This paper shares results from a national ‘familiarisation trial’ of a mental mathematics intervention focused on assessing and encouraging strategic calculation methods with Grade 3 students in South Africa. Successful smaller pilots refined the intervention into 6 foci and this paper draws on assessment results from the four provinces that trialled one focus: adding and subtracting using jump strategies. Findings from pre- and post- test results of 1379 students show statistically significant gains in both the fluencies underlying calculating strategically and in items assessing strategic competency. The results indicate that scaling up this model into national implementation is feasible, and that the intervention package can support improvements in mental mathematics learning outcomes