Calculate
Power Bank Calculator: How Many Charges Will You Get?
Enter what is printed on your power bank and on your device, and get an estimated number of full charges as a range. The calculator shows the formula, the efficiency assumption and every step, so you can argue with the number instead of trusting it.
Estimate full charges
Start with a model, or type the figures yourself. The required figures come first; the optional ones are folded away.
RequiredPack capacity and the device batteryOptionalEfficiency range and the pack’s maximum output, folded into “More options”
Start with your model
OptionalPick a brand, then narrow the list and pick a model. The capacity figures below are filled in from the published figures for that pack. Nothing is calculated until you submit the form, and every field can still be typed by hand.
Estimated full charges
- Pack energy used—
- Device battery—
- Efficiency range—
- Pack max output—
The arithmetic
Assumptions and limits
What to do next
Estimated charges, not a guaranteed result. The number describes full charges of the battery you entered. It does not promise hours of use, and it cannot account for a battery that has aged.
How the estimate is worked out
Two steps, in this order. First the pack's stored energy is established in watt-hours. Then that energy is discounted by the efficiency of the whole charging path and divided by the energy in the device battery.
Wh = mAh × V ÷ 1000
Estimated charges = power bank Wh × overall efficiency ÷ device battery Wh
Voltage here is the cell nominal voltage printed on the battery itself — 3.7 V or 3.85 V on most lithium packs. It is not the 5 V, 9 V or 20 V that appears on a USB output. Using a USB rail in this formula produces a number that does not describe stored energy, which is why the calculator flags those values.
A worked example
A 10,000 mAh pack rated at 3.7 V holds 10,000 × 3.7 ÷ 1000 = 37 Wh. If the device battery holds 15 Wh and the whole path runs at 70–75%, the range is 37 × 0.70 ÷ 15 ≤ 1.7 charges at the low end and 37 × 0.85 ÷ 15 ≤ 2.1 charges at the high end.
| Power bank | Device battery | Efficiency | Estimated charges |
|---|---|---|---|
| 37 Wh (10,000 mAh at 3.7 V) | 15 Wh | 70–75% | 1.7 — 2.1 |
| 37 Wh | 15 Wh | fixed 80% | 2.0 |
| 74 Wh (20,000 mAh at 3.7 V) | 15 Wh | 70–75% | 3.5 — 4.2 |
| 37 Wh | 60 Wh (tablet or small laptop) | 70–75% | 0.4 — 0.5 |
The single fixed percentage in the second row is shown on purpose: it is the sort of number marketing pages quote. It is not more accurate than the range — it hides the uncertainty.
Why the answer is a range and not one number
- Conversion loss is not constant. It changes with current, temperature and how full the pack is.
- The device decides how much it pulls. A phone that accepts 10 W will not charge faster because the pack can deliver 30 W.
- Charging while using the device splits the energy between the battery and the running phone.
- Battery age and temperature reduce what the device battery can actually store.
Charges are not hours
A 37 Wh pack and a 15 Wh battery give roughly two charges. That says nothing about how long the phone will run. If you want runtime, the useful calculation is available energy divided by average load in watts: hours ≤ usable Wh ÷ average W. A device drawing 2 W on average behaves very differently from one drawing 8 W.
Where to find the numbers on your labels
| Value | Where it is printed | Watch out for |
|---|---|---|
| Power bank capacity (mAh) | Front of the pack, or the spec panel on the back | "Rated capacity" (usable output) is often lower than the headline figure |
| Power bank energy (Wh) | Back label, required for air travel | If it is missing, calculate it from mAh and the cell voltage |
| Cell nominal voltage | Battery printing such as 3.7 V or 3.85 V | Not the 5 V / 9 V / 20 V in the USB output table |
| Device battery | Manufacturer spec page, or the battery itself | mAh alone is not comparable across devices with different voltages |
When this calculator refuses to answer
Empty fields, zero, negative numbers, text, and non-finite values such as Infinity or NaN are all rejected — each with a message next to the field, and without clearing what you already typed. A calculation that silently treats a missing value as zero is worse than no calculation at all.
If you do not have a trustworthy battery figure for your device, the honest answer is that the charge count cannot be narrowed down. Entering a guessed value produces a confident-looking number that means nothing.
Related tools
- mAh to Wh converter — when only one of the two figures is printed.
- 65W vs 100W charger — for the charger side of the same trip.
- Travel adapter finder — for the socket side.
- How we calculate — the full method and its limits.
Formulas last reviewed 6 November 2026. This page contains no product recommendations and no affiliate links.