10mm (i think pneumatic) to 1/2 inch (for garden hose etc.)
Don't take my word for it,
But you're matching "
millimeter" and "
inches" which are from different measuring systems that's why you're not getting an exact match.
However, a 1/2 inch garden hose's internal diameter of 12.7mm is pretty close to 12mm, and there's a clamp which should secure the fit.
But as I mentioned, I'm not a plumber so DYOR still.