The classic heuristic formula for calculating the mean arterial blood pressure (MAP) in the human cardiac cycle using 1/3 of the systolic pressure (P_S) plus 2/3 of the diastolic pressure (P_D) importantly neglects the nonlinear effect of heart rate (HR) on blood pressure. Regression analysis on blood pressure data from patients experiencing a variety of heart rates captured a logarithmic relationship between mean arterial pressure and heart rate, F_S=0.086 *ln(HR-41.2)+0.079, where F_S is the MAP form factor corresponding to the fraction of time the cardiac cycle spends in ventricular systole. The logarithmic relationship is qualitatively distinct from the linear and exponential relationships previously suggested in the literature yet better aligns with the known human physiology. This heart rate-dependent formula for the mean arterial pressure recapitulates the classic intuition as a special case (HR=60bpm implies F_S =1/3). In addition, the model suggests several other MAP heuristics: HR=45bpm implies F_S = 1/5, HR=50bpm implies F_S = 1/4, HR=175bpm implies F_S = 1/2. A calculator for the MAP can be found at https://mngu265.shinyapps.io/map\_calculator/