mAh to Wh: How Many Charges Will a Power Bank Really Give? (Calculator)
Convert mAh to Wh and see how many times a 10,000 or 20,000 mAh power bank really charges your phone. Formula, losses, airline rule, free calculator.
ON THIS PAGE
- Free mAh to Wh calculator
- What mAh means (and why it is not enough)
- The two hidden facts on a power bank box
- 1. The mAh is counted at 3.7 V
- 2. Every step loses a bit as heat
- How many charges? Quick table
- Can I take a power bank on a plane?
- Does a bigger power bank charge faster?
- How to choose a power bank (checklist)
- Frequently asked questions
A 20,000 mAh power bank and a 5,000 mAh phone. Simple maths says four full charges. Real life gives you about two and a half. Nobody is lying to you — the box just counts in a way that hides two important facts. Here is how to read it properly.
Free mAh to Wh calculator
Type the power bank's mAh and your phone's battery size. You get watt-hours, the real number of charges, and whether it is allowed on a plane.
⚡ LOADING THE CALCULATOR…
Free, no sign-up. Prefer a bigger screen? Open the mAh to Wh (power bank) calculator in the Lab →
What mAh means (and why it is not enough)
mAh (milliamp-hours) measures charge: how much current a battery can give for how long. 5,000 mAh = 5 A for 1 hour, or 0.5 A for 10 hours.
But charge is not energy. Energy also depends on the voltage: the same 1,000 mAh at 12 V carries more than three times the energy of 1,000 mAh at 3.7 V. That is why engineers compare batteries in watt-hours (Wh):
The two hidden facts on a power bank box
1. The mAh is counted at 3.7 V
Inside, a power bank holds lithium cells with a nominal voltage of about 3.6–3.7 V. The big mAh number on the box is measured at the cell voltage. But USB gives out 5 V (or 9 V and more with fast charging). Boosting 3.7 V up to 5 V means fewer mAh come out of the USB port:
- 20,000 mAh × 3.7 V = 74 Wh inside.
- At 5 V that is at most 74 ÷ 5 = 14,800 mAh — before any losses.
Many good brands print a "rated capacity" like 12,000 mAh (5 V ⎓ 2.4 A) on the back — that is the honest output number.
2. Every step loses a bit as heat
- The boost converter in the power bank (≈ 85–92 % efficient)
- The cable (thin, long cables lose more)
- The phone's own charging chip and battery (≈ 85–90 %)
Together, about a third of the energy turns into heat. Expect 60–70 % of the printed Wh to end up in your phone's battery.
How many charges? Quick table
Phone battery assumed: 5,000 mAh at 3.85 V ≈ 19.3 Wh, real-world efficiency 65 %.
| Power bank | Energy (at 3.7 V) | Reaches the phone | Real charges |
|---|---|---|---|
| 5,000 mAh | 18.5 Wh | ≈ 12 Wh | ≈ 0.6 |
| 10,000 mAh | 37 Wh | ≈ 24 Wh | ≈ 1.2 |
| 20,000 mAh | 74 Wh | ≈ 48 Wh | ≈ 2.5 |
| 27,000 mAh | 99.9 Wh | ≈ 65 Wh | ≈ 3.4 |
A phone with a 4,000 mAh battery gets about 25 % more charges; a 6,000 mAh phone about 17 % fewer.
Can I take a power bank on a plane?
Airline rules (based on IATA guidance, used worldwide) are set in watt-hours, not mAh:
- Up to 100 Wh (≈ 27,000 mAh at 3.7 V): allowed in your cabin bag.
- 100–160 Wh: only with airline approval, usually a maximum of two.
- Over 160 Wh: not allowed.
- Never in checked luggage.
Some airlines now also ban using or charging power banks during the flight — check your airline before you fly.
Does a bigger power bank charge faster?
No. Capacity (Wh) decides how many charges; output watts decide how fast. A small 10,000 mAh bank with 20 W USB-C Power Delivery charges a phone faster than a big 30,000 mAh bank that only gives 10 W. Look for both numbers: Wh and output W.
How to choose a power bank (checklist)
- Check the Wh (and the rated capacity at 5 V), not just the mAh.
- Match the output watts to your phone: 18–25 W for most phones, 45–65 W+ for laptops (USB-C PD).
- Weight: 20,000 mAh banks are about 350–450 g.
- Safety marks: CE / UKCA (Europe/UK), FCC / UL (USA), BIS (India).
- Under 100 Wh if you travel by air.
Turn mAh into watt-hours and see how many real phone charges a power bank gives — and if it can fly.
Frequently asked questions
How do I convert mAh to Wh?
Multiply mAh by the battery voltage and divide by 1,000. For a power bank use 3.7 V: 10,000 mAh × 3.7 ÷ 1,000 = 37 Wh.
How many times can a 10,000 mAh power bank charge a phone?
About 1.2 times for a 5,000 mAh phone, or about 1.5 times for a 4,000 mAh phone, because roughly a third of the energy is lost as heat.
How many times can a 20,000 mAh power bank charge a phone?
About 2.5 times for a 5,000 mAh phone — not 4 times. 74 Wh inside, about 48 Wh reaches the phone, and one charge needs about 19 Wh.
How many Wh is a 20,000 mAh power bank?
About 74 Wh, because power banks are rated at the cell voltage of 3.7 V (20,000 × 3.7 ÷ 1,000).
Is a 30,000 mAh power bank allowed on a plane?
At 3.7 V it is about 111 Wh, which is above 100 Wh, so most airlines need you to get approval first. Banks up to about 27,000 mAh (100 Wh) are allowed in cabin bags without approval.
