Ratio decompression (usually referred to in abbreviated form as ratio deco) is a technique for calculating decompression schedules for scuba divers engaged in deep diving without using dive tables, decompression software or a dive computer. It is generally taught as part of the "DIR" philosophy of diving promoted by organisations such Global Underwater Explorers (GUE) Innerspace Explorers (ISE) and Unified Team Diving (UTD) at the advanced technical diving level. It is designed for decompression diving executed deeper than standard recreational diving depth limits using trimix as a "bottom mix" breathing gas.
There have been three iterations of UTD Ratio Deco, The latest as of 2021 is RD 3.0, which has less emphasis on deep stops than RD 2.0.