mAh → Wh
Capacity to energy. Wh = mAh × V ÷ 1000.
Wh → mAh
Energy back to capacity. mAh = Wh ÷ V × 1000.
Also 10.00 Ah.
The formula
mAh = (Wh ÷ V) × 1000
That is the whole thing. The part people get wrong is not the arithmetic, it is the voltage.
Why mAh alone tells you almost nothing
mAh measures charge - how many electrons - and says nothing about the push behind them. Energy is charge multiplied by voltage, so mAh is only comparable between cells at the same voltage. Compare a 2000mAh NiMH AA against a 2000mAh 18650 and the mAh figure suggests they are equals. They are not: at 1.2V the AA holds 2.4Wh, and at 3.7V the 18650 holds 7.4Wh. Same headline number, three times the energy.
This is why the cell comparison tool ranks USB-C lithium AA cells in mWh rather than mAh. Those cells are 1.5V, so putting their mAh next to a 1.2V NiMH mAh would flatter them for the wrong reason.
Use the nominal voltage, not the charged voltage
A cell's voltage falls as it discharges. Nominal voltage is the manufacturer's stated average across a typical discharge, and it is the number these conversions assume. A "3.7V" 18650 leaves the charger at 4.2V and is empty around 3.0V - using 4.2V would overstate its energy by about 13%.
- NiMH / NiCd: 1.2V
- Alkaline: 1.5V
- Li-ion (18650, 21700, most power banks): 3.6V or 3.7V
- LiFePO4: 3.2V
- Lithium coin (CR2032): 3.0V
Power banks, the 100Wh limit, and the missing mAh
Power banks are rated at their internal 3.7V cell voltage but hand you 5V at the USB port. Energy is conserved, so pushing the voltage up drops the available mAh, and conversion losses take a further cut. A 10000mAh bank is 37Wh, which at 5V is about 7400mAh before losses and typically 6000 to 7000mAh in practice. Nothing is wrong with your bank - the mAh figure is just being quoted at a voltage you never actually see.
Airlines sidestep the whole mess by regulating Wh, because Wh is the same number no matter what voltage you quote it at. Power banks go in carry-on baggage only. Up to 100Wh is generally fine, 100 to 160Wh usually needs approval from the airline, and over 160Wh is normally refused. Airlines set their own rules on top of the IATA baseline and several tightened them recently, so check your carrier before you fly rather than at the gate.
Common questions
How do you convert mAh to Wh?
Multiply the capacity in mAh by the nominal voltage, then divide by 1000. Wh = (mAh x V) / 1000. A 2000mAh cell at 1.2V holds 2.4Wh.
Why do I need the voltage to convert mAh to Wh?
mAh measures charge, not energy. Energy depends on both charge and voltage, so two cells with the same mAh can hold very different amounts of energy. A 2000mAh NiMH AA at 1.2V holds 2.4Wh; a 2000mAh 18650 at 3.7V holds 7.4Wh - three times the energy from the same mAh number.
What voltage should I use for my battery?
Use the nominal voltage, not the fully charged voltage. NiMH is 1.2V, alkaline is 1.5V, standard Li-ion (18650, 21700) is 3.6V or 3.7V, LiFePO4 is 3.2V, and a lithium coin cell like CR2032 is 3.0V. Most power banks contain 3.7V cells even though they output 5V over USB.
Why does my power bank claim more mAh than it delivers?
Power banks are almost always rated at the internal cell voltage of 3.7V, but deliver at 5V over USB. Converting up to 5V costs energy, so a 10000mAh power bank (37Wh) realistically delivers around 6000 to 7000mAh at 5V. The Wh figure is the honest one, which is exactly why airlines use it.
Can I take my power bank on a plane?
Under IATA rules, power banks must go in carry-on baggage, and those up to 100Wh are generally allowed without airline approval. Between 100Wh and 160Wh usually needs airline approval, and above 160Wh is normally not permitted. Rules vary by airline and change - always confirm with your carrier before flying.
Next: work out how long a cell will run your device, or size a pack with the series and parallel calculator.