Ring gauge is the diameter of a cigar, measured in sixty-fourths of an inch. A cigar with a ring gauge of fifty is fifty sixty-fourths of an inch across, or roughly three-quarters of an inch in plain measurement. The number is part of how every premium cigar is described; a Robusto at five by fifty is five inches long with a fifty ring gauge.
Ring gauge matters because it changes the ratio of wrapper to filler in each puff. A thin-ring Corona at forty-two presents more wrapper per draw than a thick-ring Gordo at sixty. The wrapper carries roughly forty to sixty percent of a cigar’s flavour, depending on the blend, so changing the ring gauge changes which voice in the blend comes through loudest.
The contemporary trend has been toward thicker ring gauges. Forty years ago the industry default was the forty-two-ring Corona; today it is the fifty-ring Robusto, and Gordos at sixty plus are common. Thicker is not necessarily better; it is simply a different cigar. Many serious smokers eventually return to thinner rings precisely because the wrapper shows more clearly.
For more on construction, see The Anatomy of a Cigar.