We present the first public code for semi-analytical calculation of the γ-ray flux astrophysical J-factor from dark matter annihilation/decay in the Galaxy, including dark matter substructures. The core of the code is the calculation of the line of sight integral of the dark matter density squared (for annihilations) or density (for decaying dark matter). The code can be used in three modes: i) to draw skymaps from the Galactic smooth component and/or the substructure contributions, ii) to calculate the flux from a specific halo (that is not the Galactic halo, e.g. dwarf spheroidal galaxies) or iii) to perform simple statistical operations from a list of allowed DM profiles for a given object. Extragalactic contributions and other tracers of DM annihilation (e.g. positrons, anti-protons) will be included in a second release. Program title: CLUMPY Catalogue identifier: AEKS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEEF_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 207 466 No. of bytes in distributed program, including test data, etc.: 6 342 889 Distribution format: tar.gz Programming language: C/C++ Computer: PC and Mac Operating system: UNIX(Linux), MacOS X RAM: Depends on the requested size of skymaps ( ∼ 40 Mb for a 500 × 500 map) Classification: 1.1, 1.9 External routines: CERN ROOT library ( http://root.cern.ch/drupal/), Doxygen ( http://www.doxygen.org) (optional) Nature of problem: Calculation of γ-ray signal from dark matter annihilation (resp. decay). This involves a particle physics term and an astrophysical one. The focus here is on the latter. Solution method: Integration of the DM density squared (resp. density) along a line of sight. The code is optimised to deal with the DM density peaks encountered along the line of sight (DM substructures). A semi-analytical approach (calibrated on N-body simulations) is used for the spatial and mass distributions of the dark matter substructures in the Galaxy. Restrictions: Some generic dark matter annihilation spectra are provided but are not included in the calculation so far as it is assumed that the particle physics is independent of the astrophysics of the problem. Running time: This is highly dependent on the DM profiles considered, the requested precision ε and integration angle α int : • about 60 mn for a 5 ° × 5 ° map towards the Galactic centre, with α int = 0.01 ° , NFW dark matter profiles and ε = 10 − 2 ; • about 2 h for the same set-up towards the anti-centre; • 0.1 to 10 DM models per second, depending on integration angle and DM profile. ► Gamma-ray annihilation from dark matter structures. ► Public code for skymaps and statistical analyses. ► Full documentation and examples.