Fitting for 1/4" male flared connections—size up from the tubing ID for a tighter fit.
Metal flare connections will need a 1/4" Flare Washer to make a seal.
Recommended sizing for lines:
1/4" barb: for 3/16" ID lines, most common for beer & beverage line
5/16" barb: for 1/4" ID lines, commonly used in longer draft runs
3/8" barb: for 5/16" ID lines, commonly used for CO2 or other gas supply.
1/2" barb: for 3/8" ID lines, commonly used for uncarbonated liquid transfer