Half-plate = 4.75 x 6.5 inches = 121 x 165mm rounded to whole numbers. Allowing for the normal precision with which film sizes are given, especially by some Americans ('3x4 inch' for 3.25 x 4.25 inch quarter-plate), yes, you're right. I'm pretty sure that my one half-plate holder is marked 12,1 x 16,5 cm. The millimetre ain't gonna make much difference...
Originally Posted by Jerevan
Incidentally, half-plate ISN'T half of whole-plate (6.5 x 8.5 inches) -- it's half an inch (12.7mm) bigger on the short dimension -- but quarter-plate IS a quarter of whole-plate.