Is 18650 battery capacity from 3V or 0V?
The rated capacity of a battery is measured between its rated full charge and rated end of life voltages. With lithium polymer, these are usually 4.2&nbsp;V for full charge, but the end of life quoted varies between manufacturers. I have seen 2.5, 2.7 and 3.0&nbsp;V all used as the end of life voltage. There is no 'right' voltage, as higher end point voltages extend the cycle life of the battery, at the expense of capacity, leaving the capacity/lifetime tradeoff as your choice.

If you are lucky, the manufacturer quotes the life of the battery, the number of expected charge/discharge cycles, between the same voltages. If you are unlucky, the number of cycles will be quoted to a higher voltage than the Ah capacity.