From TF2 Wiki
Critical hits or Crits are attacks that do extra damage and have distinctive sounds both when fired and when hitting a target. When you hit a player with one, the words "CRITICAL HIT!!!" appear over the target's head in green letters (see image), accompanied by a loud "ding." Critical hits with melee weapons usually have a unique swinging animation (for example the bottle has an overhead swing). Explosives projectiles (such as rockets or sticky bombs) have a team colored glow and sparkle effects and bullets have team-colored trails.
Critical hits usually deal 300% of the base damage of the weapon, and ignore both the distance modification and the random damage modification normal shots are susceptible to. Explosive weapon Crits still suffer from splash damage fall-off.
When a player is killed by a Critical hit, the class will let out a loud scream that differs from ordinary deaths and the kill icon becomes surrounded in a red glow.
Critical hit chance
Standard weapons have a 2% base chance to Crit, while melee weapons have a 15% chance. An additional bonus percentage is added based on the total damage done by the player in the last 20 seconds, which scales from 0% at 0 damage to 10% at 800 damage. For example: dealing 200 damage in the past 20 seconds, the additional bonus percentage is 2.5%.
Among those weapons that trigger randomly, Critical hit chance is rolled differently based on the type of weapon. Single-shot weapons, which, for example, include Shotguns, the Revolver, and the Rocket launcher, roll the chance to Crit with every shot. Thus, every shot of the Shotgun will have a 2% chance to launch a flurry of Critical pellets, assuming no previous damage done. Rapid-fire weapons, however, which include the Minigun, Syringe gun, and Flamethrower, check for Critical hits once a second. With weapons like these, a successful Crit roll results in all shots for the next two seconds being Critical.
The critical hit chance for melee weapons scales more sharply than that for ranged weapons. Melee critical hit chances can reach up to 65%, meaning the bonus they receive scales from 0% to 50%.  A demo of the test can be found at the referenced blog post.
- A Knife only does Critical hits on a Backstab and during crit boost (humiliation, captured intelligence, kritzkrieg)
- Backstabs instantly kill the enemy by dealing damage equal to 6x their current health. When forced to crit, the knife does the backstab animation*, dealing 120 damage.
- The Sniper rifle and Huntsman do not roll for Crits, but rather automatically score Critical hits on successful headshots. The Sniper rifle can only Crit via headshot while scoped, or during humiliation, captured intelligence or kritzkrieg.
- The Axtinguisher scores a Critical only if the victim is on fire, and does half damage otherwise. It may still randomly perform a Crit animation, but will not inflict Crit damage unless the enemy is burning. When forced to Crit, it plays the Crit animation, but does normal (half) damage.
- The Backburner scores normal Critical hits like any other weapon, but also has a guaranteed chance to Crit from behind.
- The K.G.B. roll for Critical hits like a standard melee weapon. However, upon killing a target, they grant 5 seconds of Crit boost to all of the Heavy's weapons.
- The Ambassador rolls for Critical hits like a normal one shot weapon. However, it always scores a Crit on a headshot, except during cooldown.
- The Eyelander does not roll for random Critical hits, but will score Critical hits when under the effects of the Kritzkrieg, humiliation, or when using a full charge of the Chargin Targe. The Chargin Targe can also be used to get a Critical hit for the Bottle, the Pain Train or the Scotsman's Skullcutter.
- Jarate may appear Crit, though this has only a cosmetic effect.
- Some items do not work as conventional weapons and so do not Crit under any circumstance. These include Bonk! Atomic Punch, the Sandvich, PDAs, the disguise kit, Razorback, Invisibility watch, Cloak and Dagger, Dead Ringer and Electro sapper. Taunt attacks also do not Crit, however taunt killing a player under effects of Jarate will result in a mini-crit.
- The Flare gun will always score a critical if it hits a player who is already on fire.
- The Frontier Justice gains guaranteed critical hits if the possessor's sentry gun is destroyed after killing an enemy (or assist in killing one). These are called Revenge Crits.
- The Bushwacka will crit when it would normally mini-crit, specifically if you are near a Soldier using the Buff Banner, hitting an enemy coated in Jarate or hitting a scout using Crit-a-Cola.
Crit boost is the state of having a weapon Critical hit on every fire.
During humiliation every shot fired by the winning side is forced to Critical hit, including body shots with the Sniper rifle and Huntsman. However, swings with the Knife and Axtinguisher appear to Crit during humiliation (with the backstab and overhead swing animations, respectively), but hit for normal damage unless the proper trigger is met.
All shots are Critical while ÜberCharged with the Kritzkrieg. This does affect the Huntsman and the Sniper rifle, but not the Knife or Axtinguisher.
A 5 second Crit boost is received upon getting first blood in an Arena map. If you happen to switch weapons during this period, the electricity around your weapon will disappear, but you will still be able to fire Criticals until the end of the Crit boost.
Capturing the flag in a CTF game mode gives the entire capturing team 10 seconds of Critboost (this value can be tweaked or disabled in the host's console via a command).
Mini-crits add an extra 35% damage to a weapon's base-damage (more accurately, they ensure that a weapon does at least 135% its base damage). Like normal Critical hits, they ignore damage variance and fall off, but unlike Critical hits, they are still given ramp-up. Full Crits override Mini-crits.
Currently there are eight ways to achieve a Mini-crit:
- Jarate coated enemies take Mini-crits,
- Melee hits midway through a Chargin' Targe charge,
- Hitting a burning enemy with a flare at mid-range or closer,
- Attacking an enemy while under the effects of a friendly Soldier's Buff Banner,
- Hitting an airborne target with the Direct hit,
- Attacking an enemy as a Scout under the effects of Crit-a-Cola,
- Additionally, reflected projectiles always do Mini-crit damage.
- Also hitting while under the effects of Buffalo Steak Sandvich on Heavy will result in Mini-crits.
A Mini-crit boost is the state of having all damage being dealt modified in to Mini-crit damage. Currently this is achievable while under the effects of Crit-a-Cola, an active Buff Banner, or a Buffalo Steak Sandvich. While being Mini-crit boosted, the player's active weapon glows a faint yellow (if they are on RED team) or a faint blue-green (if they are on BLU team) and has the same color effect of particles swirling around their feet.
All damage sources (including Afterburn and Sentries - but apart from hit-scan weapons) only deal Mini-crit damage when the player dealing the damage is being boosted. i.e. If the player leaves the boost (for whatever reason) after firing a projectile and while the projectile is in mid-flight, it will lose it's status as Mini-critical. Conversely, after a Pyro sets fire to several players, returning to a Mini-crit boost will immediately transform any Afterburn damage to Mini-crit status. Sentries will only Mini-crit while the Engineer is being Mini-crit boosted.
Random Crits can be disabled on the PC by using the the server cvar command tf_weapon_criticals 0 and on the Xbox 360 through an options menu available to the host. Note that this does not affect situational Crits, Mini-crits or Crit boosts.
- Buildings are not affected by Critical hits.
- Critical Rockets, Grenades and Stickies used on yourself will cause the "ding" sound. However, they don't deal any additional self-damage.
- If you taunt as a Soldier with the Rocket launcher equipped, the Soldier will make an "L" sign with his hand while yelling "Cuh-rit".
- Killing someone with a Critical rocket will sometimes result in the Soldier yelling: "Cuh-rit" or "Crocket!".
- Upon dominating a Soldier, the Scout will sometimes shout "Dere ain't enough crits in da world ta kill me!"
- If a player covered in Jarate is set on fire, each afterburn tick with play the Mini-crit sound and animation, and will deal the extra damage.
- Mini-crits do not stack to make a Crit. e.g. A scout under the effects of Crit-a-Cola covered in Jarate who is hit midair with a Direct Hit by a Soldier being Mini-crit boosted will only receive an additional 35% damage.
- Mini-crits can boost the amount you heal from the Mad Milk while damaging an enemy player.