The Operational Guidance: Estimating the Costs of Interventions to Reach Zero-Dose Children is a practical toolkit that equips researchers and monitoring, evaluation, and learning staff with clear methods for costing interventions targeting zero-dose and under-immunized children in low- and middle-income countries. You do not need to be a health economist to use this toolkit. It is designed for program managers, MEL specialists, and implementers with basic project management, data collection, and spreadsheet skills. Using a transparent “ingredients costing” approach, breaking interventions down into their core inputs like staff time, supplies, and transport, the toolkit provides step-by-step templates and guidance to calculate both financial and economic costs, ensuring the full value of resources is captured. By focusing on incremental and total costs, it supports better budgeting, resource optimization, and planning for scaling effective interventions, while aligning with WHO’s global standards for vaccine delivery costing to ensure comparability across studies.