Milliamp-hours measure charge, watt-hours measure energy. To compare batteries or check airline limits you need the nominal voltage too. Enter both and the converter gives the energy in Wh, kWh and joules.
Wh = mAh ÷ 1000 × V, kWh = Wh ÷ 1000, J = Wh × 3600.
A 10000 mAh pack rated at 3.6 V stores 10 Ah × 3.6 V = 36 Wh, which is 0.036 kWh or 129,600 J.
Divide the mAh by 1000 to get ampere-hours, then multiply by the voltage. Wh = mAh ÷ 1000 × V. A 5000 mAh battery at 3.7 V holds 5 × 3.7 = 18.5 Wh.
Reverse the formula. mAh = Wh × 1000 ÷ V. For 18.5 Wh at 3.7 V this gives 18.5 × 1000 ÷ 3.7 = 5000 mAh.
Use the nominal voltage printed on the battery or in its datasheet. Single lithium-ion cells are usually rated at 3.6 or 3.7 V and a 12 V lead-acid battery at 12 V. A power bank's mAh rating is normally quoted at the cell voltage, not at the 5 V USB output.
Milliamp-hours ignore voltage. A 1000 mAh cell at 3.7 V stores 3.7 Wh, while a 1000 mAh pack at 12 V stores 12 Wh. Watt-hours are directly comparable.
No. It is the rated energy. Conversion losses, such as boosting a cell to 5 V, and ageing mean you get less, so for runtime estimates also use the battery life calculator.