The Department of Energy’s National Nuclear Security Administration (NNSA) uses the cost estimating methodology in this paper to produce long-term budget projections for its construction portfolio. NNSA produces early-stage and defensible cost estimates across its eight laboratories, plants, and sites to ensure that adequate resources are available to support its modernization plans. A parametric cost estimating relationship (CER) developed from historic data across the nuclear security enterprise is used to produce cost estimates using gross square footage, facility hazard classification, and programmatic equipment as the primary parameters. The CER accounts for technical uncertainty in the range of project inputs and cost uncertainty via ranges of historic cost actuals. The CER is submitted to a Monte Carlo simulation to produce an “s-curve” distribution for each project. A schedule estimating relationship (SER), based on historic schedule durations with respect to total project costs, was also developed. The combination of the CER and SER enables the development of planning scenarios essential to federal planning and identifies long-term portfolio needs. NNSA annually updates and publicly releases these estimates as part of the updated 25-year programmatic line-item plan in the NNSA strategic planning document titled the Stockpile Stewardship and Management Plan (SSMP).