2-1/4" hose end adapters are the most common size for the most popular wet / dry vacuums, including: Shop-Vac, Craftsman, Ridgid, Dustless, Wap, Nilfisk- Alto, Workshop, Festool, Pro Team and Genie. These vacuums come with 1-1/4", 1-7/8" or 2-1/2" diameter hoses, but the adapter that plugs into the tank of the vacuum is usually 2-1/4". To check the size of your hose adapter, measure across the center of the adapter that is on the end of vacuum hose or measure across the hose port opening on the vacuum tank.
Some Craftsman and Ridgid vacuums have 2-5/8" tank openings. The common 2-1/4" adapter shown above slides into the larger 2-5/8" adapter. To check the size of your hose adapter, measure across the center of the adapter that is on the end of vacuum hose or measure across the hose port opening on the vacuum tank.