In diet studies, stomach contents from predatory fish may be difficult to identify due to digestion. The Gizzard Shad Dorosoma cepedianum is an important prey species for sport fish; thus, determining the size of ingested shad can assist with evaluating competitive interactions, bioenergetic patterns, and niche partitioning and can add precision to predictive models. The gizzard organ of clupeids appears to be more resistant to digestion compared to other tissues and can often be found in the stomachs of predatory fish after other tissues from Gizzard Shad are digested. If the gizzard diameter is proportional to Gizzard Shad weight or length, it could be a useful structure for estimating the size of partially digested Gizzard Shad when other structures that are traditionally used to estimate weight or length (e.g., backbones) are damaged due to advanced digestion. For this reason, we evaluated the allometry relating gizzard diameter and Gizzard Shad weight and TL. We sampled a total of 936 Gizzard Shad from nine Oklahoma reservoirs. Fish were frozen and later thawed; they were measured for weight (±0.01 g) and TL (±1 mm), and the gizzard was then removed. Gizzard diameter was measured (±0.1 mm) at its widest point using calipers. Eight different equations were evaluated to find the best relationship (lowest Akaike’s information criterion) between gizzard diameter and weight or length. The relationship between gizzard diameter and fish weight was best modeled as a second‐order polynomial, whereas the relationship between gizzard diameter and fish TL was best described by a five‐parameter Richard’s equation. Both relationships explained over 80% of the variation in Gizzard Shad size. The 95% CIs for weight (±4–7%) and TL (±2–7%) indicated good overall precision for mean fish size based on gizzard diameter. Therefore, we recommend using gizzard diameter to determine weight and TL from diet samples when advanced digestion of Gizzard Shad limits the use of more traditional metrics (TL, backbone length, etc.).