The Power Meter (K.O.ゲージ, K.O. Gauge) is a fighter ability unique to Little Mac in Super Smash Bros. 4 and Super Smash Bros. Ultimate that is shown above his damage meter. It is filled as Little Mac takes or deals damage (even to shields), and each level of charge (represented by the color of the bar) plays a soft, specific sound to notify the player of its charge.
Once the bar is full, a loud bell sound reminiscent of those in a boxing ring plays, and the bar becomes a flashing "K.O." sign, causing his neutral special move, the Straight Lunge, to be replaced with a powerful move named the KO Uppercut. While intended to be a one-hit KO per its name, its base knockback is not quite high enough to OHKO at 0%, and reaches KO potential at about 30% when performed on the ground, although this is dependent on the weight of the character; the lightest character, Pichu, can get KO'd at 10%, where as heavyweights like Bowser or King K. Rool do so at about 40%.
When Little Mac is inhaled by Kirby, Kirby copies Straight Lunge, but does not gain a Power Meter and so does not get the KO Uppercut move.
When Little Mac transforms into Giga Mac, the power meter disappears for the entire duration of the Final Smash.
Filling the Power Meter
The Power Meter fills when it reaches 100 units. To visualize this, the meter is separated by ten arrows that represent ten units each. When receiving damage, the Power Meter is filled by units equivalent to how much damage the attack deals. For example, taking 10% will fill one arrow. However, when Little Mac deals damage, the power meter fills with units equivalent to the damage he deals with a ×0.3 multiplier. So if Little Mac deals 10%, he will only fill the Power Meter by three units. As a result, Little Mac will get KO Uppercut much more slowly should he be at an advantage, while he will get it quicker if he is losing. Overall, Little Mac is guaranteed to get KO Uppercut if he either deals 333.33...% or takes 100%. In the context of a regular game, Mac will usually get KO Uppercut from around 65%.
The meter is emptied when the KO Uppercut move is used, successfully or not, and immediately empties once the player presses the special button. This means it can be depleted if the player spams the special button during hitstun. The meter will also be emptied if Little Mac is KO'd or traverses through a door in Smash Run. After four seconds, Little Mac can also lose KO Uppercut if he is sent tumbling.
The Power Meter appears in the Punch-Out!! series, where successfully landing punches to Little Mac's opponents fills the meter up, while getting hit decreases the meter. Once the meter was full, Little Mac can perform a powerful knockout blow to his opponent. In Mike Tyson's Punch-Out!! and the Wii game, the meter was replaced with the Star Punch System.