A glass composition of the present disclosure includes, in wt %, 50≤SiO2≤56, 20≤B2O3≤30, 10≤Al2O3≤20, 3.5≤MgO+CaO≤10, and 0≤R2O≤1.0, further includes Fe2O3, and has a permittivity of less than 5.0 at a frequency of 1 MHz. R is at least one element selected from Li, Na, and K. The glass composition of the present disclosure is a low-permittivity glass composition with which the occurrence of fiber breakage during fiber forming can be reduced even when glass fibers to be formed have a small fiber diameter, and the occurrence of defects such as fiber breakage and fluffing during processing of the glass fibers can be reduced.