Mobile & Electronics

Power Bank Charges Calculator

How many times a power bank will really charge your phone, using a realistic efficiency rather than the number printed on the case.

3 inputs Free, no sign-up

Your figures

The rated figure printed on the power bank.

Most Indian phones sold today are between 4,500 and 6,000 mAh.

%

60-70% is typical. Energy is lost converting the cell voltage up to 5V and back down again inside your phone, and some is lost as heat.

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Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A 20,000 mAh power bank does not give a 5,000 mAh phone four full charges. It gives about two and a half. The rated capacity is measured at the cell voltage of around 3.7V, but the power bank has to raise that to 5V to charge over USB, and your phone then converts it back down again. Each step loses energy, and some is lost as heat.

This calculator applies a realistic efficiency so the answer matches what you will actually experience.

How to use this calculator

  1. Enter the capacity printed on the power bank.
  2. Enter your phone's battery capacity. It is in the specifications for your model.
  3. Leave efficiency at 65% unless you have reason to change it. Move it up for a high-quality bank charging a phone over a short cable; down for a cheap bank, a cold day, or charging while you are using the phone.

The formula

Usable capacity = Rated capacity x Efficiency / 100
Full charges    = Usable capacity / Phone battery capacity
Rated energy    = Rated capacity (mAh) x 3.7V / 1000   (watt-hours)

Why efficiency is around 65%. A power bank's cells sit at roughly 3.7V. USB output is 5V, so a boost converter raises the voltage, losing perhaps 10–15%. Your phone then steps 5V back down to its own battery voltage, losing a similar amount again. Add cable resistance and heat, and 60–70% of the rated capacity reaching the battery is a realistic expectation.

Manufacturers quote the cell capacity because that is a real measurement of the cells. It is not dishonest — it just is not the number that answers "how many charges do I get".

SymbolMeaningUnit
Rated capacity Capacity printed on the power bank mAh
Efficiency Share of capacity reaching the phone %
Phone capacity Your phone battery capacity mAh

Worked example

A 20,000 mAh power bank charging a 5,000 mAh phone at 65% efficiency:

Usable   = 20,000 x 0.65      = 13,000 mAh
Charges  = 13,000 / 5,000     = 2.6 full charges
Energy   = 20,000 x 3.7 / 1000 = 74 Wh

Two and a half full charges, not four. At 74 Wh it is also comfortably under the 100 Wh airline limit for cabin baggage.

Notes and limits

  • These are estimates. Real results vary with cable quality, temperature, charging speed and whether you use the phone while it charges.
  • Fast charging is less efficient than slow charging, because more energy is lost as heat. A power bank delivering 65W will get fewer charges out of the same cells than one delivering 10W.
  • Airlines limit power banks by watt-hours. Under 100 Wh is generally fine in cabin baggage; 100–160 Wh usually needs approval; above 160 Wh is normally refused. Power banks are not allowed in checked baggage.
  • Capacity falls with age. After a couple of years of regular use, expect noticeably less than when new.

What this calculation assumes

  • Rated capacity is measured at a nominal cell voltage of 3.7V.
  • Efficiency covers boost conversion, the phone charging circuit, cable losses and heat.
  • A "full charge" means from empty to 100%.

Frequently asked questions

How many times will a 20000 mAh power bank charge my phone?

For a typical 5,000 mAh phone, about two and a half full charges - not four. Roughly 35% of the rated capacity is lost converting the cell voltage up to 5V and back down inside your phone.

Why does my power bank give fewer charges than its rating suggests?

The rating measures the cells at about 3.7V. Charging over USB requires 5V, so the power bank steps the voltage up and your phone steps it back down, losing energy at both stages. Sixty to seventy per cent of the rated capacity reaching your battery is normal.

Can I take a 20000 mAh power bank on a flight?

Usually yes. Airlines measure by watt-hours: 20,000 mAh is about 74 Wh, which is under the 100 Wh limit most airlines allow in cabin baggage without approval. Power banks must be carried in the cabin, never in checked baggage.

Does fast charging reduce how many charges I get?

Yes, a little. Higher power means more energy lost as heat, so a power bank used at 65W delivers fewer total charges than the same bank used at 10W.

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