A simple model for bubble domain propagation in a field‐access device is obtained from a new analytical approach which, in contrast to previous treatments, concentrates on the energy of the b u b b l e rather than of the overlay, and emphasizes flux flow rather than field distributions. This leads to a simple magnetic‐circuit model for the bubble interacting with an overlay, and to a n a l y t i c a l expressions for the trapping potential and effective drive field of permeable overlay features. These expressions are then applied to predicting the quasistatic operating margin (region of operation in the bias field‐drive field plane) of a T+I bar device, the inputs being the device geometry and the bubble material parameters. Comparison with experimental results for a garnet device with 5 μm dia. bubbles shows remarkably good agreement.