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. This toolkit directly addresses the costing data gap in immunization planning by providing guidance on calculating both the financial and economic costs associated with field activities. Interventions like targeted outreach, house-to-house registration, and community mobilization are often overlooked or imprecisely estimated, which leads to challenges in accurate budgeting, strategy comparison, and successfully scaling up interventions. Readers will find practical, step-by-step tools to adapt an “ingredients costing” methodology to their planning. By using this toolkit, users will be able to capture the full economic value of all inputs used in the intervention, including donated time, in-kind contributions, and operational expenses.