Coin pools

I’ve heard a few game designers say that, as a base line, dice rolls should succeed at least half the time. That got me to thinking... What if we start with a base 50% chance and each increase moves it half-way towards 100%? Is there a simple dice mechanic for that?

Yes, there is, and it was already sitting on my RPG shelf. Prince Valiant, The Story Telling Game (1989) by Greg Stafford uses coin pools. The player flips a number of coins and, if any come up heads, it’s a success. (Sometimes you want more than one heads, but let’s ignore that for now.) The higher the character trait being tested, the more coins the player tosses.

(Since I usually have more dice than coins these days, I just use dice and “even” counts as “heads”. Don’t grab those odd dice—d3, d5, d7—though. Loan them to your opponent.)

It’s a nice, simple mechanic. It is open-ended (on the “high” end), which can be nice. And the probability increments start course and only get finer as you need it to.