  Several searches for Beyond Standard Model physics rely on an accurate and
highly precise theoretical description of the allowed $\beta$ spectrum.
Following recent theoretical advances, a C++ implementation of an analytical
description of the allowed beta spectrum shape was constructed. It implements
all known corrections required to give a theoretical description accurate to a
few parts in $10^4$. The remaining nuclear structure-sensitive input can
optionally be calculated in an extreme single-particle approximation with a
variety of nuclear potentials, or obtained through an interface with more
state-of-the-art computations. Due to its relevance in modern neutrino physics,
the corresponding (anti)neutrino spectra are readily available with appropriate
radiative corrections. In the interest of user-friendliness, a graphical
interface was developed in Python with a coupling to a variety of nuclear
databases. We present several test cases and illustrate potential usage of the
code. Our work can be used as the foundation for current and future
high-precision experiments related to the beta decay process.
  Source code: https://github.com/leenderthayen/BSG
  Documentation: http://bsg.readthedocs.io
