HMS4 is a the software part of a system to measure holdup. "Holdup" refers to the accumulation of special nuclear material (SNM) inside the processing equipment of nuclear facilities. Holdup must be minimized and quantified: for radiation and criticality safety, safeguarding against theft or diversion, and economic reasons. Accurate holdup measurements, while simple in principle, are demanding on the operator. The measurements by their nature are made in situ at specified points in the plant — often in awkward locations where SNM may be deposited, such as valves and ducts. The operator often must wear protective clothing, resulting in additional discomfort during the performance of the measurement under already unpleasant conditions of high temperature and humidity — perhaps up a ladder while holding a detector against a pipe with an outstretched arm. Beside those physical demands, the operator must keep track of the nuclear counting data and associated parameters (e.g., wall thickness, measurement distance). The HMS4 holdup measurement system makes it all as easy as possible for the operator, who need carry only a small, lightweight mobile unit which guides the operation and automatically logs the data with unique coding. This nearly eliminates the expense of having to repeat a measurement due to a data entry error. The mobile unit is set up ready to go at the base station. When the operator completes the measurements, the unit is returned to the base to download the data. The host computer maintains all history and current data in an easy-to-use database. All needed reports and QA are available there.