Last Updated: 220204 … 220611 (heavily revised)
Back stabbing is the striking of a blow from behind, be it with club, dagger, or sword. The damage done per hit is twice normal for the weapon used per four experience levels of the thief, i.e. double damage at levels 1-4, triple at 5-8, quadruple at levels 9-12, and quintuple at levels 13-16. Note that striking by surprise from behind also increases the hit probability by 20% (+4 on the thief’s “to hit” die roll).
There’s a lot of uncertainty and misunderstanding about the relative “power” of back stab. As the thief’s abilities often boil down to back stab, many DMs limit thief abilities. So just how good is back stab, really?
Let’s perform a reasonably direct comparison to the fighter:
- A fighter, 0 XP, Level 1, Strength 17, THAC0 20 –1 (Str), armed with a long sword (average 4.5 points of damage +1 (Str))
- A thief, 0 XP, Level 1, Dexterity 17, THAC0 20–4 (back stab), armed with a longsword (average 4.5 points of damage × 2 (back stab)) ). MS: 15%+5% (Dex), HiS: 20% + 5% (Dex).
- Our generic monster, AC: 5.
Both have a 17 in their Prime ability (Dexterity and Strength respectively), increasing the fighter’s To Hit and Damage, and the thief’s chance to MS and HiS. The thief’s backstab, on average, decreases an opponent’s AC by 1 (no Dex or shield bonus).
In a discussion of THAC0, you have to consider the early repeating 20 in the thief’s to hit chart, and adjust appropriately. A further consideration when comparing classes is relative level for commensurate XP. Generally, a thief should be one level higher than a fighter starting at level 3:
THAC0
| Level | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter | 19 | 19 | 17 | 17 | 15 | 15 | 13 | 13 | 11 | 11 | 9 | 9 |
| Thief (+4)* | 15 | 15 | 15 | 15 | 13 | 13 | 13 | 13 | 10 | 10 | 10 | 10 |
| T (+4,lvl adjusted)* | 15 | 15 | 15 | 13 | 13 | 13 | 13 | 11 | 11 | 11 | 11 | 9 |
| * Adjusted for the thief’s 20,20,19 … in the THAC0 chart, Str bonus, Dex benefit lost. | ||||||||||||
At low levels, the +4 bonus to hit makes a thief more likely to hit than the fighter, but the fighter’s more rapid THAC0 advance overwhelms the +4 bonus by level 7.
Number of Attacks
While the fighter attacks (at least) every round, back stab is limited in frequency. For the thief to back stab, they must approach, back stab, then retreat in order for their opponent to be no longer aware of their presence. Assuming that takes but 2 rounds:
| Attacks / Level | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter | 1 | 1 | 1 | 1 | 1 | 1 | 3/2 | 3/2 | 3/2 | 3/2 | 3/2 | 3/2 |
| Thief | .5 | .5 | .5 | .5 | .5 | .5 | .5 | .5 | .5 | .5 | .5 | .5 |
Damage
The thief gains a damage multiplier on back stab, but fewer attacks/round. If we assume both the fighter and thief are using a long sword (average 4.5 hp of damage), then for each hit our fighter does 5.5 hp damage, where the thief using back stab will average 9 damage (levels 1-4), 13.5 damage (levels 5-8), or 18 (levels 9-12).
Putting the Pieces Together
Now, considering our “average” AC:5 monster, that brings us to a simplistic chance to hit per attack of:
| Chance to hit AC: 5 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter (Str +1) | 35% | 35% | 45%* | 45%* | 55%* | 55%* | 65%* | 65%* | 75%* | 75%* | 85%* | 85%* |
| T (lvl adj. +4 back) | 70% | 70% | 70% | 80% | 80% | 80% | 80% | 90% | 90% | 90% | 90% | 100% |
* Ignoring that our fighter will almost certainly specialize. And again…
Given our fighter also has +1 to damage from strength, the average damage per attack is:
| Damage/attack | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter (Str +1) | 1.9 | 1.9 | 2.5 | 2.5 | 3.0 | 3.0 | 3.6 | 3.6 | 4.1 | 4.1 | 4.7 | 4.7 |
| Thief (lvl. adj w/ mult) | 6.3 | 6.3 | 6.3 | 7.2 | 10.8 | 10.8 | 10.8 | 12.2 | 16.2 | 16.2 | 16.2 | 18.0 |
| Damage ratio/attack | 3.3 | 3.3 | 2.5 | 2.9 | 3.6 | 3.6 | 3.0 | 3.4 | 3.9 | 3.9 | 3.5 | 3.9 |
Or, put another way, the thief’s back stab averages 3.4 times the damage of a fighter hit-for-hit. Not bad! However, while the thief can only (best case) back stab once every two rounds, the fighter’s attacks per round increase, and the thief’s relative chance to hit decreases. A more accurate ratio looks more like:
| Damage/round | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter (Str +1) | 1.9 | 1.9 | 2.5 | 2.5 | 3.0 | 3.0 | 3.6 | 3.6 | 4.1 | 4.1 | 4.7 | 4.7 |
| T (lvl. adj) | 3.2 | 3.2 | 3.2 | 3.6 | 5.4 | 5.4 | 5.4 | 6.1 | 8.1 | 8.1 | 8.1 | 9.0 |
| Damage/round | 1.6 | 1.6 | 1.3 | 1.5 | 1.8 | 1.8 | 1.5 | 1.7 | 2.0 | 2.0 | 1.7 | 1.9 |
Using back stab most generously (every other round), the thief averages 1.7 times damage as a fighter. Nothing earth-shattering—if the thief can back stab every other round, he’s in the ball park of a fighter, but far more vulnerable (AC, HP) once engaged in melee. Hopefully their opponent lets them withdraw. Otherwise the thief remains locked in combat after the initial back stab attempt, with his worse hit points, THAC0 and armor class—not ideal. Or flee, providing their opponent a +4 bonus in return!
The GM likely requires the thief to move around to their opponent’s back again, for an additional round for the opponent to “lose awareness” (assuming their opponent doesn’t pursue!), bringing that ratio down to 1.1 times the fighter.
Skill Checks
The Level 1 fighter requires a 14 To Hit, so: 35% (TH) * 5.5 Damage (Str) = 1.9 DPR.
Assume the best case: back stab every other round, and no other checks:
Our Level 1 thief requires an 10 To Hit, and: (45% (TH) * 9 Damage (Back stab) ) = 2 DPR.
In other words, back stab DPR looks awesome—better than the fighter! Back stab includes one more consideration—thief abilities. Move Silently is used “to approach to back stab”. Failure means that movement was not silent, and thus back stab fails. The ability to move silently ranges from 10% to 99%, but even a 9th level thief will only succeed 70% (barring race and Dexterity modifiers). That brings our thief back stab / two round damage number down to 50% of the fighter:
| Damage/round w/ MS | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Fighter (Str +1) | 1.9 | 1.9 | 2.5 | 2.5 | 3.0 | 3.0 | 3.6 | 3.6 | 4.1 | 4.1 | 4.7 | 4.7 |
| Thief (lvl. adj) | 1.1 | 1.6 | 1.6 | 1.8 | 2.7 | 2.7 | 2.7 | 3.0 | 4.1 | 4.1 | 4.1 | 4.5 |
| Move Silently (level adj) | 0.2 | 0.3 | 0.3 | 0.4 | 0.5 | 0.6 | 0.6 | 0.7 | 0.8 | 0.9 | 0.9 | 1.0 |
| Damage ratio/round | 0.1 | 0.2 | 0.2 | 0.3 | 0.4 | 0.5 | 0.5 | 0.6 | 0.8 | 0.8 | 0.8 | 1.0 |
(For other comparisons, the chance to Hide in Shadows is worse than Move Silently)
In other words, a lot of math later, on a round-by-round basis, the thief, using his premium back stab ability, with back stab every other round, averages 50% of the damage of a fighter. To back stab, he further exposes his poorer AC and hit points to attack. Under the assumption he’s using a long sword, instead of the perhaps more frequent club or dagger.
Multiple Skill Checks:
Some GMs require MS and HiS rolls instead of, or in addition to Surprise. If the GM requires MS (or HiS depending on the GM), we get to the meat of the issue:
| Fighter melee DPR (d8) | 1.9 |
| Thief back stab DPR | 2.03 |
| Thief back stab DPR w/ MS | 0.41 |
| Thief back stab DPR w/ HiS | 0.30 |
| Thief back stab DPR w/ MS × 2 | 0.08 |
| Thief back stab DPR w/ His + MS | 0.06 |
With only a single MS check, the fighter averages 429% more damage. Should the GM require two successive checks, the thief’s back stab DPR drops through the floor—the fighter averages 20× more damage than a thief with back stab! If you ask yourself, “Why do my thieves never back stab?”, this would be why.
Surprise
By the Book, Surprise—not MS, not HiS—is required for back stab. Let’s evaluate the thief’s back stab DPR with Surprise. The default chance for Surprise is 2 in 6. A silent thief increases that range to 3 in 6. A silent hidden thief increases that Surprise range to 4 in 6.
“Assume the party of characters, moving silently and invisibly, comes upon a monster. They have 4 of 6 chances to surprise.” PHB, p. 103.
With a generous GM, a thief approaching from behind could be considered invisible (at least hard to see since the thief is by definition at their opponent’s back), receiving the benefit of being hidden after a successful MS.
The Level 1 thief’s back stab DPR under Surprise:
| Thief Back stab DPR w/ Surprise (2 in 6) + To Hit | 0.60a |
| Thief Back stab DPR w/ Surprise (w/ MS bonus) (3 in 6) + To Hit | 0.76a |
| Thief Back stab DPR w/ Surprise (w/ MS bonus + auto Hide from rear) (4 in 6) + To Hit | 1.07a |
aSurprise includes a 16% chance for one attack, a 16% chance for two additional attacks, and (potentially) chances for 3 or even 4 attacks depending on the modified Surprise roll. Additional back stabs don’t multiply damage, and are only +2 to hit—see Polyhedron #31, p. 29.
If the L1 thief rolls Surprise for a successful back stab, their DPR is .6. If you incorporate a MS check before the Surprise roll to increases the chance of Surprise to 3 in 6, the DPR increases. If you assume that MS includes being hidden (hard to see someone approaching successfully from behind), then their DPR is better yet. Our thief’s DPR is less than that of the fighter, but using Surprise is better than a single MS check.
And the same comparison with our PCs leveled up to the thief’s triple damage? One can quibble about the thief’s speed of advancement for levels, but the levels for the two classes still overlap in this range, so any difference between the two classes is brief.
A thief, 19,000 xp, Level 5, Dexterity 17, THAC0 19 – 4 (back stab). MS: 40%+5% (Dex), HiS: 31% + 5%(Dex), armed with a longsword (average 4.5 points of damage × 3 (Back stab))
A fighter, 19,000 XP, Level 5, Strength 17, THAC0 16 –1 (Str), armed with a long sword (average 4.5 points of damage +1 (Str))
Here’s all the data in totality:
| Prime: 17, Long sword, Opponent AC:5 | Level 1 | Level 5 |
| Fighter Melee DPR | 1.93 | 3.03 |
| Thief Melee DPR | 1.13 | 1.58 |
| Thief Back stab DPR (only To Hit, no other mods) | 2.03 | 3.71 |
| Thief Back stab DPR w/ MS + To Hit | 0.41 | 1.67 |
| Thief Back stab DPR w/ HiS + To Hit | 0.30 | 1.34 |
| Thief Back stab DPR w/ MS + MS + To Hit | 0.08 | 0.75 |
| Thief Back stab DPR w/ His + MS + To Hit | 0.06 | 0.60 |
| Thief Back stab DPR w/ Surprise (2 in 6) + To Hit | 0.60 | 0.96 |
| Thief Back stab DPR w/ Surprise (w/ MS bonus) (3 in 6) + To Hit | 0.76 | 1.41 |
| Thief Back stab DPR w/ Surprise (w/ MS bonus+auto Hide from rear) (4 in 6) + To Hit | 1.07 | 2.32 |
Other considerations should you desire more math:
- A 1e fighter likely has additional bonuses to hit beyond a base 17 Strength. If the campaign uses Specialization, the fighter will have additional bonuses to hit in addition to additional attacks / round.
- At L6 the fighter’s attacks go to 3/2 without specialization (or 2/1 with specialization), increasing DPR by 50%.
- If the GM limits the thief to only one back stab every three rounds, all of the thief DPRs decrease 33%.
- If the attack renders a creature dead, who cares if you compare a back stab 10 hp / round vs 4 hp / round when the creature has only 4 hp?
- A good argument can be made that Surprise can, at most, provide only 2 additional attacks, as defined as “Surprise” (1 segment) and “Complete Surprise” (2 segments), where the above assumes the possibility of up to 4 segments of Surprise.
The net result is that calculating back stab damage by Surprise has a higher DPR for the thief, yet still considerably less than a fighter of comparable level. Calculating back stab taking more than two rounds, or requiring multiple rolls for success, almost guarantees futility for the thief and back stab.
And of course, back stab isn’t always an option. Certain creatures (otyughs, slimes, molds, etc.) either negate surprise or have no definable “back”, thus negating this ability. Back stab isn’t possible against those “aware” of the thief. Depending on the GM, the opponent could be deemed aware after the first attack, so only once per melee. Results swing further when you start to include a more usual fighter to hit/damage bonus from Strength.
UA further lessened back stab with weapon specialization providing fighters a 50% increase in number of attacks (and thus a 50% increase in damage), an increased chance to hit, and a ≥ 20% increase in damage (with at least +1 damage from weapon specialization). And that’s ignoring specializing more than once.
Recommendations
Don’t, just don’t, require multiple skill checks. Use Surprise, improved by MS (or HiS).
Don’t fear the thief getting to back stab more than once in a fight. The time he spends moving around limits the overall damage per round.
While +4 to hit looks good, over levels it loses value in comparison to the fighter’s THAC0 table. The thief’s bonus to hit should increase at the same rate as the damage multiplier. While you could permit a thief to specialize, that’s not quite the advantage it appears given the thief’s fewer proficiency slots. A flat additional +1 to hit at level 5-8, +2 to hit at levels 9-12 makes back stab scale better, especially if playing with weapon specialization.
While back stab has the occasional wild swing (rolling max damage with a 4× multiplier), the reverse is also true when the high-level thief rolls a “1” for 4 points of damage. Back stab is hard to pull off, and exposes the thief to significantly more peril. If the back stab is successful, a minimum of damage would help offset the risk—a 2-point minimum at level 5, a 3-point minimum at level 9, etc.
