The purpose of this study was to develop practical prediction equations for estimating adult mid-arm circumference (AC) using self-reported height and weight data from NHANES III 1988-1994 and NHANES 1999-2000. Both surveys used a complex sample design to obtain nationally representative data for the US civilian noninstitutionalized population. The analytic sample consisted of 4801 men and 4854 women in NHANES III and 1960 men and 2180 women from NHANES 1999-2000. Self-reported weight, height, and age data from NHANES III were used for model building, and similar data from NHANES 1999-2000 were used for validation. An all-possible regressions procedure by gender was used to derive the mid-AC prediction equations. The final prediction equations for adult mid-AC are (for self-reported weight in pounds and height in inches) for men: AC (cm) = 32.52145 + 0.10975 x (wt)-0.26057 x (ht)-0.03028 x (age), R2 = 0.76; and for women: AC (cm) = 30.22126 + 0.13534 x (wt)-0.34121 x (ht) + 0.09014 x (age)-0.00082565 x (age2), R2 = 0.81. Based on these equations, tables were created to predict mid-AC using self-reported height and weight. Clinicians can refer to our prediction equations and reference tables to determine mid-AC and proper BP cuff sizes.