Take the number of points one team is ahead. Subtract three. Add a half-point if the team that is ahead has the ball, and subtract a half-point if the other team has the ball. (Numbers less than zero become zero.) Square that. If the result is greater than the number of seconds left in the game, the lead is safe.
(If you don't have a calculator handy, use the tool below to do the calculations for you.)
If you've got a 10-point lead and the ball with 10 minutes left, is that a safe lead?
Of course not; teams come back from a 10-point deficit all the time. A 10-point lead, plus the ball, gives you a 7.5-point safety margin. It's safe for 56.25 seconds—56, rounded down. With 600 seconds to play, a 10-point lead (with the ball) is 9 percent safe. That doesn't mean a team with a 10-point lead and the ball with 10 minutes to go has only a 9 percent chance of winning. Rather, it means they're 9 percent of the way to having a completely insurmountable advantage.
An 11-point lead with nine minutes to play—we'll let you keep the ball. That's an 8.5-point safety margin with 540 seconds to play; it's 13 percent safe (72.25 divided by 540).
A 12-point lead with eight minutes to play ... that's a 9.5 point margin. It's 19 percent safe (90.25 divided by 480).
A 13-point lead with seven minutes to play ... 26 percent safe.
A 16-point lead with four minutes to play ... 76 percent safe, assuming the team with the lead also has the ball. It's really unusual for a team to come from 16 back with four to play and win, but it does happen. I would guess it happens twice a year somewhere in the world of college basketball.
A 17-point lead with three minutes to play ... bingo. That's a safe lead. Seventeen points with three minutes to play is a safe lead whether you have the ball or not, actually; a 17-point lead with the ball is safe at 3:30; a 17-point lead without the ball is safe at 3:02.