Background: Childhood speech, language and communication needs (SLCN) impose a significant burden on individuals, families and society. There are explicit costs related to increased health utilization and expenditure. Additionally, there may be indirect costs associated with a child's employment prospects in the long term because of the child's low literacy and numeracy, which in turn affects adult labour force participation (LFP). Several reviews have identified that there is paucity of published evidence on the costs of SLCN. Motivated by broad policy implications, and the lack of prior research in this area, this paper calculates the indirect costs and workplace productivity losses of children with SLCN. Aims: To estimate the indirect costs of SLCN associated with a child's reduced long-term productivity. Methods & Procedures: Using 12 years of data from a longitudinal study of Australian children, we employed a panel fixed-effects model to estimate academic achievement at 14-15 years of age. Using these estimates, we employed a human capital approach (HCA) to estimate the projected LFP for children with SLCN, measured by workforce participation and foregone wages. LFP is estimated by extrapolating a child's academic achievement at 14-15 years of age to adulthood outcomes. Outcomes & Results: The results showed that a 1 SD (standard deviation) decrease in SLCN is equivalent to 0.19 (95% confidence interval (CI) = 0.09, 0.30) SD decrease in academic achievement at 14-15 years, 0.79% (95% CI = 0.37, 1.21) decrease in work participation and A$453 (95% CI = A$207, A$674) per annum in lost wages. The average work participation penalty across all level of SLCN (-3, -2, -1) is A$628 (95% CI = A$236, A$894) per person per year. Based on the prevalence of 8.3% from our sample, this equates to lifetime costs of A$21.677 billion (US$14.28 billion, €13.08 billion, £11.66 billion) for children with SLCN in Australia. Speech pathology treatment appears to have a positive impact on work participation and wages. On average A$355 (95% CI = A$346, A$355) per person per year could be saved through treatment or identification (the difference in lost wages for children with and without speech pathology treatment at each SLCN level (-1, -2 ,-3) calculated as a weighted average). This equates to lifetime savings of A$5.22 billion (US$3.44 billion, €3.15 billion, £2.81 billion) for children with SLCN in Australia. Conclusions & Implications: Overall, the findings showed that SLCN are associated with increased indirect costs through reduced workforce participation. The evidence from this study can be used to inform policies on the societal costs of SLCN.