Pick a Number 1 Through 20: Roll a d20, Plain or with Advantage: how it works
Pick a number 1 through 20 and you have rolled a d20, the twenty-sided die that settles whether a sword swing, a bluff or a leap succeeds in tabletop role-playing games. Dice left at home, or playing over a call? Press once for a plain roll, or ask for two and keep the higher for advantage.
Advantage and disadvantage from two numbers
Set how many to 2 and press. Under the pair the page prints the highest and the lowest: read highest for advantage, lowest for disadvantage. A plain roll averages 10.5, advantage lifts that to about 13.8 and disadvantage drags it to about 7.2. Needing 11 or better is a coin flip on one die and a 75% shot with advantage. A natural 20 goes from 5% to just under 10%.
The solid behind the odds
A d20 is a regular icosahedron: 20 equilateral triangles, five meeting at every corner, with 30 edges and 12 corners. It is one of five Platonic solids, so all 20 faces are alike and none is favored. When the faces run 1 to 20, each one and the face opposite total 21. A twenty-sided die survives from Ptolemaic Egypt, its faces inscribed with Greek letters.
Tips
- Initiative for six goblins? Ask for 6, leave repeats on and read down the list. Expect ties: six rolls hold a matching pair 56% of the time.
- Dice notation writes keep highest as kh and keep lowest as kl, so advantage is 2d20kh1 and disadvantage 2d20kl1.
- Off the game table, try higher or lower: five guesses always corner the number, since five halvings cover 32.
Questions and answers
Will my table accept a random number 1 to 20 instead of a dice roll?
That is the table's call, but the odds match: 5% for each number, and no press remembers the last, so three low rolls running are bad luck, not a fault. Press where others can see the screen. Your browser makes the pick, which suits game night and nothing with money riding on it.
Should repeats be off for an advantage roll?
No, leave them on. Two real dice can show the same face, and do so once in 20 rolls. Switching repeats off rules out doubles and quietly bends the odds: a pair of 20s, due once in 400 presses, could never happen. Save no repeats for dealing each player a different number.
How long until a natural 20 comes up?
Twenty rolls on average, but luck is lumpy. A 20 is more likely than not within 14 presses, yet about 36% of 20-roll stretches pass without one. Seeing all 20 numbers takes about 72 presses.