Every gamer has muttered it after a bad night: “these dice are cursed.” Usually it’s just variance — but not always. Real dice can carry a genuine, measurable bias. Here is what “fair” actually means, why some dice miss the mark, and how you can test your own.
What “fair” really means
A fair die is one where each face is equally likely to land uppermost. For a d6 that means a 1-in-6 chance per face; for a d20, 1-in-20. Fairness is about the physics of the object: its shape, its mass distribution and its edges. A perfectly fair die has its centre of mass exactly at its geometric centre and identical geometry on every face. In the real world, nothing is perfect — the question is whether the imperfection is big enough to matter.
Casino dice vs mass-market RPG dice
Casino craps dice are the gold standard of fairness, and they look nothing like your average RPG d6. Three features do the work:
- Precision machining. Casino dice are cut to tolerances measured in thousandths of an inch, so all edges and faces are as close to identical as manufacturing allows.
- Sharp edges and corners. No rounding — square edges make the die tumble unpredictably and stop cleanly rather than favouring a rolled-off corner.
- Flush, filled pips. The spots are drilled and then back-filled with paint of the same density as the body, so removing material for a “6” doesn’t make that face lighter. Many casino dice are transparent so no hidden weights can be inserted.
Mass-market RPG dice are made for looks and durability, not laboratory fairness. They’re usually injection-moulded plastic with rounded corners, swirled colours and painted-in numbers. They’re fine for play — but small biases creep in.
Where bias comes from
| Source | What it does |
|---|---|
| Internal voids | Air bubbles trapped during moulding shift the centre of mass, nudging heavier faces to land down and lighter faces up. |
| Rounded corners & edges | Uneven rounding lets the die “roll on” past some faces and settle on others more often. |
| Injection flashing & sprue marks | The nub left where plastic entered the mould adds a tiny lump of extra material on one face. |
| Uneven paint fill | Deep-cut numbers filled with lighter or heavier paint change the balance face by face. |
| Tumbling & polishing | Finishing that smooths one region more than another leaves subtly unequal faces. |
The salt-water float test
You can check a d20 at home with the classic buoyancy test. Fill a glass with water and stir in salt until a die floats. Now flick the floating die and watch: on a well-balanced die, it settles showing a random face each time. If the same few numbers keep bobbing to the top, the opposite (heavier) side is sinking — a sign the centre of mass is off. Spin it a couple of dozen times and note the pattern. It’s not a laboratory measurement, but a strongly biased die usually gives itself away quickly.
Sanity check: on most dice, opposite faces are numbered to sum to one more than the number of sides — 7 on a d6, 21 on a d20. This balances the pip weight across the die and gives you a quick way to spot a mis-numbered or knock-off set. The d10 is the exception (see our dice types guide).
Why digital dice are fair
A physical die fights gravity, air bubbles and worn corners. A digital roll has none of those problems. A good roller draws from a high-quality pseudo-random number generator — and increasingly a cryptographic one — where each outcome is mathematically uniform: every face genuinely has an equal chance, every roll, forever. There is no centre of mass to drift, no edge to wear, and the result can’t be influenced by how you throw. That is why groups often reach for a digital roller to settle a disputed or high-stakes roll.
So should you worry?
For casual play, no. Most bias in a decent modern set is small enough to vanish beneath ordinary variance — you’ll feel “unlucky” long before any real imbalance shows. If you play competitively, or you just enjoy the rabbit hole, float-test your d20 and retire any die that clearly favours a face. And if you want a roll that is fair by construction, let the maths do it.
Try it yourself. Dice Drop rolls real physics dice in your browser — provably fair RNG under the hood, with advantage, modifiers and mixed pools built in.
🎲 Roll dice freeKeep reading: probability, variance & luck — why “hot dice” is a story your memory tells you, not physics.