How the calculation works
Full charges = power bank mAh × 3.7 V × efficiency ÷ (phone mAh × phone voltage)
A worked example
A 10,000 mAh power bank holds 37 Wh; at 65 % efficiency about 24.1 Wh reach the phone. A 4,000 mAh phone battery at 3.85 V holds 15.4 Wh, so the bank gives about 1.6 full charges.
Why 10,000 mAh is not 2.5 charges of a 4,000 mAh phone
Dividing mAh by mAh ignores two things. The power bank's mAh is counted at 3.7 V while modern phone batteries run at about 3.85 V, and every conversion loses energy: the bank steps its cells up to 5–9 V, the phone steps it down again, and both turn some of it into heat. Around 65 % of the bank's energy usually ends up in the phone.
Wireless costs more
Magnetic and wireless power banks lose more on the way, often reaching only 50–60 % efficiency, so they give fewer charges than a cable-connected bank of the same capacity. Lower the efficiency field to see the difference.
Common questions
How many charges does a 20,000 mAh power bank give?
About three full charges of a 4,000 mAh phone at 65 % efficiency: 20,000 × 3.7 × 0.65 ÷ (4,000 × 3.85) ≈ 3.1.
Does fast charging change the count?
Slightly: higher power means more heat, so fast charging can lose a little more energy than slow charging. The efficiency field covers it.
Method: arithmetic from the label values you enter. Reviewed .