How bad is rolling for health in DnD 5e?

2021/03/08

Progression in Dungeons and Dragons revolves around gaining levels, in which your character becomes more powerful. One component of leveling up is increasing your maximum hit points each level. The rules as written provide two alternate ways of determining how much of an increase you get:

Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up). — Player’s Handbook p. 15

An interesting bit here is that the fixed health rule uses the rounded up average of the roll. This means that the fixed value will be better than rolling, on average. In this post, we’ll dig in a bit deeper, talk about the math behind each rule, and compare the practical differences.

Defining the strategies

Our approach to comparing these rules is to define them mathematically as random variables. As for notation, we will use $X_{\mathrm{roll}}^k$ and $X_{\mathrm{fixed}}^k$ , to represent the amount of health gained after leveling up $k$ times for the rolling and fixed health rules, respectively.

The fixed health rule increases your character’s health pool by the same amount each level, which we write as:

X_{\mathrm{fixed}}^k = k\left(\frac{d+2}{2} + C_{mod}\right)

Where $d$ is the value of your class’s hit dice and $C_{mod}$ is your constitution modifier. As an example, suppose you are playing a Bard with a +1 Constitution modifier, then $X_{\mathrm{fixed}}^k = 6k$ . Each level you gain 6 health (until your constitution modifier changes).

The rolling strategy is a bit more complex. If we let $Y_d^k$ represent the sum of $k$ $d$ -sided dice rolls, we can write the health gained from the dice rolling rule as:

X_{\mathrm{roll}}^k = k C_{mod} + Y_d^k

Since we are primarily interested in comparing these two rules, we can look directly at the difference between them, $\Delta X^k_d = X_\mathrm{fixed}^k - X_\mathrm{roll}^k$ , which represents how much better the fixed strategy is compared to rolling after $k$ level ups of a class with hit dice $d$ . We can rewrite it using the definitions above:

\Delta X^k_d = k \frac{d+2}{2} - Y_d^k

Our dependence on the constitution modifier drops completely and we have a random variable defined in terms of the sum of dice rolls.

Expected differences

A simple step from here is to take a look at the expectation of this difference between the rules. Applying the tools that we looked at in a post on expectations of dice rolls we can tackle this:

\begin{aligned} E\left(\Delta X^k_d\right) &= k \frac{d + 2}{2} - E Y_d^k \\ &= k\frac{d+2}{2} - k \frac{d+1}{2} \\ E\left(\Delta X^k_d\right) &= \frac{k}{2} \end{aligned}

Simply put: the fixed health increase will give you 1 more hit point every 2 levels, on average. It’s no surprise that this value comes up, because it is exactly the amount that is gained by rounding up the average roll each level.

The probability distribution of how much better fixed health is

The expectation is not the whole picture, of course. The whole point of rolling is the element of randomness in the process. We can do a bit of algebra to write out the probability distribution of $\Delta X^k_d$ in terms of the sum of dice rolls:

P(\Delta X^k_d = x) = P\left(Y_d^k = k\frac{d+2}{2} - x\right)

Using the results from our recent post on dice roll sums, we can write out the exact form:

P(\Delta X^k_d = x) = \frac{1}{d^k}\sum_{l=0}^{\left\lfloor\frac{k}{2} - \frac{x}{d}\right\rfloor} (-1)^l {k \choose l} {k\frac{d+2}{2} - x - dl - 1 \choose k - 1}

It’s a bit messy, but we can calculate some interesting quantities, such as the probability that the fixed health increase on level up is strictly better than rolling — $P(\Delta X^k_d > 0)$ — by adding up terms when $x > 0$ . The below graph shows how this quantity changes as you level up:

There are a few interesting features here. The first is how the benefits of taking the fixed health increase are greater with a low hit dice — by 5th level a Wizard or Sorcerer has a 2-in-3 chance of having more health by taking fixed increases. On the other hand, a class with a d10 hit dice doesn’t reach that same likelihood until around 10th or 11th level.

Conceptually, this arises because the $\frac{1}{2}$ average benefit is fairly large relative to the range of possible outcomes on a d6, but on a d12? Not so much.

The second interesting point on this graph (to me) is that there is no difference between fixed health and rolling if you only do it once! So if you’re one of those people who wants to mathematically min-max your character to the greatest degree possible, you can still have a little fun by rolling for health just one time.

Just how big is the difference?

While the above is more focused on whether there is any difference at all, probably what would be more interesting is looking at the range of possible outcomes and seeing how the actual hit point increases vary between the different rules.

In the below figure, we compare the fixed health rule (horizontal line) with the 50% central interval of outcomes from the rolling strategy, you can think of the interval as the range of possible outcomes that are pretty close to average.

We are only showing the results for d6 and d12 hit dice up to 11th level to avoid cluttering the graph, but the behavior we see is consistent for the other hit dice as well. The key feature is that the health pool of the fixed increases inches its way to the upper end of the central interval. By 11th level, fixed health increases are either just outside the 50% interval as is the case for d6 hit dice, or at the upper end of the range as we see for d12.

In all cases, the fixed health outcome is relatively close to the central intervals, which means that the difference between the two rules isn’t that big. If I had to sum it up, I’d say that although the odds favor the fixed health strategy, it wouldn’t be surprising if someone happened to roll higher health in the long term.

Which strategy to pick?

The way you choose to advance your character’s health is one of the more minor decisions you have to make in DnD. But hey. That doesn’t stop us from talking about the pros and cons of each rule:

Why choose fixed increases?

Mathematically, it wins on average
Especially when playing a low hit dice class like Wizard, hit points come at a premium and taking the fixed value is a prudent choice
At low levels, rolling a 1 or 2 for health can have fatal consequences — avoiding that by taking fixed increases could be the difference between life and death for your character

Why choose rolling for health?

Rolling is fun!
The benefits of fixed increases for are smaller for high hit dice classes, so you risk less by choosing to roll
Fixed health prevents you from getting low values, but it also prevents you from getting high values as well — if you want to press your luck, there’s no better way to get a big health pool than some lucky rolls

Outside of any number-based arguments, just pick the strategy that is most likely to give you a character/style of play that you have the most fun with. It is a game, after all.