people seem to assume these numbers often.
FYI As of the last known Dev announcement:
Bronze is bottom 1%-8%
Silver is the bottom 8%-29%
Gold is from 29%-61% making it the largest rank with below average, average, and above average players all in the same rank
Platinum is 61%-86% (from top 39% to top 15%)
Diamond is 86%-96% (from top 14% to top 4%)
Masters is 96%-99% (from top 4% to top 1%)
Grand Master is 100% (top 1%)
Per Jeff Kaplan himself on Feb 20. This probably has changed slightly, but not much, as this list had not changed all that much from the previous one.