  The dark matter halo mass function (HMF) is a characteristic property of
cosmological structure formation models, quantifying the number density of dark
matter haloes per unit mass in the Universe. A key goal of current and planned
large galaxy surveys is to measure the HMF and to use it to test theories of
dark matter and dark energy. We present a new web application for calculating
the HMF -- the frontend HMFcalc and the engine hmf. HMFcalc has been designed
to be flexible, efficient and easy to use, providing observational and
theoretical astronomers alike with the means to explore standard functional
forms of the HMF or to tailor their own. We outline the theoretical background
needed to compute the HMF, we show how it has been implemented in hmf, and
finally we provide worked examples that illustrate HMFcalc's versatility as an
analysis tool.
