  The Simons Observatory (SO) is an upcoming experiment that will study
temperature and polarization fluctuations in the cosmic microwave background
(CMB) from the Atacama Desert in Chile. SO will field both a large aperture
telescope (LAT) and an array of small aperture telescopes (SATs) that will
observe in six bands with center frequencies spanning from 27 to 270~GHz. Key
considerations during the SO design phase are vast, including the number of
cameras per telescope, focal plane magnification and pixel density, in-band
optical power and camera throughput, detector parameter tolerances, and scan
strategy optimization. To inform the SO design in a rapid, organized, and
traceable manner, we have created a Python-based sensitivity calculator with
several state-of-the-art features, including detector-to-detector optical
white-noise correlations, a handling of simulated and measured bandpasses, and
propagation of low-level parameter uncertainties to uncertainty in on-sky noise
performance. We discuss the mathematics of the sensitivity calculation, the
calculator's object-oriented structure and key features, how it has informed
the design of SO, and how it can enhance instrument design in the broader CMB
community, particularly for CMB-S4.
