'Quarter-range' rule of thumb

For many data sets, the standard deviation is just under a quarter of the range.

This is a simple rule, but is only very approximate. The standard deviation can be more than a quarter the range in distributions with short tails or much less if there are long tails or outliers.

The 70-95-100 rule of thumb

The 70-95-100 rule is more accurate. In many distributions,

The 70-95-100 rule holds approximately for most reasonably symmetric data sets, but for skew data or distributions with long tails, outliers or clusters, it is often less accurate.