Fox Go Server rank calculator
The Fox Go Server has an interesting way of handling ranks, but many players are unclear about the subtleties of it, so I wrote an online calculator to help visualize the criteria for promotion and demotion.
The Fox system
Western Go players may be most familiar with having a fairly fine-grained rating, regularly updated with some mathematical formula based on recent results, which is then discretized to produce a rank.
The Fox server uses a different method, which I like to think of as a graded league system rather than a rating. If I'm currently in the 3d league, then I face other 3d players, and based on my recent record I might be promoted to the 4d league or demoted to the 2d league.
The actual criteria are summarized in a table you can see by hovering over “Ranked game rules” in your stats window, but they can be a little hard to interpret.
I'll use the 3–4D row as an example. To determine if your rank should change, only your most recent 20 games at your current rank are considered. If you have won at least 14 of those games, you are promoted (unless you still have a chance of a double promotion), and if you have lost at least 13 of those games, you are demoted (unless you still have a chance of a double demotion).
If you have won at least 18 of your first 20 games at this rank, you are promoted two ranks instead of one. To give you a chance to meet the double-promotion criterion, if you have earned a promotion but still have a chance at a double promotion, your promotion is delayed until you either earn the double promotion or it becomes impossible, at which point you are immediately granted the single promotion you already earned. Similarly, losing 17 of your first 20 games will cause a double demotion.
Interpreting the stats screen
Your recent results can be seen in your stats window:
This shows that this account of mine has gone 9–11 in my last 20 games, and the Os (wins) and Xs (losses) show those results in order, most recent on the right. After my next game, the new result will be added to the right of that string and the oldest result in the window will be removed from the left. Since a win will be removed, my new record will be 9–11 if I win and 8–12 if I lose.
The bottom row is often misinterpreted. It shows that if I win my next 8 games straight I will be promoted (you can verify that that would make my recent record 14–6), and if I lose my next 4 games straight I will be demoted (and I have no way to be doubly promoted or demoted). That's all it says. In particular, it doesn't say anything about what will happen if I go 8–1, or 1–4.
The calculator
If you want the full rundown on how future results will affect your rank, that's what the calculator is for. Enter your rank and recent results and click on Go, and you'll see something like this:
Here I can confirm that winning my next 8 games will earn me a promotion to 4d, but I can also see that if I lose a single game I'll have to go 10–1 to earn the promotion, not 8–1 or 9–1. You need to enter your entire string of recent results and not just your won-loss record, because it matters in what order old results fall off the edge of the window.
For completeness, here's what the top of the table of someone with a chance of double promotion looks like:
10–0 or 10–1 from this point on earns you a double promotion. 6–0 (or 6–1) means that you have earned a single promotion for sure but will remain at your current rank until you either earn that double promotion or are disqualified from it by picking up a second loss.