Knockback is the measure of how far an attack sends its target. For most attacks, knockback increases as damage on a character increases. An example of an attack with high knockback is Bowser's forward smash; it launches opponents very far, so far that at high damages it's more powerful than a Home-Run Bat's smash attack. Pikachu's neutral attack, on the other hand, has very low knockback - it hardly sends opponents anywhere, even at ludicrous damages. Some attacks with relatively low knockback are good for combos.
The knockback of almost every attack works in tandem with the target's damage - it increases as the target's damage gets higher. It is also affected by the target's weight, gravity, and a few other conditions (such as type effectiveness). The damage dealt by an attack is a significant factor in how much knockback it deals, but it is not the primary factor - this is the reason stale moves have less knockback than fresh ones, and why many chargable special moves deal less knockback when fully charged (the knockback values of the fully-charged move are altered to compensate for the higher damage, whereas the same is often not true when not fully charged).
Each hitbox of a move has two knockback values: a base knockback and a knockback scaling (also known as knockback growth). Base knockback is the minimum amount of knockback the attack can deliver (in normal circumstances), and knockback scaling is a factor that controls how much the knockback increases as damage increases. Moves with high base knockback deal high knockback under any circumstances, such as the swing of a Home-Run Bat. They additionally tend to be more effective at KOing when less knockback is needed to KO (such as against lighter characters, when near the edge of the stage, or when an opponent is handicapped to sustain more knockback). On the other hand, moves with high knockback scaling take less damage to reach KO potential, such as Luigi's forward smash. They additionally tend to be more effective at KOing when the opponent requires more knockback to be KO'd (such as against heavier characters, when far away from a stage's blast line, or when an opponent is handicapped to sustain less knockback). Advanced techniques to extend survival such as DI and momentum cancelling are also less effective the stronger a move's knockback scaling is.
In Melee, the highest knockback delivered and received by each character is given in a match's results screen, labeled as "Fastest Pitch" and "Top Speed" respectively, though the numbers lack meaning. In Brawl, the velocity applied (in units per 1000 frames) is provided instead of the knockback value (though it's simply knockback divided by 0.03). The unit is given as "mph"; while this would presumably mean "miles per hour", this is obviously not the case, as a 1,000 mph hit barely sends characters anywhere. In Super Smash Bros. for Nintendo 3DS, knockback units are again shown without a meaning, while in Super Smash Bros. for Wii U, they are measured in "mph" as in Brawl, though in a lower scale. Strangely enough, knockback for a same move between the two games is actually shown in different scales: Ganondorf's Warlock Punch, for instance, scores 102 units of knockback against Mario at 0% damage (without other modifiers) in Smash 3DS, while scoring 53 mph under the same conditions in Smash U.
The following table gives an idea of various knockback strengths:
Visual effects known as angle indicators appear in Super Smash Bros., Super Smash Bros. Brawl, and Super Smash Bros. 4 when a hitbox collides with an opponent. As its name suggests, the angle indicator shows the angle of hitboxes. In Brawl, angle indicators show different colors depending on who or what they hit. With player one, the angle indicator is red. The second player's angle indicator is blue, with the third and fourth players' angle indicators yellow and green, respectively. Computer-controlled opponents and objects like Sandbag have white angle indicators. Angle indicators appear in Super Smash Bros. as small "bubbles" appearing in the direction of the angle. In Smash 4, angle indicators appear as colorful, stylized "debris" flying off of an impact. Smoke trails (appearing in all four games to date) also indicate the angle at which a character is knocked away, but only appear when a character has taken a lot of damage, and are thus much less useful.
Brawl uses a second formula to alter knockback based on the target's gravity:
The result of this formula is added to the vertical component of the knockback. As a result, characters with higher gravity take more knockback than those with lower gravity, depending on how vertical the angle is.
If the move has a weight-based knockback value set (so the move deals set knockback), then d is set to that value, and p is always 10.
Other physics of knockback
Set knockback, also referred to as fixed knockback, is a property of some attacks where the knockback dealt by the attack does not depend on the opponent's percentage or how much damage the attack does. This results in it dealing the same amount of knockback regardless of stale-move negation. Usually, set knockback is used for the first hits of a natural combo or multi-hit move, while the last hit deals normal knockback (sometimes called "scaling knockback" to contrast), to make it easier for the entire attack to hit regardless of the opponent's damage. While damage is not a factor for set knockback, the character's weight still is, so the term weight-dependent set knockback is sometimes used.
List of set knockback moves
From Melee onward, there are certain attacks that deal zero knockback, causing damage as usual but not flinching, meaning that the hit character can still perform any actions while being attacked. Most attacks with no knockback can be used for racking up damage quickly, as they can hit foes repeatedly without flinching them or sending them away; other attacks that do cause knockback have hitboxes with no knockback in the middle of them for a similar purpose, an example being Roy's Blazer in Melee. These moves can also be used to "steal" KOs from opponents in free for all battles, as while they do not cause knockback, they will take ownership of the KO on an already fatally struck opponent. Fox's Blaster is an infamous example of an attack with no knockback. Due to the fact that it does not deal knockback, it will not interrupt an opponent's moves or force them out of a punishable state such as being asleep or being stuck in the ending lag of a missed Rest. Idle characters in Brawl will also turn around if they are repeatedly struck with moves that deal no knockback, which is noticeable with Fox's Blaster.
In Melee, phantom hits damage foes, but without any knockback. It is possible for any character to receive no knockback from attacks by certain factors, such as by being giant and metal simultaneously on a low damage ratio battle, or during the actions of certain attacks in Brawl, such as Charizard during the first frames of Fly. This is known as launch resistance.
List of moves with no knockback
Knockback stacking is the means by which the game applies knockback to a character who has already been sent flying. The new knockback will sometimes replace the old knockback entirely, sometimes have no effect at all, and sometimes a merge will be performed between the two velocities. This has a large effect on combos, as the character being comboed will often have already been struck when another hit lands.
In Melee, the method of knockback stacking used varies based on how many frames separate the first and second hit in addition to the relative angle between the two knockback directions. When the two hits are separated by fewer than 10 frames, the second hit's knockback always replaces the first's. This prevents attacks that hit repeatedly from accelerating the character that they hit with each strike, which would be the case were they added. This can also lead to some interesting scenarios that allow strong hits to be canceled into weaker ones, although this is mostly prevalent in team battles and with certain items. If the two hits are separated by 10 or more frames, the knockback is either set to the stronger value or the two are merged. The relative angle between the two hits' launch angles comes into play here: if the angle is large enough (meaning the hits are in opposite directions), then the two hits are merged via vector addition. However, if the relative angle is small (meaning the hits are in the same direction) the stronger hit is used and entirely replaces the weaker one. Since merging occurs when the hits are in opposite directions, merged knockbacks will often be in a direction completely unrelated to either of the previous two. For example, if a character is struck up and right by a strong hit and then up and left by an equally strong hit more than ten frames later, the character will be sent straight up (as the horizontal values cancel out and the vertical ones stack). This often leads to there being residual knockback from a previous hit in a combo which must be taken into account when trying to follow up.
In addition, hitstun duration is always refreshed by the second hit regardless of the time and angle between them, and the amount of hitstun applied is the amount that would be applied if the opponent had not already been hit. For example, the additional knockback put on the opponent from any merging does not factor in to the equation used to calculate the amount of hitstun that should be put on the opponent.
In Brawl and Smash 4
In Brawl, the second hit always completely replaces the first hit's direction unless the first hit was much stronger, in which case the direction does not change. No merging is ever performed. This is carried over to Smash 4.
The following is a chart demonstrating how much knockback each character sustains in Brawl. While weight is the primary attribute for determining how much knockback a character sustains, there are other attributes that factor in, such as a character's gravity. This results in some characters sustaining more knockback than lighter characters.
The knockback value shown is how much knockback a character sustains when hit with Marth's Critical Hit at 0%, arranged from least knockback sustained to most knockback sustained.