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Explainer • 2026

How Many Charges Do You Get From A 20000mAh Power Bank?

Plain English • No sponsored placements
Written and checked by Simple Product Picks · last updated October 2026
✓ Plain-English explainer ✓ No sponsored placements
a hand holding a phone charged by a small power bank outdoors

The short answerA 20000mAh power bank never delivers the full 20000mAh to your phone, because that figure is the capacity of the cells at their own voltage, usually 3.7V, and energy is lost converting it up to the 5V or more your phone is fed at, then a little more in the cable. Worked through from Anker's and UGREEN's own figures, a 4000mAh phone gets about four full charges from a 20000mAh bank, not five.

Why the bank and the phone count mAh differently

A mAh figure only means something at a stated voltage, and the bank and your phone don't share one. UGREEN puts the bank's side plainly: "The internal lithium cells usually sit at 3.7V, but your phone needs power delivered at 5V, 9V, or higher via USB," and "Stepping up this voltage consumes energy, which is why your gear gets warm, that's literally your battery percentage turning into wasted heat." Your phone's battery is counted at a third voltage, and Anker's figure for it is "The typical voltage of a phone is 3.87V." Compare the two only after putting them on that same footing.

The sum for a 4000mAh phone

Using those makers' own figures:

  1. Restate the bank at the phone's voltage: 20000mAh x 3.7V / 3.87V = about 19,100mAh.
  2. Take off the conversion loss Anker puts at "typically, 10-15%": about 16,250 to 17,200mAh.
  3. Take off the cable loss Anker puts at "typically around 5%": about 15,400 to 16,350mAh.
  4. Divide by the phone's 4000mAh: about 3.9 to 4.1 full charges.

Anker's own example makes the same voltage step on a smaller bank, "approximately 9,300mAh from 10,000mAh at 3.6V", and a bank whose cells are rated at 3.6V rather than 3.7V lands a little lower, at about 3.8 to 4.0 charges. For your own phone, swap in its battery size from the phone maker's spec page.

If your bank quotes a rated capacity

Anker defines rated capacity as "the battery's capacity at the rated voltage", so a rated figure follows the same rule: it only compares with your phone's mAh once both sit at the same voltage, which means repeating step 1 with the voltage printed beside it.

Watt-hours are a separate number. If you're packing for a flight, our power bank rules for UK flights guide converts mAh to watt-hours and sets out the cabin limits. For choosing a bank of this size, the best 20000mAh power bank guide has the picks.

FAQ

Q: How many charges do I get from a 10000mAh power bank? A: Half as many, by the same sum: 10000mAh x 3.7V / 3.87V, less 10-15% and then 5%, is about 7,700 to 8,200mAh, or about two full charges of a 4000mAh phone.

Q: Does a bigger mAh number always mean more charges? A: Broadly yes, but only comparing like with like. Anker's conversion loss alone runs from 10 to 15%, so two banks both labelled 20000mAh can hand over different amounts.

Sources

The maker, standards and support pages behind the facts on this page, with the month we last checked each one.