Actually a 5/8" hose barb isn't actually 5/8" dia. Think about it, if the barb was exactly the same size as the hose inside dia they would just slide on and off very easily. I just looked up one manuf. hose barb sizes and for a 5/8" barb the largest part of the barb is .712" or 18.08mm.
So yes 18mm is the same as a 5/8" 'barb' connection.
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