Der Vermis Matrizen – Converting d20 to d100

Converting d20 to d100 in Classic Games & Retro Clones

The d20 tables used in classic games and retro clones can be easily converted to d100. One reason for doing this is that each pip of a d20 conceals 5% within it and converting the value to d100 makes the hidden numbers between 1% and 5% available for use if you apply modifiers or construct other tables to take advantage of it.

The known mechanic for a d20 table is that one must roll equal to or greater than the number to succeed. That is if 15 is indicated as the target number you are required to roll 15 or better using d20 and may add +1 or more to the dice roll in the form of dice roll modifiers. This mechanic is easily comprehended which helps explain its popularity. It also conceals the actual nature of the chance of success for were I to tell the reader the chance of success is only 6/20, or 30%, it would be difficult to believe without doing the math; few like to do the math.

In order to follow same mechanic for d100 we have to convert the values of d20 in such a way as to make possible rolling equal to the target number or better. The following table converts d20 values to d100. The 5% formerly hidden in the number 1 is now broken down to a range; 01, 02, 03, 04, and 05. This is true of each pip of the dice.

d20
1
2
3
4
5
6
7
8
9
10
d100
01
06
11
16
21
26
31
36
41
46
d20
11
12
13
14
15
16
17
18
19
20
d100
51
56
61
66
71
76
81
86
91
96

Once we have the d100 values, how do we then apply them? In the following d100 Attack Matrix the player must roll equal to or greater than the target number. A +1 bonus equates to a +5% added to the dice roll.

Example: in order for a character of 1st to 3rd level to attack an opponent with an AC of 4 they must roll equal to or better than 71 using d100, 71 through 00. If the character were to possess a +1 weapon they would add 5% to their dice roll.
d100 Attack Matrix
Character’s
Level
Defender’s
Armour Class
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
Normal
Man
51
56
61
66
71
76
81
86
91
96
96
96
96
1st
to 3rd
46
51
56
61
66
71
76
81
86
91
96
96
96
4th
+ Higher
36
41
46
51
56
61
66
76
81
86
91
96
96

There are some interesting potential applications for d100 vis-a-vis role playing as Characters might seek training with a martial arts master for example and become skilled with a particular weapon. This would grant attack benefits of +1% through +5% as the training progresses for a fee of course, or performing tasks for the master.

Magical weapons might also have ranges of values below the standard 5% benefit, or these may apply to different creatures or classes of creatures, +5% for some, +1% for others. For example, a magical sword which is +5% vs. the living dead but possibly +3% vs. a certain type, and +1% vs. something really nasty.

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GregMacKenzie

Greg MacKenzie is the author of the novella Seven Crows a Secret, the Fenris 2d6 role playing game, adventure modules Gloomland, and Wildwood, as well as countless other adventures.

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