All li-ion batteries are considered to have an output of 3.7 volts, whether they are of 14500, 18350, 18490, or 18650 size. Right off the charger they should be 4.1 - 4.2 volts, and can be used down to 3.4 volts when they should be replaced with another charged battery.
If you are using a mechanical mod or a non-variable voltage eGo battery, the voltage is totally dependent on the batteries output (3.7 volts). If you are using a regulated mod (VV/VW), voltage or wattage raised or lowered from 3.7 volts by setting the processor to a certain voltage or wattage setting.
To help visualize the relationship between resistance (ohms), voltage, and watts the Safe Vaping Power Chart can be used as a general guideline: