The aim of this study was to explore what arithmetical understanding is needed when students with mild intellectual disabilities use recipes during practical cooking lessons in Home Economics. The settings were compulsory schools in Sweden attended by students with intellectual disabilities. Sixteen lessons in Home Economics during which cooking took place were observed. In total, 37 students and three teachers participated. All students had a mild intellectual disability. Their ages varied, but most were between 13 and 14 years old. The sociocultural perspective on learning, combined with a literacy framework, was used as a theoretical foundation for the study. Main findings are that students need an arithmetical understanding of (i) how to interpret numbers, (ii) how to interpret and use units, and (iii) how to compute when using recipes. The knowledge and skills needed to be able to use a recipe are featured in the concept recipe literacy, capturing both theoretical, declarative knowledge and the more practical, procedural knowledge. Recipe literacy can be used to theorize the use of recipes when learning to cook, as in Home Economics.