Only One Way

A common feature in many tabletop games is that of fractional numbers.  Commonly these come in halves, but they can also frequently be thirds.  There are even games that ask you to use square roots in some situations, so you can really end up with just about any rational non-whole number.

Non-whole numbers aren't convenient to work with in games, so players are most often asked to round these numbers.  Typically, games will have a consistent method of "Up" or "Down" but I remember one 90s era game that had three different rounding methods requested within the character creation process alone:  "Up", "Down", and "To the nearest whole number".

I was never sure if the designers felt strongly about how to arrive at the desired whole number, or if they were just having a laugh.

Getting around to it

With this in mind, I remember my first Ars Magica game.  Someone asked the GM (whom was a software developer) how the game wanted them to round some number.  Clearly this player anticipated a response of either "up" or "down".

To this day I am not sure what the actual instruction is in the rule book, because our GM leveled a perfectly flat gaze and growled: "Round."

I was really enamored with this response, and for years I used it myself whenever I was facilitating a game where rounding was required.  After all, my public school education taught me that there was a correct way to round, and everyone else knew this also.  For almost 20 years I went about happily replacing the default rule assumptions in any game with the simple imperative: "Round".

It is a kind of dinosaur

Mercifully, I eventually picked this incredibly tiny hill to die on when there were better educated people around, and they made me aware of the error of my ways.  I went to Wikipedia as the first step in educating myself, and discovered a veritable plethorasaurus of rounding methods that were all valid in different ways.

Of course this led me down the rabbit hole of finding the most bestest kind of rounding that I could, and so I did and now I present it to you as a drop-in rule that you can use any time a game asks you to round something!

Zeno was Stotastic

Stochastic rounding uses probability to determine how to round a number.  Instead of a hard rule such as "less than 5 goes down" or whatever, you use probability to round 4 down 6/10th of the time and up 4/10th of the time.  Like in game theory, you get the best results when you iterate the process.

So this is how you do it, specifically:

Take a D10.  No, not that one with the unequalateral faces.  Use a D20 (it is a platonic solid) and count the values as 0-9 twice.

When rounding a number, choose a degree of accuracy.  This may be to a Whole Number, otherwise known as "the ones place".  You can also round to lower degrees of accuracy (also known as a more significant digit), for example "the tens place" or "the thousands place".

Now start with the least significant digit, and roll the die.

If the result is less than the digit, round this digit "up" such that the next most significant digit increases by one and the least significant digit is removed.

Otherwise, that is to say: if the result is equal to or greater than the digit, round this digit "down" such that the next most significant digit remains the same and the least significant digit is removed.

Continue this process with the next least significant digit until you have reached your desired degree of accuracy.

A simple way to think about this, is that you just roll a D10 for each digit that you want to get rid of, starting with the least significant and moving up.