One hectogram is exactly 100 grams. The unit is little used in English, but it is standard at the delicatessen counter across the Nordic countries and Italy, where it is the natural size for buying cold cuts and cheese.
About the units
The hectogram is the gram with the prefix hecto, from the Greek hekaton, a hundred. It is one of the metric prefixes that SI keeps but discourages, since the system prefers steps of a thousand — gram, kilogram — with nothing in between. Like the centimetre, it survived because it matches a real human scale that the preferred prefixes miss: a hundred grams is about a portion, and neither ten grams nor a kilogram serves that purpose. In Swedish and Norwegian it is called hekto and is used constantly in food shopping; in Italian, the etto plays exactly the same role.
The exact factor
The factor is exactly 100, a two-place decimal shift. 2.5 hg is 250 g; 400 g is 4 hg. One kilogram is 10 hectograms, which is the relationship that makes the unit convenient at a counter: a price per hectogram multiplied by ten gives the price per kilogram, and comparing the two is a single mental step. The unit sits between the decagram, ten grams, and the kilogram, both of which see far less everyday use.
Where you meet this conversion
Delicatessen and market shopping in Sweden, Norway, Denmark and Italy. Ham, salami, cheese and prepared salads are ordered by the hectogram — "two hekto of ham" — and priced the same way. Swedish recipes use it for meat and cheese quantities. The unit occasionally appears in nutritional comparisons, where values per 100 g are effectively per hectogram, which is why nutrition tables are so often built on that number even in countries that never use the word.