Here is the size chart for selecting Speedo swim trunks for men. For these swim trunks, your hip, and waist measurements are used as a basis to find your correct size.
If you happen to fall between two sizes, the most important measurement is usually the hip measurement. Additionally, you should consider the desired fit. For a tight fit, choose the smaller of the two sizes; conversely, opt for the larger of the two sizes if you prefer a looser fit.