Milliamp-hours is not a capacity. It is a charge — and the same battery can honestly be labelled 10,000 or 7,400 depending on which voltage the seller picks.
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The default for power banks, laptops and most phones. Runs 4.2 V full to about 3.0 V empty; 3.7 V is the nominal average used for labelling.
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Energy37 Wh
10000 mAh at 3.7 V. Watt-hours is the figure that means something on its own — it is energy. Milliamp-hours is a charge, and a charge only becomes an amount of energy once you know the voltage it sits at.
The same 37 Wh, labelled at other voltages
Identical energy. Seven different “capacities”. A seller quoting mAh can pick the voltage that produces the biggest number, and nothing they print is untrue.
Lithium-ion (Li-ion) · 3.7 V
10,000 mAh
Li-ion, high-voltage phone cell · 3.85 V
9,610 mAh
Lithium iron phosphate (LiFePO₄) · 3.2 V
11,563 mAh
NiMH (AA / AAA rechargeable) · 1.2 V
30,833 mAh
Alkaline (AA / AAA) · 1.5 V
24,667 mAh
USB output (5 V) · 5 V
7,400 mAh
Lead-acid (12 V block) · 12 V
3,083 mAh
Taking it on a plane
Energy37 Wh
StatusNormally allowed in carry-on
Under 100 Wh — normally allowed in carry-on baggage without airline approval. Spare batteries must go in the cabin, never in checked baggage.
The rule is written in watt-hours, not mAh, for exactly the reason above — mAh cannot say how much energy is in the battery. At 3.7 V the 100 Wh line falls at 27,027mAh, which is why the largest widely sold “airline safe” bank is 26,800 mAh: 99.2 Wh, deliberately just under. Check your own airline before flying; this is a conversion, not their policy.
What this converter covers
mAh to watt-hours at any cell voltage, the airline limit checked against the number that actually governs it, and an honest count of how many charges a power bank gives once the voltages and the losses are accounted for.
mAh ↔ Wh at seven cell voltages, from NiMH to a 12 V block
The same energy shown as every mAh figure it could be labelled with
The 100 Wh and 160 Wh airline thresholds, checked on energy
Real power-bank charges against the number the labels imply
Runtime at a steady draw, and C-rate current
mAh ↔ Wh Airline limit checked Real charges, not label maths Seven chemistries
Free, no signup — exact by definition, not an estimate.
Updated 7 September 2026
At a glance
Formula shown
Wh = mAh × V ÷ 1000 · charges = bank Wh × efficiency ÷ device Wh
Scenario support
10000 mAh at 3.7 V = 37 Wh · 26,800 mAh = 99.2 Wh, just under the 100 Wh limit
Educational estimate
Planning support from the values you enter — not professional advice.
mAh is a charge, not a capacity
A milliamp-hour measures how much electrical charge a battery can deliver — how many electrons, in effect. It does not measure energy, and energy is what actually runs a device. Energy is charge multiplied by the voltage it is delivered at, which is why watt-hours is the figure that means something on its own and milliamp-hours is not.
The consequence is that an mAh figure only compares two batteries at the same voltage. Take 37 watt-hours of energy — a typical power bank — and label it honestly at different voltages:
One battery holding 37 Wh, labelled at six different voltages
Quoted at
Would be labelled
1.2 V (NiMH)
30,833 mAh
3.2 V (LiFePO₄)
11,563 mAh
3.7 V (Li-ion cell)
10,000 mAh
3.85 V (phone cell)
9,610 mAh
5 V (USB output)
7,400 mAh
12 V (lead-acid)
3,083 mAh
Every one of those numbers is true. None of them is a lie. And a seller who quotes mAh gets to choose which voltage the number is measured at — which is why capacities are almost always quoted at the low cell voltage, where the figure comes out biggest, and almost never at the 5 V the device actually receives.
Watt-hours removes the choice, because it is the energy regardless of how it is delivered. If you want to compare two batteries, compare their watt-hours; if only mAh is given, find the voltage before believing it.
Why a 10000 mAh bank is not 2.5 phone charges
The obvious sum is to divide the labels: a 10,000 mAh bank into a 4,000 mAh phone gives 2.5 charges. In practice you get about two, and often fewer. The gap is not the manufacturer lying; it is two real effects, and the first one catches people even when they know about the second.
The voltages differ. The bank’s cell is nominally 3.7 V. A modern phone cell is nominally 3.85 V, because it charges to a higher peak. So the bank holds 37 Wh and the phone holds 15.4 Wh, and the honest ratio is 2.40 rather than 2.50 — before anything is lost at all. That 4% is exactly the ratio of the two voltages, and it exists purely because the two mAh figures were never the same unit.
Then there are the conversions. The bank cannot send 3.7 V down a USB cable, so it boosts to 5 V or negotiates higher. Your phone cannot charge its cell at 5 V, so it drops back to about 4 V. Each step is a switching converter running at perhaps 85 to 95% efficiency, and the losses multiply. Charging fast makes it worse, because higher currents mean more resistive loss and more heat — and heat is precisely the energy that did not reach the battery.
A good bank on a short cable at a moderate rate might return 85% round trip, giving 2.04 charges. A cheap one fast-charging a warm phone might manage 60%, giving 1.44. The realistic band for a 10,000 mAh bank and a 4,000 mAh phone is therefore about one and a half to two full charges, and any listing promising two and a half is quoting arithmetic rather than experience.
Airline limits are in watt-hours for a reason
Aviation rules for lithium batteries are stated in watt-hours, and now it is clear why: a limit in mAh would be meaningless, because the same limit would permit four times the energy in a 1.2 V pack as in a 5 V one.
The internationally adopted thresholds are 100 Wh, below which a spare battery normally travels in carry-on baggage without asking anyone, and 160 Wh, between which and 100 Wh you generally need the airline’s prior approval and are limited to two spares. Above 160 Wh a battery is not accepted as passenger baggage at all. Spare batteries always travel in the cabin, never in the hold, because a fire in the cabin can be dealt with and one in the hold cannot.
At a 3.7 V cell, 100 Wh works out at 27,027 mAh. Which explains a number you have probably seen without wondering about: 26,800 mAh is the standard largest power bank on sale. It is 99.2 Wh — deliberately, precisely, just under the line, with enough margin that nobody has to argue at a security desk.
A useful habit follows. Look for the watt-hour figure printed on the battery itself, which is required on cells of this type; if only mAh is shown, the voltage will be printed next to it and the calculator above will do the rest. And check your own airline, because carriers add their own conditions on top of the international minimum.
Rechargeable AAs and the 1.2 volt surprise
A NiMH rechargeable AA is 1.2 V. The alkaline it replaces is 1.5 V. Same shape, same slot, 20% less voltage — and since energy is charge times voltage, that is 20% less energy for the same mAh figure.
A 2,000 mAh NiMH AA therefore holds 2.4 Wh, while a 2,500 mAh alkaline holds 3.75 Wh: a much bigger gap than the mAh numbers alone imply. This is why some devices — particularly ones with a low-voltage cutoff designed around alkalines — declare a fresh set of rechargeables flat, or run visibly dimmer on them.
The picture is not one-sided, and the comparison is more interesting than it looks. An alkaline’s voltage falls steadily from 1.5 V as it discharges, so it spends most of its life below its nominal figure and much of its stated energy sits below the voltage a device will still run at. NiMH holds close to 1.2 V for most of its discharge and then drops sharply. So for a high-drain device — a camera flash, a motorised toy — the rechargeable often does better in practice despite the arithmetic, while for a low-drain one — a clock, a remote — the alkaline’s extra energy and far lower self-discharge win comfortably.
Runtime, C-rate and what the arithmetic misses
Two more figures come out of a battery’s capacity, and both depend on the charge rather than the energy — so unlike everything above, the voltage does not enter them.
Runtime at a steady draw is simply capacity divided by current: 3,000 mAh at 250 mA is 12 hours. C-rate is a multiple of the capacity expressed in ampere-hours, so 1C on a 3,000 mAh cell is 3 A, 0.5C is 1.5 A, and 2C is 6 A. Manufacturers quote charge and discharge limits this way because it scales across cell sizes.
Both are upper bounds rather than predictions, for reasons worth knowing. Capacity falls as the discharge current rises — Peukert’s effect — so a cell that gives 3,000 mAh at a gentle draw may give appreciably less at 2C. Capacity also falls in the cold, which is why phones die at what they claim is 30% on a winter morning and recover when warmed. And a battery loses capacity permanently as it ages, typically to around 80% of its original figure after a few hundred full cycles.
So the arithmetic gives you the best case. Halve it for a rough real-world planning figure on anything that matters, and measure rather than calculate if the answer is load-bearing.
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The arithmetic is charge times voltage and needs no authority. What the sources settle is everything around it: the watt-hour thresholds that airlines actually apply and why they are written in watt-hours, what “nominal voltage” means for a cell whose voltage falls through its discharge, and the fact that USB charging happens at 5 volts and above rather than at the cell voltage — which is where the conversion losses on this page come from.
Two things this page cannot tell you. First, it is a unit conversion and not an airline policy: the 100 Wh and 160 Wh figures are the widely adopted international thresholds, but individual carriers and countries impose their own additional rules on quantity, packing, terminal protection and whether a device may be used or charged in flight, and those change. Check your own airline before you travel, and never put a spare lithium battery in checked baggage. Second, the efficiency in the power-bank estimate is an assumption you supply, not a measurement of your equipment — real round-trip efficiency depends on the bank, the cable, the charging speed and the temperature, and can be well below the figures suggested here when fast-charging a warm phone. Manufacturers' printed capacities are also nominal and are measured under favourable laboratory conditions; a cell delivers less at high current, less in the cold, and less as it ages, so treat every figure on this page as an upper bound rather than a promise. Damaged, swollen or overheating lithium batteries are a fire hazard and should be taken to a proper disposal point rather than binned, charged or carried.
Published the Battery Capacity Converter: mAh to watt-hours at seven cell voltages, real power-bank charge counts, the airline limit, runtime and C-rate.
Makes the voltage a required, prominent input, because mAh is a charge rather than a capacity. The same 37 Wh is shown as every mAh figure it could honestly be labelled with -- a tenfold spread from 3083 at 12 V to 30833 at 1.2 V.
Gives three power-bank answers in descending order: what dividing the two labels suggests, what the energy actually allows, and what survives a stated round-trip efficiency. The gap between the first two is exactly the ratio of the two cell voltages, so the label overstates even at 100 percent efficiency.
Checks the airline thresholds on energy rather than mAh, which is why the rules are written that way -- the same 26800 mAh is allowed at 3.7 V and forbidden at 12 V. Explains that 26800 mAh is the standard largest bank because 100 Wh falls at 27027 mAh.
Covers the NiMH 1.2 V against alkaline 1.5 V case, where the same mAh buys 20 percent less energy, and why the rechargeable still often wins in a high-drain device despite that.
Keeps runtime and C-rate on the charge alone, since neither involves voltage, and states plainly that both are upper bounds -- capacity falls with current, with cold and with age.
Verified by 59 automated cases, including that an efficiency above 100 percent is refused rather than clamped.
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