The invention discloses a method of calculating an external diameter of a hot rolling checkered plate steel coil. The method of calculating the external diameter of the hot rolling checkered plate steel coil includes the following steps: A. a hot rolling checkered plate steel coil finished product is sampled, and sample relevant coiling diameter data are measured; B. the compactness of a hot rolling checkered plate steel coil sample is calculated; C. a compactness mathematical model of the hot rolling checkered plate steel coil is established; D. a calculation formula of the external diameter of the hot rolling checkered plate steel coil is obtained according to the sample compactness and a compactness mathematical module; and E. the external diameter of the hot rolling checkered plate steel coil is calculated through the calculation formula of the external diameter of the hot rolling checkered plate steel coil. The method of calculating the external diameter of the hot rolling checkered plate steel coil starts from checkered coil external diameter actually measured data. Legendre polynomials are applied for a first time, a steel coil density calculation model is established, and then a novel method of checkered coil external diameter calculation is deduced. The method of calculating the external diameter of the hot rolling checkered plate steel coil is convenient and flexible to use, accurate in calculation result and capable of greatly improving the calculation accuracy of the checkered plate steel coil external diameter.