To ensure the safety of formwork engineering, and to accurately calculate the lateral pressure of fresh concrete on formwork, the problems of the formulas for lateral pressure of fresh concrete on formwork, provided by the national current standards, were analyzed firstly. Subsequently, based on the basic physical quantities of the international system of units, five key factors affecting the lateral pressure of fresh concrete on formwork were identified, the range of concrete slump was emphatically analyzed. By taking concrete slump as an important factor, it was directly introduced into the derivation. According to the relation between the depth of the concrete from the top of the placement to the point of consideration in the formwork, and the product of the rate of placing concrete in forms and initial setting time, formula for calculating the lateral pressure of fresh concrete on formwork was derived. Finally, the accuracy of the proposed formula was verified by using the experimental data in the literature. The results show that the proposed formula accords with the results of dimensional analysis, and the formula is more accurate in the application range of the formula provided by the national current standards. Moreover, beyond the applicable range of the formula provided by the national current standards, the proposed formula is of high accuracy and is generally safe.