Onion is one of the most productive vegetables in Japan, and onion growth models have been developed to predict its yield. However, onion growth is complex, and the selection of simple parameters is required to estimate bulb weight. This study utilized a 2-year dataset from a sequential growth study of spring-sown onions in the northeastern region of Japan to evaluate the correlation among growth parameters and to determine suitable parameters for estimating the dry weight of bulbs via multiple regression analysis. The results indicated that when bulb dry weight was the objective variable, the coefficients of determination of the single regression equation with plant height and leaf area as explanatory variables were 0.553 and 0.508, respectively, and the leaf parameter explained more than 50% of variance in bulb dry weight. Furthermore, the coefficient of determination was 0.912 when the leaf sheath basal diameter (ball width of the bulb) was used as an explanatory variable to explain the bulb dry weight. This indicates that there is a gap between the top and bulb growth. This study revealed the important parameters for predicting onion yield and demonstrated the switch in onion development from tops to bulbs.