Math computation is a foundational skill for students developing higher-level mathematics problem-solving skills and represents a primary deficit for students struggling in mathematics. Students with mathematics difficulties also tend to have difficulties with other executive functions. One domain that is systematically part of most mathematical computation tasks but is not directly related to mathematics is handwriting. Handwriting fluency serves as a proxy for fine motor and domain-general cognitive functions (like executive functions) that are important for math fact fluency, yet handwriting carries very little domain-specific skill on its own. Handwriting can potentially be used to partial out the aspects of fact fluency that are not domain specific. In the current study, we tested the robustness of basic fact fluency in relation to general math fluency when accounting for handwriting. Third-grade students (N = 144) participated in the current study with measures of handwriting, basic fact fluency, and general math fluency taken at pretest prior to an intervention. We found that basic math fact fluency was a robust predictor of general math fluency across quantiles of the outcome distribution. Handwriting fluency did not improve model predictive performance beyond basic fact fluency. Limitations, as well as implications for future research and practice, are discussed.