The Rule of 16 is a way to estimate the 1-day expected move of any security based on evaluating its implied volatility and dividing that number by 16. For the market in general (SPX), this can be done by dividing the VIX by 16 since the VIX is the implied volatility of SPX (roughly 23-37 days out). For a market week, a similar calculation can be made dividing the VIX by 7.2.

Implied volatility is based on the projected percentage move that the options market is pricing in for a year out with 68.3% confidence (one standard deviation). This rule works well for approximating the single day move because there are roughly 252 market days in a year, and the square root of 252 is approximately 16 (15.87). The active principle here is how the decay of implied volatility is proportionate to the square root of time (remaining).