Understanding the d100
A d100 is a hundred-sided die: it produces a whole number from 1 to 100, and every value is equally likely. This roller simulates that by drawing a uniform random integer in the range 1–100 for each die. It is used in tabletop role-playing games such as Call of Cthulhu and older editions of Dungeons & Dragons (for percentile checks and random tables), and any time you need a fair one-in-a-hundred outcome.
How the roll is generated
For each die, the tool computes Math.floor(random × 100) + 1, which maps the interval [0, 1) into the integers 1 through 100. When you roll several dice, it reports each individual result, their total (the sum), and the average (total ÷ number of dice).
Percentile ("d100") dice in real life
A true 100-sided die exists (a "Zocchihedron"), but at the table a d100 is almost always rolled as percentile dice: two ten-sided dice, where one die is read as the tens digit (00, 10, 20, … 90) and the other as the ones digit (0–9). You add them, and the special reading of "00" + "0" counts as 100 rather than 0. The result is still a uniform number from 1 to 100.
The probability math
- Each outcome: P(roll = k) = 1/100 = 1% for any k from 1 to 100.
- Expected value (mean): E = (1 + 100) / 2 = 50.5. For N dice the expected total is N × 50.5.
- Ranges: because outcomes are uniform, P(roll ≤ k) = k/100. So P(roll ≤ 30) = 30%, and P(roll > 95) = 5% — a common "critical" threshold.
- Variance of one die: for a discrete uniform 1…100, Var = (100² − 1)/12 = 9999/12 ≈ 833.25, giving a standard deviation of about 28.87.
Common reference points
- Roll ≥ 96 (top 5%): 5% chance — often used for rare successes or critical results.
- Roll ≤ 5 (bottom 5%): 5% chance — often used for fumbles or rare failures.
- Roll of exactly 100 (or 00): 1% chance.
- Roll ≤ 50: 50% chance — a coin-flip threshold.
Frequently Asked Questions
What is a d100?
What is the average roll of a d100?
Is this roller fair and random?
How do I roll a d100 with two ten-sided dice?
Common Tip Percentages
| Service | Typical Tip Range | Notes |
|---|---|---|
| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
| Taxi / rideshare | 10–20% | More for assistance with luggage |
| Hotel housekeeping | $2–5/night | Leave daily, not just at checkout |
| Barber / hair salon | 15–20% | On pre-discount total |
| Pizza delivery | 10–15% or $3–5 minimum | Higher in bad weather |
General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
|---|---|---|
| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
| Savings / Debt | 20% | Emergency fund, retirement, extra payments |
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |