D8 Roller

The d8 is the damage die for many classic weapons and the hit die for several character classes. Roll as many as you need with a modifier, see the exact odds of every total, and — for tabletop fights — find out how much damage an attack deals on average against a given armour class.

The d8 in numbers

A single d8 is flat — every face from 1 to 8 has a 12.5% chance, and the average is 4.5. Add more dice and the totals bunch up around the middle, forming a bell-shaped curve, which is why 2d8 rolls a 9 far more often than a 2 or a 16.

Where the d8 is used

• Damage for many one-handed martial weapons in d20 fantasy games — longswords, battleaxes, rapiers, warhammers and morningstars — and for longbows and light crossbows.
• Hit dice for several middle-toughness character classes.
• Some spell and healing effects that scale with extra d8s.
• Random tables with eight entries, such as compass directions or weather.
• Teaching probability — eight equally likely outcomes make clean fractions.

Worked example

A character with a +5 attack bonus swings a longsword (1d8 + 3) at a creature with armour class 15. They need a 10 or higher on the d20, so they hit 55% of the time, and 5% of attacks are critical hits that roll 2d8 + 3. The average damage per swing is 4.35 — compared with 7.5 on every hit. With advantage the hit chance rises to 79.75% and average damage to 6.42.

Critical hits and resistance

In many d20 systems a critical hit doubles the number of damage dice — 1d8 + 3 becomes 2d8 + 3 — but not the flat modifier, and resistance halves the final damage, rounding down. Tick the boxes to apply either, and the distribution chart updates to show the new range and average. House rules vary: some tables maximise the first die on a crit instead of rolling twice; adjust the modifier to model that.

Is the roller fair?

Each die uses crypto.getRandomValues, the browser’s cryptographic random number generator, with rejection sampling so all eight faces are exactly equally likely. The history panel counts every face rolled, and after 80 dice a chi-square test tells you whether the spread is consistent with a fair die. Expect streaks: in a fair sequence, three of the same number in a row happens about once every 64 rolls.

Reading the distribution chart

The bar chart shows the exact probability of every possible total for the dice you’ve chosen, including the modifier and any critical doubling. Hover over a bar to see its percentage. For 1d8 the bars are all the same height; for 2d8 they form a triangle peaking at 9; for 3d8 and more they approach a smooth bell curve. The standard deviation tells you how spread out the results are: about two-thirds of rolls land within one standard deviation of the average.

Hit points with a d8 hit die

Many groups let players take the fixed average (rounded up to 5) instead of rolling, which avoids unlucky low rolls. Use the roller if your table rolls for hit points, and keep the results in the hit-point log from the pack.

Probability questions for the classroom

• What is the chance of rolling 8 on one d8? (1/8 = 12.5%)
• What is the chance of rolling at least one 8 with two d8s? (1 − (7/8)² ≈ 23.4%)
• Which total is most likely with 2d8, and why? (9 — it has the most combinations)
• What is the chance two d8s show the same number? (1/8)
• Roll 80 times and compare your counts with the expected 10 per face.

Advantage and disadvantage

Rolling the d20 twice and keeping the higher result (advantage) or the lower (disadvantage) changes hit chances a lot. If you normally hit on 55% of attacks, advantage raises that to about 80% and disadvantage lowers it to about 30%. Advantage also nearly doubles the chance of a critical hit, from 5% to 9.75%, which matters for weapons with bigger damage dice.

Physical d8s

A real d8 is an octahedron — eight triangular faces, with opposite faces usually summing to 9. Cheap dice can be slightly lopsided; a common home test is to float them in salt water and spin them to see whether one face keeps coming up. Precision-edged casino-style dice are the most balanced.

Privacy

Rolls happen in your browser and are not sent or stored.