This paper presents a spreadsheet design tool for barriers in streams used for natural flood management. Retention times in such barriers should neither be too short (they fill and empty too quickly) or too long (they might already be full when a flood occurs). Previous work has shown the order of 10 h to be effective. The tool is freely available for download at https://www.jbatrust.org/how-we-help/publications-resources/rivers-and-coasts/nfm-leaky-barrier-retention-times.