Weight is a measurement of how much a character can resist knockback. Weight is one of several factors used in calculating the amount of knockback a character experiences. Characters with a higher weight (heavy) tend to suffer less knockback, and characters with a lower weight (light) tend to suffer more knockback, all other factors controlled.
Weight is, in practice, understood as how difficult it is to send a character flying away. In this sense, it is generally considered an advantage for a character to be heavy, as less knockback makes it harder to KO a character. Additionally, many throws take longer to execute on heavier characters, giving the opponent being thrown more time to properly react to throws and to DI effectively. Because throwing heavier characters causes the throwing animation to continue longer once the target is released, the viability of throw combos may be affected; for example, Mr. Game & Watch's down-throw-to-down-smash combo isn't guaranteed on heavier characters who don't tech the throw; the Ice Climbers' infinite chain grabs in Brawl are more difficult to perform on heavyweights; and in Melee, Captain Falcon is not vulnerable to many of the up throw chain throws (such as from Marth) that the space animals are vulnerable to, since although equivalent falling speed to them, his heavier weight reduces knockback.
Lighter characters do have a few advantages. Weight-sensitive platforms such as those in Mushroom Kingdom and Rainbow Cruise fall slower while holding less weight, making them safer to use for lighter characters. Many combos are less effective on lighter characters because their higher received knockback makes it easier to escape — these combos include some of Fox's waveshine combos in Melee and King Dedede's chaingrab in Brawl, where characters below a certain weight usually receive enough knockback to escape. Finally, certain weight-based throws have hitboxes that appear for very short lengths of time, and if the target is very light, the animation may progress so quickly that the hitbox appears and vanishes in less than a frame without hitting — this causes the lightest characters to take less damage, notably with Bowser's down throw in NTSC Melee and Link's down throw in Brawl, both of which miss their pre-throw hit on Jigglypuff. As of Ultimate, weight dependent throws are no longer present. In addition, weight is now a factor in determining how much a character can be pushed by an opponent running into them, and how far they will push other opponents.
Heavier characters tend to have stronger attacks, longer range, worse recovery (that is; recoveries that are slower or cover less distance), bigger hurtboxes, and slower movement, while lighter characters tend to have weaker attacks, shorter range, better recovery, smaller hurtboxes, and faster movement. However, this is a a loose trend that many characters defy in some way — Captain Falcon is heavy but dashes incredibly fast, Yoshi is heavy but has a good jump and weaker attacks, Wario is heavy but short and highly maneuverable, Zelda is light-medium with slow and strong attacks, Falco is light with a very high falling speed and lackluster vertical recovery, and Little Mac is light but is considered among the worst at recovering in his games.
In single player modes, sometimes unnaturally high weight is introduced to challenge the player, such as when fighting Metal Mario; this is often paired with additional knockback resistance, since even characters with infinite weight will still take knockback from any attack with a base knockback greater than 0.
On an additional note, some attacks are weight-independent, and will always act as if the target fighter's weight is set to 100; the amount of knockback dealt will remain consistent, no matter how light or heavy the target is. This is especially notable for attacks that have the bury, paralyze, or stun effects. However, other factors, such as knockback taken multipliers, and knockback resistance, would still affect the amount of knockback dealt.
Super Smash Bros. weight values
These values use a different scale than the later games - heavier characters have lower numbers, representing a direct multiplier in the knockback formula. For comparison purposes, the equivalent value in the newer system is also listed.
Super Smash Bros. Melee weight values
Following are the characters in Super Smash Bros. Melee, ranked in order of heaviest to lightest. The Metal Box multiplies a character's base weight by 3, the Super Mushroom by 1.6, and the Poison Mushroom by 0.625.
Super Smash Bros. Brawl weight values
This is a list of characters' weights in Super Smash Bros. Brawl.
Super Smash Bros. 4 weight values
This is a list of characters' weights in Super Smash Bros. 4.
According to the game, weight is the base factor in determining how many powers can be equipped in Smash Run. There are other factors — namely a character's speed — so it is not a direct linear correlation, but as a general rule weight definitely carries a positive correlation: the higher the weight, the greater the number of powers, with the only exceptions being Miis and the DLC fighters having a power limit of 25.
Certain updates have made slight alterations to the weights of characters, affecting both survivability and resistance to combos. These changes have not had a major effect on most characters in the metagame, with the exceptions of Sheik and Bowser, who are considered to be worse and better after their weight alterations, respectively.
Super Smash Bros. Ultimate weight values
This is a list of characters' weights in Super Smash Bros. Ultimate.
Mii Fighters now have their weight determined by their class rather than their size modifiers from their creation.
The average weight value for all fighters is rounded down to 96.01, meaning that Pit, Dark Pit, Ivysaur, and Wii Fit Trainer are the characters whose weights are closest to the average.