HRmax (maximum heart rate)
HRmax is the highest rate your heart can beat during maximal effort. Every age-based formula for it is an estimate with substantial individual scatter, and the common ones disagree with each other by more as age rises.
Example
At 50, the Fox formula (220 − age) gives 170 bpm while Tanaka (208 − 0.7 × age) gives 173. Tanaka reported a standard deviation of about 10 bpm, so treat any single figure as the middle of a band.