Predicted intensities for the saturated components of the magnetic field as a function of the accreted mass using the formalisms of Fuller et al. (2019; Eqs. (37)–(39); solid lines) and Spruit (2002; Eqs. (18)–(19); dash-dot lines). The shear rate is set at q = 1. These intensities are compared to the magnetic field reached at maximum intensity (blue and green plus signs) and at q = 0 (blue and green cross signs) by integrating Eqs. (42)–(46) for several fallback masses. Grey areas represent the estimated range of the dipolar magnetic field strength from regularly observed magnetars (dark grey) and from the three detected low-field Galactic magnetars (light grey). The vertical lines show the lower limit on the fallback mass (upper limit on the rotation period) needed to form a magnetar with a radial field stronger than B_Q ≃ 4.4 × 10¹³ G for the prediction of Fuller et al. (2019; dashed red) and Spruit (2002; dotted red).