Background & Aims: Previous prognostic scores for transarterial chemoembolization (TACE) were mainly derived from real-world settings, which are beyond guideline recommendations. A robust model for outcome prediction and risk stratification of recommended TACE candidates is lacking. We aimed to develop an easy-to-use tool specifically for these patients. Methods: Between January 2010 and May 2016, 1,604 treatment- naive patients with unresectable hepatocellular carcinoma (HCC), Child-Pugh A5-B7 and performance status 0 undergoing TACE were included from 24 tertiary centres. Patients were randomly divided into training (n = 807) and validation (n = 797) cohorts. A prognostic model was developed and subsequently validated. Predictive performance and discrimination were further evaluated and compared with other prognostic models. Results: The final presentation of the model was "linear predictor = largest tumour diameter (cm) + tumour number", which consistently outperformed other currently available models in both training and validation datasets as well as in different subgroups. The thirtieth percentile and the third quartile of the linear predictor, namely 6 and 12, were further selected as cut-off values, leading to the "six-and-twelve" score which could divide patients into 3 strata with the sum of tumour size and number <= 6, >6 but <= 12, and >12 presenting significantly different median survival of 49.1 (95% CI 43.7-59.4) months, 32.0 (95% CI 29.9-37.5) months, and 15.8 (95% CI 14.1-17.7) months, respectively. Conclusions: The six-and-twelve score may prove an easy-to-use tool to stratify recommended TACE candidates (Barcelona Clinic Liver Cancer stage-A/B) and predict individual survival with favourable performance and discrimination. Moreover, the score could stratify these patients in clinical practice as well as help design clinical trials with comparable criteria involving these patients. Further external validation of the score is required. Lay summary: There is currently no prognostic model specifically developed for recommended or ideal transarterial chemoembolization (TACE) candidates with hepatocellular carcinoma, despite these patients being frequently identified as the best target population in pivotal randomized controlled trials. The six-and-twelve score provides patient survival prediction, especially in ideal candidates of TACE, outperforming other currently available models in both training and validation sets, as well as different subgroups. With cut-off values of 6 and 12, the score can stratify ideal TACE candidates into 3 strata with significantly different outcomes and may shed light on risk stratification of these patients in clinical practice as well as in clinical trials. (C) 2019 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.