In this study a new functional form, r = r0[2/(1 + cos θ)]α, is used to fit the size and shape of the magnetopause using crossings from ISEE 1 and 2, Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM), and IMP 8 satellites. This functional form has two parameters, r0 and α, representing the standoff distance and the level of tail flaring. The value r is the radial distance at an angle (θ) between the Sun-Earth line and the direction of r. It is found that r0 varies with the interplanetary magnetic field (IMF) Bz component and has a break in the slope at Bz = 0 nT. The best-fit value of r0 decreases with increasing southward IMF Bz. For northward IMF Bz, the best-fit value of r0 increases slightly with increasing Bz. The best-fit value of α increases monotonically with decreasing IMF Bz. The dynamic pressure (Dp) also changes r0 and α. The parameters Dp and r0 are related by a power law of −1/(6.6±0.8). The best-fit value of α is slightly larger for larger dynamic pressure, which implies that Dp also has a role in flux transfer from the dayside to the nightside, but the size of this effect is small. An explicit function for the size and shape of the magnetopause, in terms of Dp and Bz, is obtained by using multiple parameter fitting in a form that is useful for operational space applications such as predicting when satellites at geosynchronous orbit will be found in the magnetosheath.