Paladin smites are equivalent-ish to Fighter action surge, and Barbarian's added rage damage + other rage benefits.
No, a paladin with 2 attacks at level 20 is not equivalent to a Fighter with 4. For every "cast a spell" there's an equivalent action surge of 8-9 attacks in a single round nova from the Fighter.
Afaik, the DMG numbers are wrong in spirit and in practice, which is why we use averages per level, such as the ones by Teos. If you can find something in the DMG numbers however, I'd be fascinated to see them!
Hope that helps
Very few monsters have features that make them worse than their raw DPR and toughness.
Many monsters have features that make them stronger than their raw DPR and toughness.
Using monster averages gives you deflated DPR and Toughness values for a given level, because the DMG numbers effectively apply to "dumb brutes".
Gorilla: 44 DPR, +9 ATK, 12 AC, 157 HP
44*157*2.1/6 = 2418 XP (vs 2900)
Averaging HP/DPR/etc of creatures with non-dumb-brute features gives you bad input data. Even the t-rex has restrained and a 2nd attack that can't target the same target, both of which change how dangerous they are in some way that isn't fully clear.
Hill giant 36 DPR, +8 ATK, 13 AC, 105 HP. 2.1 * 105 * 36/6 = 1323 XP (vs 1800)
Gladiator 33 DPR, +7 ATK, 16 AC (+3 parry), 112 HP - 1602 XP (vs 1800) (parry is almost as good as +3 AC - better than +2 - as to be worse you have to be hit by 3 or less
twice in the same round. At about 10 incoming swings on the same turn it becomes as bad as +2 AC; before that it is better)
Dragon Turtle 58 DPR, 104 dragon breath (1.67 uses/3 rounds), +13 ATK +10 save DC, 341 HP.
83.7 DPR, +11 ATK, 20 AC, 341 HP
3.1 * 83.7 * 341 / 6 = 14746.545 XP (vs 18000).
I get, for brutes, CR values within 1 of the described CR using a simple formula.
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A baseline sword and board fighter who has 1 fight per day. They have +3 weapon, dueling fighting style, 20 strength. Paladin has defensive fighting style (for 1 more AC). Subclasses are ignored for now.
Fighter attacks are 1d8+10 x 4 per action, or 58. They get 2 action surges so over 3 rounds do 290 (modulated for accuracy). Assuming 75% accuracy and 5% crit rate, they deal 217.5 + 4.5 from crits (20 attacks, 5% chance per attack landing a crit), for 222 damage.
Paladin casts holy weapon
before the fight, as it lasts an hour, and is also sword and board. They drop level 1 and 2 smites no normal attacks, and level 3 and 4 smites on crits. Over 3 rounds they get 6 attacks.
Each attack deals 4d8+8 damage, has a 75% chance of hitting and 5% chance of critting. They get 0.3 crits over the 3 round fight, and 4.5 hits. Each hit deals +2d8 to 3d8 damage from smite. Each crit deals +12d8 to +14d8 damage. To be conservative we'll use the lower level smite.
So 4.2 hits for 6d8+8, 0.3 crits for 16d8+8, or 30d8 + 36 = 143 damage over a 3 round fight. This paladin probably only used 1st level slots.
Going all out, it gets 4.5 hits and uses 3 4th and 1.5 3rd level slots in its 4.5 hits, an average of +4.67d8 per hit. Add in 4d8 from improved smite and holy weapon and we get 47 damage per hit, times 4.5 is 211.6 damage, basically matching the above naive fighter.
Over a day with 1 short rest, the fighter gets 4 action surges worth 44.4 damage each (total 177.6), plus at-will damage of 44.4 per round.
The paladin with a holy sword deals 40.8 nearly at-will damage (4d8+8 times 2, 75% accuracy, 5% crit rate). Over the day they can dump out 15+12+9+8 = 44d8 of smite damage. If they smite without saving big smites for crits, 1/15 are crits, so this comes to 198 smite damage per day.
So on a 1 short rest day, this naive paladin and a naive fighter are doing ... almost the same damage. Within 10% at-will damage (where the paladin self-buffs with an hour+buff), and action surge vs smite ends up being pretty close.
Add in a 2nd short rest and the fighter pulls ahead, not the least because the paladin runs out of holy weapon slots.
TL;DR; The paladin's damage boost from 11 to 20 is from their spell slots and spells. If you give them a substantial boost at 17, naive paladins will outclass naive fighters.
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Now, the Fighter does scale nicely with many magic items, better than the paladin does. A +3 weapon that adds +2d6 elemental damage massively boosts the fighter's damage output and doesn't nudge the paladin's nearly as much. And GWF ups the fighter's damage output as well.
But I think optimization and itemization causing a problem should be treated differently than the baseline class causing a problem.