Understanding the Odds

D&D Next adds in two new odds mechanics (Advantage and Disadvantage) that can lead to very interesting results. Understanding these and other odds is fairly important when making decisions during character building and play.

First, you must understand the basics of a d20 roll. For any target DC, you have 20 – (DC – 1) * 5% chance for success. Thus if the DC is 1, you have a 20 – (1-1 or 0) * 5%, or in this case a 100% chance of success. If the DC is 10, you have a 20 – (10 – 1 or 9) * 5%, or 55% chance of success.

Knowing that, each + 1 you have to the roll adds a straight 5% to your chance of success. In the case of DC 10, a +1 gives you a modified 60% chance, and a +5 gives you a modified 80% chance. Each – 1 subtracts 5% from your chance, so a – 1 on a DC 10 is a 50% chance, and a – 5 is a 30% chance.

Advantage is a mechanic whereby you roll two d20s and take the highest result. Disadvantage is a mechanic whereby you roll two d20s and take the lower result. Advantage adds upwards of 25% to your chance of success over a straight d20 roll, and Disadvantage subtracts upwards of 25% from a straight d20 roll.

Below is a chart that shows the odds of getting or beating a specific DC with a d20 roll, a roll with advantage and a roll with disadvantage:

Target DC Straight 1d20 Advantage Disadvantage
1 100.00% 100.00% 100.00%
2 95.00% 99.75% 90.25%
3 90.00% 99.00% 81.00%
4 85.00% 97.75% 72.25%
5 80.00% 96.00% 64.00%
6 75.00% 93.75% 56.25%
7 70.00% 91.00% 49.00%
8 65.00% 87.75% 42.25%
9 60.00% 84.00% 36.00%
10 55.00% 79.75% 30.25%
11 50.00% 75.00% 25.00%
12 45.00% 69.75% 20.25%
13 40.00% 64.00% 16.00%
14 35.00% 57.75% 12.25%
15 30.00% 51.00% 9.00%
16 25.00% 43.75% 6.25%
17 20.00% 36.00% 4.00%
18 15.00% 27.75% 2.25%
19 10.00% 19.00% 1.00%
20 5.00% 9.75% 0.25%

Understanding the Odds

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