INTRODUCTION: Domestic hand hygiene could prevent over 500 000 attributable deaths per year, but 6 in 10 people in least developed countries (LDCs) do not have a handwashing facility (HWF) with soap and water available at home. We estimated the economic costs of universal access to basic hand hygiene services in household settings in 46 LDCs. METHODS: Our model combines quantities of households with no HWF and prices of promotion campaigns, HWFs, soap and water. For quantities, we used estimates from the WHO/UNICEF Joint Monitoring Programme. For prices, we collated data from recent impact evaluations and electronic searches. Accounting for inflation and purchasing power, we calculated costs over 2021-2030, and estimated total cost probabilistically using Monte Carlo simulation. RESULTS: An estimated US$12.2-US$15.3 billion over 10 years is needed for universal hand hygiene in household settings in 46 LDCs. The average annual cost of hand hygiene promotion is US$334 million (24% of annual total), with a further US$233 million for 'top-up' promotion (17%). Together, these promotion costs represent US$0.47 annually per head of LDC population. The annual cost of HWFs, a purpose-built drum with tap and stand, is US$174 million (13%). The annual cost of soap is US$497 million (36%) and water US$127 million (9%). CONCLUSION: The annual cost of behavioural change promotion to those with no HWF represents 4.7% of median government health expenditure in LDCs, and 1% of their annual aid receipts. These costs could be covered by mobilising resources from across government and partners, and could be reduced by harnessing economies of scale and integrating hand hygiene with other behavioural change campaigns where appropriate. Innovation is required to make soap more affordable and available for the poorest households.