When installing a new bathroom sink or drainpipe, you'll need to know what the standard drain hole size is for every bathroom sink. If you want to change your drainpipes due to a crack or leak in the drain, you will need to buy the proper size pipe fittings.
Standard drain sizing
Bathroom taps have a 3.1 cm (1 1/4 inch) diameter drain hole, which is the diameter for all bathroom sinks today. The size is suitable for a bathroom tap because it allows for proper drainage during hand washing, shaving and brushing your teeth. You have no real need for a larger drain size, even in commercial bathroom taps, because the function of the tap does not demand it.
Why standardised drain sizes
All bathroom sinks have standard size drain holes to facilitate the installation of a new drain plug. When you change a tap, the finish should match the drain plug finish. So if you had a chrome tap and install a new brushed nickel tap, the old drain will not match. Tap makers include a matching drain with every tap they sell. Install this for a perfectly matched tap and drain and to update the appliance.
Sink bowl diameters
The sink bowl has a slightly larger diameter than the drain itself. Drain holes in bathroom sinks are 3.5 cm (1.375 inches) in diameter because the drain fits inside the drain hole, and the walls of the drain itself take up some space. Any standard drain assembly fits into an opening of this size very easily. The drain is sealed with plumber's putty to fill any gaps or variances between the drain rim and the sink drain hole surface.
The drainpipe size for the bathroom is standardised at 3.1 cm (1 1/4 inches); however, it feeds into a large pipe. Typically, P-traps for bathroom drains run in the range of 3.7 to 5 cm (1 1/2 to 2 inches). An adaptor connects the two different sized pipes. Usually, the smaller drainpipe is fed into the longer end of the P-trap pipe, which is then connected to the bathroom's main drain line. This pipe is typically 5 cm (2 inches) in diameter; however, the size varies in some homes.