ATR%

A useful measure of recent volatility. I use 50 day or 50 week periods, but you can experiment with other values too. Because it measures ranges as a % of current close it can be used to make good comparisons with other historic periods of low (or high) volatility. This measure reached a new 23 year low for daily S&P 500 in July 2017.
Uses and historic examples: https://lunatictrader.com/2017/07/26/why-the-vix-is-so-low-and-why-you-shouldnt-worry-about-it-yet/