SUMMARYHalimeda is a potential carbon sink species and an important player in the global carbonate budget. The objectives of this study were to: (i) examine the CaCO3 and sediment productions of H. macroloba by measuring the density, growth rate, and recruitment; (ii) quantify the numbers of aragonite crystals; (iii) document reproductive events; and (iv) determine the life‐span. This study was carried out at Lidee Lek Island, Satun, Thailand during July 2015 to April 2016. The density was measured using quadrats (0.25 m2) and three 50 m line transects. Alizarin Red‐S marking technique was used for the growth rate and CaCO3 accumulation rate assessments. The recruitment, reproduction and life‐span were measured by tagging 500 individuals. Tagged individuals and new plants were counted. In this study, mean and the highest density of Halimeda were 44.42 ± 13.95 and 138.22 ± 11.68 thalli m−2, respectively, and Halimeda produced 1–2 new segments.thallus−1 day−1 or 0.021 ± 0.001 g dry weight.thallus−1.day−1. The annual biomass production was 1910–5950 g m−2 year.−1. There was a low rate of occurrence of sexual reproduction, observed in late July to September, ranging from 0.17% to 1.92%. For the mortality and recruitment rates, approximately 70–80% of individuals were lost during July to September 2015, probably from sexual reproduction and the recruitment rate varied from 5.36 ± 0.79% to 21.03 ± 2.33%. The highest density of new recruits was found in September 2015 right after the sexual reproductive event occurred. New recruits have been found up to April 2016 without any reproductive events, suggesting that both sexual and asexual reproduction helped maintain the population. The life span of Halimeda was 8–12 months. In addition, Halimeda accumulated CaCO3 at approximately 0.018 g CaCO3 thallus−1 day−1 and produced CaCO3 at approximately 291.94–908.11 g m−2 year−1, indicating that Halimeda contributes to CaCO3 and helps to sink carbon through calcification. The results in terms of the density, growth rate, and CaCO3 accumulation rate can be used to calculate the mass of carbonate sediment contributed by Halimeda.