Critical hits, also known as crits or full Crits, are attacks that do extra damage and have distinctive sound effects when fired and upon hitting a target. Players on the receiving end of the attack have the words "CRITICAL HIT!!!" in green, or "MINI-Crit!" in yellow, respectively, appear above their head. Buildings are immune to both types.

Critical hits can be easily distinguished from normal hits. Most melee weapons have a unique swinging animation, projectiles such as rockets or stickybombs have a team-colored glow and sparkle effect (exemptions are the Cow Mangler 5000, the Righteous Bison, and the Pomson 6000), bullets have team-colored trails, and flamethrowers produce additional team-colored particles. These visual effects are accompanied by a crackling electric sound upon firing and receiving a Critical hit.

Self-inflicted Critical hits do not deal critical damage to the user, but still play the sound effect.

Critical hits deal three times the base damage of the weapon and ignore both the distance modification and the random damage modification of normal shots, though explosive weapon Critical hits still suffer from splash damage fall-off. As a result, weapons that are normally merely nuisances at long range (such as Miniguns) can become dangerous when critical hitting. The extra damage ignores most resistances, meaning it cannot be resisted by the Vaccinator's normal healing or Upgrade Station buffs — though other mechanics can target the critical damage specifically, such as the Battalion's Backup and the Vaccinator's ÜberCharge, which fully neutralize the extra damage.

When a player is killed by a Critical hit, the class lets out a loud scream that differs from ordinary non-melee deaths, unless the class was gibbed (in which case they let out a larger "ouch" noise than explosive kills usually cause). The kill feed icon for a Critical hit surrounds the weapon icon with a red glow.

Critical hits and Mini-Crits do not stack in any form — that is, meeting multiple conditions to earn a Critical hit or a Mini-Crit do not deal any more than the normal damage boost. For example, the Frontier Justice's "Revenge" Critical hits do not deal any additional damage if the Engineer is ÜberCharged by a Kritzkrieg, nor does a player marked for death and coated in Jarate suffer more than standard Mini-Crits. This means that team coordination is required if multiple players have ways to increase their Critical hit output, or extra damage is wasted. If both a Mini-Crit and full Critical hit occur with the same attack, the Critical hit overrides the Mini-Crit.

The base Critical hit chance for standard weapons is 2%. This is modified by a bonus percentage based on how much damage the player has done in the last 20 seconds, which scales from 0% at 0 damage to 10% at 800 damage.[1] For example, if the player has dealt 200 damage in the past 20 seconds, the additional bonus percentage is 2.5% and so the overall Critical hit chance is 4.5%. As a result, the highest natural Critical hit chance attainable is 12%.

Melee weapons are designed to Critical hit far more often than ranged ones in order to encourage their usage at close range. Their base Critical hit chance starts at 15% — higher than attainable for ranged weapons with the recent damage bonus maxed out — and their recent damage bonus scales from 0% to 45%, meaning a player that has done 800 damage or more in the last 20 seconds has a melee Critical hit rate of 60%.

Among those weapons that trigger randomly, Critical hit probability is determined differently based on the type of weapon. Single-shot weapons, such as the Shotgun, Revolver, and the Rocket Launcher, determine the chance to Critical hit with every shot. Thus, every shot of the Shotgun has a 2% chance to launch a flurry of critical pellets, assuming no previous damage done. Rapid-fire weapons, which include the Minigun, Syringe Gun, Flamethrower, and the Pistol, check for Critical hits once a second. With weapons like these, a positive Critical hit determination results in all shots for the next two seconds being critical. This only applies to the active weapon; getting two seconds of Critical hits on one's Pistol does not cause their Shotgun to fire Critical hits during this time.

Crit boost is a state in which a player is guaranteed to deal Critical hits with any weapon that is not specifically unable to score Critical hits. A Crit boost is indicated by the player's weapon glowing in their team color, with sparks of electricity surging and crackling from it. This affects most weapons that do not normally deal random Critical hits, such as the Knife, Sniper Rifle, Gunslinger, and Eyelander, causing them to deal critical attacks for the duration of the boost; however, weapons that do not damage enemies (for example, Mad Milk, Jarate, Bonk! Atomic Punch, or Crit-a-Cola) do not gain any benefits from Crits. Only the Cow Mangler 5000 is exempt from this rule, as it does Mini-Crits instead of Critical hits.

A Crit boost can be attained under the following circumstances:

During Humiliation, the winning team is granted a Crit boost.
The ÜberCharge from the Kritzkrieg grants the Medic's heal target an 8 second Crit boost.
Capturing the intelligence in a game of Capture the Flag gives the entire capturing team a 10 second Crit boost. This value can be altered or disabled by the server host console via a command.
On Halloween maps during Halloween, killed players randomly drop candy-filled Halloween pumpkins. When picked up, the player is awarded around 3–4 seconds of Crit boost.
On the Map Crasher, the intelligence carrier gains Crits until death.
Upon killing an enemy with the Killing Gloves of Boxing, the Heavy is Crit boosted for 5 seconds. The Crit boost applies not only to the Killing Gloves of Boxing, but to all the Heavy's currently equipped weapons.
The Phlogistinator allows a Pyro to Crit boost itself for about 10 seconds upon activating "Mmmph", but the Crit boost only affects the Phlogistinator (or any flamethrower he picks up in that 10 second period).
In Mann vs. Machine, players can fill their Power Up Canteen with a Critical hit Boost buff, which grants them a 5 second Crit boost upon activation.
In the Versus Saxton Hale game mode, all melee weapons except the Market Gardener, Holiday Punch, and all Spy melee weapons are Crit boosted.
The last player in either Versus Saxton Hale or Zombie Infection gains Critical hits for all weapons.
First Blood
In Arena Mode, a 'First Blood' buff is granted to a player who achieves the first kill of a round. It consists of a five-second Crit boost.

If players switch weapons during the buff, the electricity around their weapon disappears, but Critical hits are still delivered.
Triggering the Dead Ringer's feigned death gives the attacker the 'First Blood' buff.
The 'First Blood' buff counts as an ÜberCharge towards the Heavy achievement Supreme Soviet.
Obtaining 'First Blood' as a Scout unlocks the First Blood achievement; killing five enemies with 'First Blood' unlocks First Blood, Part 2.
The 'First Blood' achievement can also be earned by a Scout in Competitive Mode, but no Critical hits are awarded for the first kill.

Mini-Crits add an extra 35% damage to a weapon's damage. Like normal Critical hits, they ignore damage falloff, but unlike Critical hits, they are still given ramp-up at close range and random damage spread if enabled. Full Critical hits override Mini-Crits. Like full Critical hits, Mini-Crits have a glowing red outline in their kill icons and are displayed with a larger font when combat text is enabled. If the player is under a Mini-Crit buff state (such as the Buff Banner or Buffalo Steak Sandvich), or the victim is under a Mini-Crit debuff state (such as being covered in Jarate or marked for death), all taunt attacks also Mini-Crit. Aesthetically, Mini-Crits do not trigger critical death lines from victims, un