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Computed tomography (CT) scanning of coral skeletons provides critical information about coral growth and sensitivity to environmental change. Accurately deriving skeletal density from CT depends on careful calibration and may be sensitive to instrument settings, scan resolution, or species effects. Here, I evaluate the influences of these factors on deriving bulk coral skeletal densities from CT scans. The results show that instrument settings have important effects on derived densities. Increasing the beam hardening correction and decreasing scanning voltage had large effects, increasing derived density by 44% and 39%, respectively. Conversely, the effects of changing sample thickness and scan resolution by factors of two were smaller (<10% difference in derived density), though still important. There were no apparent differences between two genera (Porites sp. and Pocillopora sp.), but this should be validated with further investigations. Introduction Much like tree rings, the calcium carbonate (CaCO3) skeletons of many tropical shallowwater corals contain annual density bands that serve as intrinsic chronometers [1]. Computed tomography (CT) is a powerful tool that enables 3-dimensional visualisation and quantification of skeletal density. Indeed, CT scans of coral skeleton have been used to analyse colony growth [2], bioerosion [3] [4], porosity [5], and surface area [6]; to reconstruct past climate variability [7]; and to identify thermal stress events [8] [9]. This wealth of information ultimately depends on accurately deriving skeletal density from CT data. Several studies have presented relationships between the X-ray attenuation data of CT scans (in “Hounsfield Units”, or HU [10]) and bulk skeletal density [11] [4]. However, there has not yet been a systematic study evaluating the sensitivity of HU to instrument settings, making it difficult to compare the data collected in different laboratories. One of the key parameters that influences the quality of CT images, and potentially the relationship between HU and density, is the scan resolution. To date, coral skeletons have been CT scanned under various resolutions, with voxel (a 3-dimensional pixel) spacing ranging from ~1.3 μm [5] to ~200 μm [2]. Further, other factors such as voltage, sample thickness, and reconstruction settingsmay affect HU, but their influences have not been tested for coral skeleton analyses. Here, I evaluate the relationship between mean HU and various factors including scan resolution (36 and 18 μm), sample thickness (~1.2 cm and ~2.5 cm), beam hardening (19% and 79% corrections), X-ray voltage (80 kV and 89 kV), and species (Porites and Pocillopora). Objective The objective of this experiment is to test the influence of CT scan parameters on the mean HU, and thus derived bulk density, of coral skeletons. Deriving coral skeletal density from computed tomography (CT): effects of scan and reconstruction settings DOI: 10.19185/matters.2017065 Matters Select (ISSN: 2297-9239) | 2