A theoretical model is proposed for roughly estimating the screening-current-induced field (SCIF) in hightemperature superconducting coils. The direct relationship between the SCIF and the magnetization of tape wires leads to a scaling law for the SCIF and coil dimensions. A simple analytical expression of the SCIF at the coil center is obtained as functions of the current load factor, tape wire width, and coil dimensions. We verify that the numerical data for the precise simulation of SCIF in the literature are roughly fitted by our theoretical results for flat coils where the height is smaller than the outer diameter.