3/8 inch tap washer.
Tap washers were originally sized according to the size of the threaded connection into the bottom of the tap, this means the name of the washer doesn't match up with it's actual physical size.
Approx physical measurement : 0.625 inch (5/8" / 15.8mm) diameter.
Price is for one washer.
This size of washer is used in some newer bathroom basin taps.