on the box. It is the CELL capacity, measured at the cell voltage, not what comes out of the USB port.

Bank cell nominal voltage (V)3.6 or 3.7 V for the lithium-ion cells inside almost every power bank.

Boost converter efficiency to the output (%)Typically 85–93% for a good design at a sensible load; lower at very small and very large currents, and lower again for USB-PD at high voltages.

Output voltage (V)5 V for plain USB. A USB-PD bank negotiating 9 V or 20 V does the same arithmetic at that voltage.

Phone battery capacity (mAh)

Phone cell nominal voltage (V)3.85 V on most recent phones, 3.7 or 3.8 V on older ones.

Phone charging efficiency (%)The phone’s own charger circuit and the battery’s charge acceptance together. 80–90% is usual; less when the phone is hot or fast-charging.

Phone’s charge when you plug in (%)0 means you start from flat. Charging from 50% takes half the energy.

Output current while charging (A)Only used for the charging-time row.

The energy chain, drawn as blocks rather than as a schematic. Energy leaves the cells at about 3.7 V, is lifted to 5 V by a boost converter that keeps some of it, travels down the cable, and is put back into a 3.85 V phone cell by a charger that keeps some more. Only what survives both steps charges anything, so the honest charge count uses watt-hours at every node, never the milliamp-hours printed on the case. The dots move at the current the bank supplies at 5 V.

2.78chargesExample

a 20,000 mAh power bank of 3.7 V cells, 85% boost efficiency, charging a flat 5,000 mAh phone whose cell is 3.85 V at 85%

Follow the energy, not the milliamp-hours

E_bank = mAh_cells × V_cell ÷ 1000 · E_port = E_bank × η_boost · E_phone = E_port × η_charge
charges = E_phone ÷ (mAh_phone × V_{phone cell} ÷ 1000) · usable mAh at the port = E_port ÷ V_out × 1000

V cell: 3.6–3.7 V, the voltage the bank’s mAh figure is measured at
η boost: the converter that lifts the cell voltage to 5 V — 85–93% in a good design
η charge: the phone’s own charger and the charge its battery accepts — 80–90%
V phone cell: 3.85 V on most current phones: the phone’s mAh is measured there, not at 5 V

Worked example

a 20,000 mAh power bank of 3.7 V cells, 85% boost efficiency, charging a flat 5,000 mAh phone whose cell is 3.85 V at 85%
Energy in the cells = 20,000 × 3.7 ÷ 1,000 = 74 Wh
Through the boost converter at 85%: 62.9 Wh reaches the USB port, which is 12,580 mAh at 5 V — 11.1 Wh gone already
The phone's own charging keeps 85% of that, so 53.47 Wh lands in its cells and another 9.435 Wh is gone
A full 5,000 mAh phone battery at 3.85 V holds 19.25 Wh
Charges = 53.47 ÷ 19.25 = 2.78, not the 4 the milliamp-hours suggest — a shortfall of 30.6%
At 74 Wh the bank is under IATA's 100 Wh threshold, so it travels in carry-on baggage without asking anyone

Where the 74 Wh goes

The same energy expressed as charge at three different voltages, which is exactly why the milliamp-hour figures cannot be compared directly. Every figure computed by this page’s own arithmetic at its defaults.
Stage	Energy	Share of the cells	Same energy as charge
In the bank’s cells at 3.7 V	74.00 Wh	100.0%	20,000 mAh at 3.7 V
Out of the USB port at 5 V	62.90 Wh	85.0%	12,580 mAh at 5 V
Into the phone’s own cells	53.465 Wh	72.3%	13,887 mAh at 3.85 V
One full charge of this phone	19.250 Wh	—	5,000 mAh at 3.85 V

The same bank against different phones

The shortfall is the same percentage every time, because it is set by the two efficiencies and the two cell voltages, not by the size of the phone. What changes is how visible it is: losing 28% of four charges costs you more than a whole charge.
Phone battery	Charges the box implies	Charges you get	Shortfall
2,000 mAh	10.00	6.94	30.6%
3,000 mAh	6.67	4.63	30.6%
4,000 mAh	5.00	3.47	30.6%
5,000 mAh	4.00	2.78	30.6%
6,000 mAh	3.33	2.31	30.6%

Why the number on the box is not the number you get

Two things go wrong between the label and the phone, and they are independent of each other.

The label is measured at the wrong voltage. A power bank’s milliamp-hours are its CELLS’ milliamp-hours, at a cell voltage of about 3.7 V. The energy is 20,000 mAh × 3.7 V = 74 Wh. To deliver that over USB the bank has to boost 3.7 V up to 5 V, and charge is not conserved when voltage changes — energy is. Even a perfect converter would hand over only 74 ÷ 5 = 14.8 Ah at the port, so the honest figure on a lossless 20,000 mAh bank would be 14,800 mAh, not 20,000. Chinese national standard GB/T 35590 requires a power bank to declare a rated capacity measured at the output for this reason; the big number on the front is usually the cell figure anyway.

Then two conversions each keep a share. The boost converter runs at perhaps 85%, so 11.1 Wh becomes heat before anything leaves the bank. The phone then does the reverse conversion, stepping 5 V back down to its own cell voltage, and its charging circuit and battery together keep perhaps another 85% — 9.4 Wh more gone. What arrives in the phone’s cells is 53.5 Wh of the original 74, about 72%. A full 5,000 mAh phone battery at 3.85 V holds 19.25 Wh, so you get 2.78 charges from a bank whose label invites you to expect four.

Reading it the other way. If a manufacturer quotes a rated output capacity, that figure already includes the first loss: 12,580 mAh at 5 V for this example. Comparing that with a phone’s 5,000 mAh is still not right, because the phone’s figure is at 3.85 V, but it is much closer. The only comparison that is always safe is in watt-hours, which is also the unit airlines use: IATA allows batteries up to 100 Wh in carry-on baggage without asking, 100–160 Wh with the operator’s approval, and nothing above 160 Wh. For the plain conversion between mAh, Wh and joules the battery capacity converter is the page to use; for what the cells inside are doing, the lithium pack builder.

What is left out. Cable resistance and connector drop, which matter at 2 A over a thin cable. The bank’s standby drain and its self-discharge, which cost a few per cent a month if it lives in a bag. The fact that neither efficiency is a constant: both fall at very low and very high currents, and the phone’s charging efficiency falls further when the battery is nearly full or the phone is hot. And ageing: the cells lose capacity with cycles, so a two-year-old bank starts from less than 74 Wh.

Frequently asked questions

How many times will a 20,000 mAh power bank charge my phone?

Fewer than the division suggests. A 20,000 mAh bank holds about 74 Wh; after a typical boost converter and the phone’s own charging losses about 53 Wh reaches the phone’s cells, which is 2.8 full charges of a 5,000 mAh phone — not 4.

Why is a power bank’s real capacity lower than the label?

The label is the cell capacity at about 3.7 V. Getting it out at 5 V converts energy, not charge, so even a perfect bank delivers only capacity × 3.7 ÷ 5 at the port. Converter losses take another 10–15% on top.

How do I convert power bank mAh to Wh?

Multiply by the cell voltage and divide by 1,000: 20,000 mAh × 3.7 V ÷ 1,000 = 74 Wh. Watt-hours are the figure airlines limit and the only figure that can be compared between batteries of different voltages.

Can I take a power bank on a plane?

Under IATA’s rules, up to 100 Wh in carry-on baggage without approval, 100 to 160 Wh with the operator’s approval, and above 160 Wh not at all on a passenger aircraft. Spares and power banks must never go in checked baggage.

Does a USB-PD power bank do better?

Not in this arithmetic. Negotiating 9 V or 20 V charges faster and reduces cable loss, but the same energy still goes up through one converter and back down through another. Boost efficiency at the higher voltages is usually slightly lower, not higher.

What efficiency should I assume?

85% for the bank’s converter and 85% for the phone is a fair middle estimate, giving about 72% end to end. A very good bank charging a cool phone at a sensible current might reach 80% overall; a cheap one charging a hot phone will be well under 65%.

Related calculators

References

IATA. Lithium battery guidance for passengers: batteries and power banks up to 100 Wh are generally allowed in carry-on baggage, 100–160 Wh require the operator’s approval, above 160 Wh are not permitted on passenger aircraft, and spare batteries and power banks must never be placed in checked baggage.
GB/T 35590-2017. Information technology — General specification for portable digital equipment used power bank, which requires a declared rated capacity measured as the effective output capacity: the discharge current multiplied by the time to the cut-off point at 23 ± 2 °C, with the measured output capacity required to be not lower than the rating.
Erickson RW, Maksimović D. Fundamentals of Power Electronics, 3rd ed. Springer, 2020. Boost converter operation and the switching, conduction and quiescent losses that set a converter’s efficiency curve against load.
Reddy TB (ed.). Linden’s Handbook of Batteries, 4th ed. McGraw-Hill, 2011. Lithium-ion nominal voltage conventions, charge acceptance and coulombic versus energy efficiency.