this is easy for an X-clammer...A peck it 1/4 bushel (dry measure) if you want liquid measure...it is about
: 2.3272943922678 gallons. (give or take

)
Striperman is right, 2 gallon is used for a dry meausure or 1/2 of your 4 gallon bucket.
Generally for clams you refer to dry measures.
Happy digging.