So the new edition of D&D came out today, or at least the Basic Rules, which is more than enough to begin actually playing. When you get hit for enough damage to knock you down to 0 hp, you need to make death saving throws.
- If you roll a 20, you are back to 1hp
- If you roll a 19-10, you have one success. Three of these and you become stable.
- If you roll a 9-2, you have one failure. Three of these and you become dead.
- If you roll a 1, it counts as two failures.
There are no modifiers to these rolls at all, so it’s just a matter of straightforward probability. There are a total of nine different ways to live:
- Roll two successes, then roll a natural 20 or a third success
- Roll two successes and a failure in any order, then roll a natural 20 or a third success
- Roll a natural 20
- Roll two successes and two failures in any order, then roll a natural 20 or a third success
- Roll a success, then roll a natural 20
- Roll a failure, then roll a natural 20
- Roll a success and a failure in any order, then roll a natural 20
- Roll two failures, then roll a natural 20
- Roll two successes and a natural 1 in any order, then roll a natural 20 or a third success
- Roll a success and two failures, then roll a natural 20
- Roll a natural 1, then roll a natural 20
- Roll a success and a natural 1 in any order, then roll a natural 20
You may be thinking that this order seems a bit strange, but it’s actually in roughly the order of probabilities.
- One way to do this, 13.75%
- Three ways to do this, 5.5% each, for a total of 16.5%
- One way to do this, 5%
- Six ways to do this, 2.2% each, for a total of 13.2%
- One way to do this, 2.5%
- One way to do this, 2%
- Two ways to do this, 1% each, for a total of 2%
- One way to do this, 0.8%
- Three ways to do this, 0.69% each, for a total of 2.06%
- Three ways to do this, 0.4% each, for a total of 1.2%
- One way to do this, 0.25%
- Two ways to do this, 0.13% each, for a total of 0.25%
Add all of those probabilities together, and you get the probability that you will survive falling unconscious in 5th edition; 59.51%. (Note: And numbers that seem off above are because everything was rounded to the nearest hundredth of a percent for this post, but the calculations of the numbers were done in Excel.)
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Odds of surviving unconsciousness in D&D 5th Edition
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In 3.5e, if you get hit to -1 hp or lower, you are Dying. If you're Dying, each turn you have a 10% chance to stabilize, and a 90% chance to lose 1 hp. If you drop to -10 hp, you die. If you get hit to exactly -1 hp (possible, but unlikely– in 3.5e there's no distinction between positive and negative instantly from positive hp by any attack that deals your current hp + 10 damage) then your odds of dying are 0.9 ^9, or 38.7%