WHIP = (BB + Hits) / IP. Sub-1.30 is good, 1.10 is elite, sub-1.00 is exceptional.
Where ERA tracks runs, WHIP captures the rate of allowing baserunners — often more stable over time.
Koji Uehara posted a historic 0.57 WHIP in 2013 (Boston relief).
Simple to compute and applies equally to starters and relievers.