Converting D&D to GURPS: Combat Comparison

[abates17]

I wanted to try my hand at a quick monster conversion from D&D to GURPS, and I checked my results with a combat example. Let’s see how it goes!

Skeleton (D&D 3.5)
Str 13, Dex 13, Con 0, Int 0, Wis 10, Cha 1
Hit Dice: 1d12 (6hp)
Armor Class: 15 (+1 Dex, +2 natural, +2 shield), touch 11, flat-footed 14
Speed: 30 ft.
Attack: Claw +1 melee (1d4+1)

Okay, let’s start the conversion! Based on suggestions from others on the forums, I am converting Str directly to ST, and halving any differences between the other attributes and 10. (So for example, Dex 14 = DX 12, Dex 8 = DX 9, and so on.) The idea here is that attributes in D&D give a bonus (or penalty) based on 1/2 the amount off from 10, and GURPS gives a straight bonus (or penalty). With that in mind, we get:

Str 13 => ST 13
Dex 13 => DX 11
Wis 10 => Will 10 => Per 10

Normally, I would convert Int to IQ and Con to HT, but both values are 0 in D&D, which makes no sense to me. Since Wis is already 10, I’ll just set IQ to 10 based on that, since I doubt it will come up anyway. As for HT, let’s see if we can reverse-engineer a value based on Basic Move (which is based off DX and HT).

The skeleton has Speed of 30 ft., so we should be able to figure out Basic Move based on that. A round in D&D is equivalent to six seconds, while a GURPS round is one second. However, a creature can take two Move actions in a D&D round, so Speed should be divided by 3 to get the equivalent GURPS Basic Move (and converted from feet to yards, of course). So that leaves us with

Speed: 30 ft. x 2 moves per round = 60 ft. / 6 seconds per round = 10 ft. = 3.33 yards => Basic Move 3.33 (dropped to 3)

Now we know that in GURPS, Basic Speed is (HT+DX)/4, which means that (HT+DX) = 13.33, which gives us an HT score of 2.

HT 2
Basic Speed 3.33
Basic Move 3
Dodge 6 (Basic Speed + 3, dropping fractions)

Okay, doing well so far! Just a few more things to do, then we can check our numbers. Let’s get some combat data in here. Based on the given AC bonus of +2 natural, divide that by two to get:

Natural DR 1

It says he has a shield, so we can figure:

Shield = DX - 4 = 7

As for Block, Dodge, and Parry values, the Parry score is 3 plus 1/2 Attack value. Let’s assume a base attack value of 10 (more on that in a minute) plus the +1 bonus makes it 11, which leads to:

Block = 3 + 1/2 Shield = 6 (8 on shield side)
Dodge = 6 (8 on shield side)
Parry = 3 + 1/2 Attack = 8 (10 on shield side)

Now, time to check our numbers with a combat example! Let’s pit skeleton vs. skeleton to figure out hit percentage and average damage. First, the D&D combat data:

D&D: Attack D20+1 vs. AC 15 means the attacker hits 35% of the time. At 1d4+1 damage on a hit, that means that the average damage is:

(3.5 * 35%) = 1.225 average damage.

Now, let’s see if we can get similar numbers from GURPS combat…

I’m assuming a basic attack skill of 10, plus the +1 Claws attack bonus puts skill at 11. With skill 11, the attacker will hit 63% of the time. Now, the defender’s Parry is 10 on the shield side (50%), and 8 on the other side (26%). I’ll figure attacks from the front will be on the shield side 2/3 of the time (I know, this is very quick and dirty), so the weighted Parry percentage is 42%. Using the same calculations, we get Block 20% and Dodge 20%. Average those all together (figure an equal chance of any defense), and we get the defender succeeding 27.3% of the time.

SO…the attacker hits 63% of the time, and the defender succeeds 27.3% of the time. This leads to:

(63% * 72.7) = Attacker damages defender 45.8% of the time.

Okay, 45.8% is higher than the D&D hit percentage of 35%, but we haven’t figured damage yet. Average damage for 1d4+1 is 3.5, minus the skeleton’s DR1 equals 2.5. This means that the average damage is:

(2.5 * 45.8%) = 1.145 average damage

Not bad! With our converted numbers (and some hand-waving), we get 1.145 average damage in GURPS, which is reasonably close to the 1.225 average damage in D&D. I think that is a good sign that our conversion (at least for the sake of combat) is pretty accurate.

Thoughts? Comments? Suggestions?