Five fields drive the estimate, and two of them cause almost all the confusion. Power bank capacity and device battery capacity are the easy ones — read them straight off the printed rating or the spec sheet, in mAh. Efficiency is the honesty dial: 85% is fair for a good bank on a short wired cable, and cold weather, wireless pads, fast-charge modes, or a long thin cable pull it toward 75–80%.
The voltage fields are where people slip. Conversion voltage is the bank's internal cell voltage — about 3.7 V — not the 5 V printed on the USB port; the climb to USB output is already folded into the efficiency number, so entering 5 V here double-counts and inflates the result. Device battery voltage is your phone or tablet's nominal cell voltage, usually 3.7 to 3.85 V, or roughly 7.4 V for a two-cell pack. To compare several banks, hold the device fields steady and vary capacity and efficiency; to weigh one bank against several phones, do the reverse. The table under your result reruns the math across common bank sizes so you never retype anything.
Everything rests on one relationship, energy = capacity × voltage, applied once to the bank and once to the device, with the efficiency factor sitting on the supply side. In symbols:
The C terms are capacities in mAh, the V terms are volts, and η is efficiency written as a decimal, so 85% becomes 0.85. Because mAh × V is proportional to energy, the ratio lands directly on the number of full refills the bank can deliver. If you would rather think purely in watt-hours, it is the same statement written charges ≈ (available Wh × efficiency) ÷ required Wh; the calculator just keeps the inputs in the mAh-and-volts form you find on packaging.
A concrete run makes it click. Take a 10,000 mAh bank at 3.7 V, a phone with a 3,000 mAh battery at 3.7 V, and 85% efficiency:
Example calculation (10,000 mAh bank to 3,000 mAh phone)
| Parameter |
Value |
| Available energy (mWh) |
10,000 × 3.7 × 0.85 = 31,450 |
| Device energy need (mWh) |
3,000 × 3.7 = 11,100 |
| Estimated full charges |
31,450 ÷ 11,100 ≈ 2.83 |
That 2.83 usually feels like two full charges plus a generous partial. Leave the screen on, run turn-by-turn navigation, or sit in a weak-signal dead zone where the radio works overtime, and you will land a little lower. Efficiency moves the answer more than most people expect: hold everything else fixed and slide from 85% to 75%, and 2.83 drops to 2.50 — a gap big enough to matter on a long travel day or when you are rationing power through an outage.
What the number does and doesn't include
The efficiency field quietly carries a lot of weight, rolling converter losses, cable resistance, heat inside the phone, and battery-management overhead into a single percentage. For a conservative read — an older bank, cold hands, a wireless pad, or high-voltage fast charging — reach for 75-80%; for a modern bank on a short cable charging normally, 85-90% is honest. Read the result as full-charge equivalents from empty to full: 1.00 is one complete refill, 1.60 is one refill plus about 60% of another. If you habitually top up from 40% to 80% rather than draining to zero, the bank will feel more generous than the number suggests, because each partial top-up spends less energy than a full cycle.
A handful of real effects sit outside this simple model, and nearly all of them nudge the true count a little lower:
- Nonlinear charging: the last stretch toward 100% is slower and less efficient than the middle of the range.
- Fast-charge negotiation: USB-PD and Quick Charge shift where converter losses fall.
- Temperature and age: cold cells and old cells both give back less usable capacity.
- Early cutoff: some banks stop feeding very low-current devices before they are truly empty.
- Use while charging: maps, video, hotspot, and gaming all draw from the socket you just plugged into.
- Reserved capacity: many devices hide a slice of chemical capacity to extend battery life.
None of that undercuts the point the model captures cleanly: the mAh printed on a bank is not the energy that ends up stored in your device.
Watt-hours, travel limits, and packing
Watt-hours are the better yardstick because they already fold voltage in, so two banks wearing the same mAh label can still hold different usable energy depending on cell chemistry and converter quality — and this calculator has been working in watt-hours the whole time. To check a label yourself, use Wh ≈ (mAh ÷ 1000) × V: a 10,000 mAh bank at 3.7 V is about 37 Wh, and a 5,000 mAh phone battery at 3.85 V is about 19 Wh.
Airlines think in watt-hours too. Spare lithium batteries under roughly 100 Wh are widely allowed in carry-on, 100-160 Wh often needs airline approval, and anything larger is usually refused — so most 10,000 to 26,800 mAh banks clear the bar comfortably, while a bulky 40,000+ mAh unit can brush the ceiling. Treat those thresholds as a starting point and confirm your carrier's current policy; this page is informational, not official guidance.
For packing, let the estimate set your buffer. A pocket 5,000 mAh bank covers one emergency top-up, while a 20,000 mAh bank is the realistic pick when you want several phone recharges, a tablet top-up, or margin for cold and poor conditions. A rough rule many people use: 1.0-1.5 estimated charges for a day out, 2-4 for a weekend with uncertain outlets, and extra reserve in the cold, where you should also dial efficiency down. If your real results fall far short of the estimate, suspect an aging or damaged bank — or the cable, since a worn, resistive cable bleeds efficiency and is the cheapest thing to swap first. When a bank finally gives out, its lithium cells belong in an electronics or battery recycling program, not the household bin.
Common questions
Why won't my 20,000 mAh bank charge a 5,000 mAh phone four times? Because the 20,000 mAh describes the bank's internal cells near 3.7 V, not a clean handoff into the phone. Voltage conversion, cable resistance, and the phone's charging circuit each skim energy off as heat, so the count usually lands nearer two-and-a-half to three.
What efficiency should I use with no measurements? Start at 85% for a solid modern bank on a short wired cable, drop to 80% to play it safe, and use 75-80% for wireless charging, fast charging, cold weather, or long cables.
Isn't the conversion voltage 5 V, since USB is 5 V? Usually not. That field is the bank's internal cell voltage — about 3.7 V — used for the capacity rating. The climb to 5 V USB output is a loss, and the efficiency field already accounts for it.
My device lists watt-hours instead of mAh — can I still use this? Yes. Convert with mAh ≈ (Wh ÷ V) × 1000, put the result in the device field, and compare banks the same way.
Does the estimate include wireless charging losses? Not on its own — wireless transfer is noticeably lossier than a cable. If you will charge from a pad, lower the efficiency input to fold that extra loss in.
