The Operational Guidance: Estimating the Costs of Interventions to Reach Zero-Dose Children is a practical toolkit from the Gavi Zero-Dose Learning Hub 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. It provides step-by-step guidance on using robust approaches, especially ingredients (micro) costing, to calculate and report both financial and economic costs, helping practitioners capture the true value of resources used, including staff time, donated goods, and volunteer labor. By focusing on incremental and total costs, the toolkit supports better budgeting, resource optimization, and planning for scaling effective interventions, while aligning with WHO’s global standards on vaccine delivery costing to ensure consistency and comparability across studies. Readers will gain a rigorous framework to strengthen their costing analyses and generate evidence that directly informs decision-making, cost-effectiveness evaluations, and immunization equity strategies.