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(1) Background: The distribution of tumor volumes is important for various aspects of cancer research. Unfortunately, tumor volume is rarely documented in tumor registries; usually only maximum tumor diameter is. This paper presents a method to derive tumor volume distributions from tumor diameter distributions. (2) Methods: The hypothesis is made that tumor maximum diameters <i>d</i> are Weibull distributed, and tumor volume is proportional to <i>d^k</i>, where <i>k</i> is a parameter from the Weibull distribution of <i>d</i>. The assumption is tested by using a test dataset of 176 segmented tumor volumes and comparing the <i>k</i> obtained by fitting the Weibull distribution of <i>d</i> and from a direct fit of the volumes. Finally, tumor volume distributions are calculated from the maximum diameters of the SEER database for breast, NSCLC and liver. (3) Results: For the test dataset, the <i>k</i> values obtained from the two separate methods were found to be <i>k</i> = 2.14 ± 0.36 (from Weibull distribution of <i>d</i>) and 2.21 ± 0.25 (from tumor volume). The tumor diameter data from the SEER database were fitted to a Weibull distribution, and the resulting parameters were used to calculate the corresponding exponential tumor volume distributions with an average volume obtained from the diameter fit. (4) Conclusions: The agreement of the fitted <i>k</i> using independent data supports the presented methodology to obtain tumor volume distributions. The method can be used to obtain tumor volume distributions when only maximum tumor diameters are available.