Rule Of 8

Rule of 8 is a guideline for whether or not to overcall an opening 1N.

1. Count the number of cards in your two longest suits.

2. Subtract your total losing tricks. (See Losing Trick Count.)

3. If the result is less than 2, pass; otherwise...

4. Count "good" HCP and if less than 6, pass.

"Bad" HCP include singleton honors and unsupported face cards in side suits such as Kxx.

With fewer than 10 HCPs, almost all of them should be in your long suits,

Sources:

BridgeGuys.com

Mel Colchamiro ACBL article.