1/4 in. barb and 1/4 in. swivel nut for flare fitting connections. If you're connecting to stainless steel or brass, you will also need a nylon flare washer to form a seal. Plastic quick disconnects (QDs) on the other hand, have a built-in washer so the additional nylon flare washer is not needed. The barb and nut are both made from stainless steel.
We recommend using 3/16" ID tubing for a tight fit.