This study evaluates the impact of undergraduate research on student achievement. It analyses graduating students from 2012 to 2016 at a UK university that requires a research project comprising 25% of final-year credits in most subjects, providing a sample of over 5000 students across the natural sciences, social sciences and humanities. It compares project grades to other final-year grades to gauge the 'research gain' or impact on students' achievement. Multiple regression analysis then determines what factors affect this impact. It finds that students achieve better grades on research projects than the average of other modules. This improvement is larger for students with lower prior achievement, women, and students in the natural sciences, though smaller for Asians and students declaring a disability. The implications are that undergraduate research provides a measurable benefit to all students, but this impact is larger for some, though not all, historically underrepresented or underachieving groups of students.