There were 4 main varieties of hammers used in Medieval combat.
First, there was the pollaxe (where "poll" refers to the hammer). This was a classic knightly weapon, used in tournament, judicial duels, and on the battlefield. This was a two-handed polearm, 6-7' long (180-210cm), often about 3kg in weight. Trusting spike, hammer, and axe blade, and often a butt spike. Some examples:
Second, there were similar polearms but with a spike instead of the axe blade. These often have a small pronged hammer head, rather than the large flat one seen on the classic pollaxe. These polearms are often simply referred to as "war hammer", but also Lucerne hammer, bec-de-corbin (crow's beak), etc. These are often somewhat lighter than pollaxes, but are still usually heavy polearms, often from 2.5kg to 3kg. Some examples:
https://collections.royalarmouries.org/object/rac-object-17000.html
and an outstandingly heavy one, at 3.85kg: https://collections.royalarmouries.org/object/rac-object-3293.html
Third, there were one-handed hammers, typically used by armoured cavalry against armoured cavalry. These typically have a hammer head and a back spike. As one-handed weapons, these are much lighter than the two-handed polearms above, often 1-1.2kg. Some examples:
https://collections.royalarmouries.org/object/rac-object-40743.html
and a lightweight (about 0.5kg) Ottoman example: https://collections.royalarmouries.org/object/rac-object-1754.html
Fourth, hammers intended mainly as tools were used as weapons. These were often described as "mauls", heavy two-handed hammers, often with wooden or iron-reinforced wooden heads. These were sometimes used as improved weapons by rebels, and were used in battle by English archers in the Hundred Years War (who had them to use to drive stakes in when preparing field fortification to protect themselves against enemy cavalry).