Random Number Generator 1-100

A percent in every number

There’s a reason 1 to 100 is the most popular range on the whole site. It’s the only one where the number you get and its probability are the same idea. Every result is worth exactly 1%. Any chance you can write as a percentage, you can roll here without doing any arithmetic.

That turns out to be useful in places you wouldn’t expect.

Making a percentage happen

Say you’re writing a board game and want a storm to hit on 15% of turns. Generate a number; 1 to 15 is a storm. Running a story game and want a 60% chance the guard is asleep? 1 to 60, he’s snoring. Teachers use the same idea for probability lessons: “the forecast says 30% chance of rain. Let’s simulate thirty days and see how many are wet.”

The rule we follow: always decide whether low or high numbers mean “yes” before you click, and say it out loud. “Under or equal to 15 is a storm” leaves no room for argument.

Percentile dice

Tabletop games call this a d100 or percentile roll, and physically it’s usually done with two ten-sided dice: one for the tens and one for the units, with “00 and 0” read as 100. Call of Cthulhu runs its entire skill system on it: your Spot Hidden skill might be 45%, so you need to roll 45 or lower. Old editions of Dungeons & Dragons used percentile rolls for treasure tables and thief skills. This generator gives you the same distribution without the “which die is the tens?” debate. The dice roller has a proper d100 if you want the dice on screen.

Raffles of up to a hundred

A hundred tickets is a common raffle book. If you sold all of them, each ticket has a 1% chance of winning one prize. With three prizes drawn without repeats, every ticket holder’s chance of winning something is 3%.

What if people bought several? Someone holding 5 tickets has a 5% chance of winning the first prize drawn. If three prizes are drawn without repeats, their chance of winning at least one is about 14.4%. That number surprises people, and it’s worth knowing if a regular buys a whole strip and then wins twice: it’s not rigged, it’s just more tickets.

Tickets held (of 100)	Chance of winning a single prize	Chance of winning at least one of 3 prizes
1	1%	3%
5	5%	14.4%
10	10%	27.3%
20	20%	49.2%

The hundred square

Every primary classroom has a hundred square on the wall, and a random number from 1 to 100 turns it into a game.

• Find it fast. Generate a number and the first child to point to it on the square wins.
• Ten more, ten less. Generate a number; the class names the numbers directly above and below it on the square.
• Odd or even, prime or not. Generate and sort into columns on the board. There are 25 primes between 1 and 100, which most kids find shockingly few.
• Guess my number. Ask yes-or-no questions about properties (“is it a multiple of 5?”) rather than higher or lower. It forces kids to think about factors.

The numbers behind 1 to 100

Probability facts for random numbers from 1 to 100

Fact	Value
Possible results	100 (1 to 100, inclusive)
Chance of any one number	1 in 100 = 1%
Even numbers / odd numbers	50 even (50%) · 50 odd (50%)
Prime numbers in range	25 (25% chance of a prime)
Same number twice in 2 picks (repeats on)	1 in 100 = 1%
A chosen number appears at least once in 10 picks	9.56%
Picks until a repeat is more likely than not	13 picks
Average (expected) result	50.5
Average picks to hit one specific number	100

The “at least once in 10 picks” figure is about 9.6%, which feels low to most people. Even with a hundred tries, there’s still roughly a 37% chance that any one number you were hoping for never appears. If your favourite raffle number didn’t come up all night, that’s not unusual.

Guess the number

The classic: someone hides a number between 1 and 100, and the others guess with higher-or-lower clues. With perfect halving, seven guesses always do it (start at 50, then 25 or 75, and so on). Most people take eight to ten, mainly because they guess round numbers like 30 and 70 instead of halving properly.

A competitive twist for a group: everyone gets one guess, closest wins, and nobody can repeat a number already said. The last players to guess have a big strategic edge, because they can tuck their guess right next to someone else’s and steal half their range. It’s a tiny lesson in game theory.

Sampling and quick checks

A less exciting but genuinely useful job: picking a random sample. If you’ve got 100 survey responses and only time to read 10 closely, generate 10 numbers with No repeats and read those. It’s fairer than reading the first ten, which are usually from the most eager respondents. For lists of a few hundred, the 1 to 1,000 page does the same job at a bigger scale.

And if you need zero included in your percentages (because 0% is a real outcome), the 0 to 100 generator gives you 101 possible results.

Low or high: pick a convention

One habit worth stealing from tabletop games: decide whether low rolls or high rolls are good. Call of Cthulhu uses “roll under,” so low is good. Plenty of homemade games use “roll over,” so high is good. Mixing them up mid-game causes more arguments than any other rule we’ve seen. Write it on the top of the score sheet.

Another habit that saves arguments: write your percentage on the sheet before generating, not after. “We said 40%, it came up 41, the storm misses” is a much calmer conversation than one about what the odds were meant to be.