What is the voltage of a D cell battery?

A standard D cell battery has a nominal voltage of 1.5 volts.

The D designation describes the battery’s physical size, not its chemistry. Common disposable D cells use alkaline or zinc–carbon chemistry, both generally rated at 1.5 volts. A fresh alkaline cell may measure slightly above 1.5 volts without a load, then gradually fall as it discharges.

Voltage is separate from capacity. D cells are much larger than AA or C cells, so they can typically store more energy and deliver current for longer, while still providing the same nominal voltage. Connecting two D cells in series produces about 3 volts; connecting them in parallel keeps the voltage near 1.5 volts but increases available capacity.

The main mix-up involves rechargeable D cells. Nickel–metal hydride and nickel–cadmium versions are usually rated at 1.2 volts because their chemistry has a lower nominal cell voltage. Thus, 1.5 volts is the canonical answer for the ordinary disposable D battery meant by this question, while 1.2 volts applies to many rechargeable alternatives.