Background <p>Syndrome of inappropriate antidiuresis (SIAD) is a common cause of euvolaemic hyponatraemia. Moderate-to-severe hyponatraemia, particularly in the neurosurgical setting, often necessitates administration of correctional fluids, typically hypertonic saline. However, predicting changes in plasma sodium levels following fluid administration is challenging and often inaccurate. In adults, the Voets equation has been shown to be superior to the classical Adrogué–Madias formula in predicting sodium levels achieved with correctional fluids, however, no studies have investigated its efficacy in children. Recent advancements in accurately estimating total body water in young children may improve this process. The current study was performed to compare the predictive accuracy of the Adrogué–Madias and Voets equations in estimating the change in plasma sodium levels following administration of correctional intravenous fluids.</p> Methods <p>This prospective observational hospital-based study was conducted in paediatrics and neurosurgical intensive care units from August 2022 to August 2024. Children aged 2&#xa0;months to 18&#xa0;years who satisfied the inclusion criteria for SIAD were enrolled. The predictive accuracy of Voets and Adrogué–Madias equations were calculated and compared.</p> Results <p>Analysis of predicted sodium correction using the Adrogué–Madias equation showed a p-value of 0.026 and an intraclass correlation coefficient (ICC) of 0.399. By contrast, the Voets equation demonstrated a p-value of &lt; 0.001 and an ICC of 0.926.</p> Conclusion <p>Both Adrogué–Madias and Voets equations were found to be effective in the prediction of hyponatraemia in patients with SIAD. However, the Voets equation was found to be more reliable and accurate in predicting the change in plasma sodium.</p> Graphical abstract <p>A higher resolution version of the Graphical abstract is available as <InternalRef RefID="MOESM1">Supplementary information</InternalRef></p> <p></p>