Background“Kooth” is a web-based mental health platform commissioned by the National Health Service (NHS), local authorities, charities, and businesses in the UK. The platform gives children and young people (CYP) access to an online community of peers and a team of counsellors. This study reports an early economic evaluation of the potential benefits of Kooth in the UK. MethodsAn early evidence cost calculator was built to estimate the potential costs and savings of implementing Kooth from a UK NHS and crime sector perspective. A decision tree structure was used to track the progress of CYP with emerging mental health needs (EMHN), comparing CYP with access to Kooth to CYP without access to Kooth. The model implemented a 12-month time horizon and followed a typical Kooth contract in relation to costing, engagement, and CYP demographics. ResultsThe base case results followed a cohort of 2160 CYP. The results of the cost calculator estimated that engagement with Kooth is associated with a cost saving of £469,237 to the NHS across a 12-month time horizon, or £236.15 per CYP with an EMHN. From a combined NHS and UK crime sector perspective, the cost savings increased to £489,897, or £246.54 per CYP with an EMHN. The largest cost savings were provided by an estimated reduction of 5346 GP appointments and 298 antidepressant prescriptions. For this cohort, the model predicted that engagement with Kooth averted 6 hospitalisations due to suicidal ideation and 13 hospitalisations due to self-harm. Furthermore, the number of smokers and binge drinkers was reduced by 20 and 24, respectively. When a crime sector perspective was taken, 3 crimes were averted. DiscussionThis early model demonstrates that Kooth has the potential to be a cost-saving intervention from both an NHS and a combined NHS and UK crime sector perspective. Cost savings were provided through aversion in clinical and social outcomes. The model used a conservative approach to balance the uncertainty around assumptions of the intermediate outcomes (GP and medication use). However, it is limited by a paucity of costing data and published evidence relating to the impact of digital mental health platforms.