In Vermintide 2, damage/cleave/stagger is calculated based on Hero Power. You can find it in your inventory.
Then you can calculate damage/cleave/stagger from there, based on the lowest Hero Power value that the game can provide(15).

Cleave is a metric that measures the amount of Effective Mass that an attack can damage/stagger.

Damage Cleave affects the amount of Effective Mass you can damage.
Stagger Cleave affects the amount of Effective Mass you can stagger.

Cleave is tied to the attack profile, not the weapon. Different attacks on the same weapon can have different Cleave. Different attacks on different weapons can have the same Cleave.

In order to cleave next unit:
(Modified Cleave) - (total Effective Mass of enemies that have been cleaved) > 0

In the Formula above:
Modified Cleave = Base Cleave x Product of Green Buff x Sum of Cyan Buff x Sum of Orange Buff
Effective Mass = Base Mass x Effective Mass Modifier value

Use Damage Cleave as Base Cleave to calculate the number of units you can damage.
Use Stagger Cleave as Base Cleave to calculate the number of units you can stagger.

Base Cleave:
Variable on different weapons and attacks. Can be found in mod Armory.

Red circle shows Damage Cleave, green circle shows stagger cleave(for 650 Hero Power).



















Green/Cyan/Orange Buff:
You can check these buff in the table down below.
































Base Mass:
Can be found in mod Vermitannica. Shield sometimes also adds mass to enemy units.

Shield added mass is in the brackets.



















Effective Mass Modifier:
Including Linesman, Heavy Linesman, Tank. Most of these values can be found in Bestiary. All exact values will be listed in the next section.

Modifiers on weapons

Modifier number on specific type of enemy

Effective Mass Modifier value

Cleave Resistant Units are units that will instantly stop your cleave once you hit them, regardless of your Cleave value.
Some attacks have the special ability to cleave these units. In that case, cleave calculation will remain the same.
Different attacks have different Cleave Resistant Units.

Cleave Resistant Units including:
Melee Attacks:
Armoured Enemies, Mauler, Monster Enemies.
Projectile Range Attacks:
Armoured Enemies, Shield Enemies, Monster Enemies.
Hitscan Range Attacks:
Armoured Enemies, Shield Enemies, Mauler Head.

Attacks that bypass Cleave Resistant Units:
Melee Attacks:

Projectile Range Attacks:

Hitscan Range Attacks:

Shotgun attacks are Blunderbuss Primary, Grudge Raker Primary, Griffon-foot Primary, Repeater Pistol Secondary, Bounty Hunter Locked and Loaded shotgun shells and Coruscation Staff Primary. Their cleave calculation has two differences compared to other attacks.

1. Effective Mass Modifiers
All these shotgun attacks except Coruscation Staff Primary all have Linesman as Effective Mass Modifier. However, unlike on Melee Attacks and Projectile Range Attacks that Linesman reduce Enemy Base Mass by percentage, on these shotgun attacks Linesman set all affected Enemy Mass to 0.6. This means with Linesman on shotgun attacks, Slave Rat/Clan Rat/Plague Monk/Fanatic/Marauder/Savage/Mauler/Ungor/Gor all have only 0.6 Effective Mass across all difficulties.

2. Dead Units Mass Removal
If units are killed by last pellet, their mass won't contribute to the total mass for the next pellet's cleave calculation.
You can think like shotgun attacks actually shoot multiple pellets one by one in a very small interval. So once the Nth pellets killed this unit, N+ pellet(s) cleave won't be consumed by the dead unit mass.

Normal Cleave Calculation Example:
650 Hero PowerXsword Foot Knight(only use talent Comrades in Arms) vs Ungor + Gor

- Xsword Light Attack 1 swing from right to left. Has Base Damage Cleave of 7.46 and Tank. The only Buff here is Comrades in Arms, so Modified Cleave = 7.46 x 110% = 8.206
- On Cataclysm, Ungor has Base Mass of 2.5, Gor has Base Mass of 7. With Effective Mass Modifier Tank, the Effective Mass of Ungor = 2.5 x 75% = 1.875, Effective Mass of Gor = 7 x 50% = 3.5.

- If you use Light Attack 1, try to cleave enemies with the composition of Ungor - Gor - Ungor - Gor - Ungor:

• 8.206 - 1.875 - 3.5 - 1.875 = 0.956, so it CAN cleave through 2 Ungor + 1 Gor.
  
• 0.956 - 3.5 = -2.544 < 0, so after that it can cleave 1 more Gor.
  
• In total you cleave through 2 Ungor + 2 Gor.

Shotgun Cleave Calculation Example:
650 Hero Power Grudge Raker/Blunderbuss(no crit no headshot) vs Massed Maulers

- One Grudge Raker shot contains 9 pellets, one Blunderbuss contains 12 pellets.
- On Cataclysm, with Enhanced Power + 40% Chaos/Infantry, for both shotguns each pellet can cleave 6 Mauler(calculation is the same as other normal weapons, remember Mauler Effective Mass = 0.6 here) and it takes 6 pellets damage to kill a Mauler. Penetrating Maulers do not reduce pellets damage.

- If you shoot one shell into a bunch of Maulers(assuming they stack perfectly & all your pellets land on one point):

• With Grudge Raker, the first 6 pellets will kill 6 maulers, and the last 3 pellets will damage 6 Mauler. Resulting in killing 6 Mauler & damage 6 Mauler.
  
• With Blunderbuss, the first 6 pellets will kill 6 maulers, and the last 6 pellets will kill another 6 Mauler. Resulting in killing 12 Mauler.

Including Sword & Shield Heavy 1, Mace/Hammer/Skull-Splitter & Shield Heavy 1, Axe & Shield Heavy 1, Bretonnian Sword & Shield Light 3 & Heavy 2, Flail & Shield Light 3, Dagger Heavy 1, Fire Sword Heavy 1, Blunderbuss Secondary Attack, Grudge Raker Secondary Attack, Griffon-foot Secondary Attack

Bash attacks are comprised of multiple separated attacks.

• The first attack has a very small hitbox under your crosshair; in real gameplay, this can only hit one enemy. It deals the most damage and highest stagger. Blunderbuss/Grudge Raker/Griffon-foot Secondary Attack does not have this hitbox.
  
• The second attack hits infinite enemies in a narrow cone around the crosshair. Enemies hit by this will take same damage and stagger (both less than the first attack).
  
• The third attack hits infinite enemies in a wider cone around the crosshair. Enemies hit by this will take no damage and lowest stagger.

Including Drakegun Primary & Secondary Attack, Drakefire Pistol Secondary Attack, Beam Staff Secondary Attack, Flamestorm Primary & Secondary Attack

Limited Target Attacks will hit a constant number of enemies in the attack hitbox; regardless of the type of enemy, difficulty, or power level.

Drakefire Pistol/Beam Staff Secondary Attack has a target limit of 10.
Drakegun/Flamestorm Staff Primary & Secondary Attack has a target limit of 50.

Including Bombs, Hagbane Shortbow Secondary Attack Explosion, Flaming Flail Heavy 1, Fireball Staff Secondary Attack Explosion etc.

Explosions can hit all enemies in the attack hitbox.

Both Soulstealer Staff attacks do not use Cleave.
The primary attack - Cursed Bolt can only hit one target a time, but can bounce twice, hit up to 3 targets per attack.
The alternative attack - Soul Rip can only hit one target a time.

*All with 650 Hero Power

Foot Knight - Valiant Charge
With 650 Hero Power, can charge through total mass of 59.68. Instantly stopped by Armoured or Monster enemies. Charge limit can only be increased by Power buff. Talent Battering Ram(wide charge) removes both mass and enemy type limits.

Slayer - Leap
Always ignore mass and enemy type limits.
During Leap buff duration, Slayer is immune to enemy stagger.

Handmaiden - Dash
Always ignore mass and enemy type limits.

Zealot - Holy Fervour
With 650 Hero Power, can charge through total mass of 17.9. Instantly stopped by Armoured or Monster enemies. Charge limit can only be increased by Power buff.
During Holy Fervour buff duration, Zealot gains phase walk.

Battle Wizard - Fire Walk
Always ignore mass and enemy type limits.

In Patch 2.0 - WoM update, FatShark introduced Stagger Bonus system.

Damage vs staggered enemies will increase by stacking staggers.
This scales from 1x bonus (Stagger Count = 0), 1.2x bonus (Stagger Count = 1) up to 1.4x (Stagger Count = 2) bonus damage.

Melee attacks now deal from 100% damage to 140% damage depends on the Stagger Count on enemies.
Ranged attacks deal 120% damage to enemies with Stagger Count = 0 or 1. Deal 140% damage to enemies with Stagger Count = 2.
DoT (burn/bleed/poison) does not receive any bonus from Stagger Count.

Stagger Count is how many times an enemy have been staggered in succession (stagger once for 1, stagger again during previous stagger for 2, further stagger will cap Stagger Count at 2).
Here, a successful stagger is defined as "If the enemy's move is interrupted".

Weapons carry three types of stagger power: Light, Medium and Heavy. You can tell the difference from the animation of the enemy.

Originally posted by Fatshark:

• Light: a flinch in place (fencing sword vs Clan rat)
  
• Medium: a stepback (sword vs Clan rat)
  
• Heavy: a throw and fall over (big hammer vs face of Clan rat)

On horde, Light/Medium attack will increase the Stagger Count by 1. Heavy attack will increase the Stagger Count by 2.

Push also set horde units Stagger Count to 2. This means push horde while killing them will greatly increase your killing speed.

During climbing/dropping animation, all enemies' Stagger Count is set to be 2. Use this opportunity to kill them safely and efficiently.

Weave mode has difficulty beyond Cataclysm(Cataclysm 1) - Cataclysm 2 and Cataclysm 3. In Cataclysm 2 and Cataclysm 3, Stagger Bonus gives different damage bonuses. In Cataclysm 2, the coefficient of Stagger Bonus becomes 0.3, and becomes 0.5 in Cataclysm 3. This means your melee weapon, by default, will deal 130% damage to enemies with Stagger Count = 1 in Cataclysm 2, and 150% damage in Cataclysm 3.

Here is a table for damage bonus comparison(for specific mechanic, check former sections):

* Modded realm difficulty Deathwish will lock your Stagger Bonus at Cataclysm 1.

The overall mechanic of stagger is very complicated. The best way to determine the stagger power of a weapon right now is go modded realm and test them one-by-one. Here are some general rules that I can provide:

Player wise:
1. Do you have enough Stagger Cleave? If you cannot cleave to this enemy with your Stagger Cleave, you cannot stagger this enemy.
2. What’s your power level? Increased Hero Power will increase stagger. Increase some other buff like power vs race/armour type will also increase stagger.
3. What type of weapon? Generally stagger power of 2h weapon > 1h/dual wield weapon. Also weapons with tank property/blunt weapons tend to have better stagger.
4. What type of attack? Generally stagger power of heavy attack > light attack.
6. Did you score a headshot/crit shot? Headshot and crit increase the attack's stagger power.

Enemy wise:
1. What’s the stagger resistance of the enemy? Values are available in the mod Bestiary.
2. What’s the armour type of the enemy? Generally monster > armour/berserker > infantry.
3. What’s the state of the enemy? Enemies that are performing attacks (especially charging attacks) are harder to stagger.

Some Stagger Breakpoints on Cataclysm:
https://steamproxy.net/sharedfiles/filedetails/?id=2200946092