<p>Radiography plays a crucial role in nondestructive testing, particularly for detecting volumetric defects. While the characterization of these defects is generally accurate, measuring their size along the direction of the X-ray remains challenging. This paper explores the relationship between image gray value, penetration thickness, process parameters, and the linear attenuation coefficient based on radiographic theory. We utilize numerical integration and curve fitting to express the linear attenuation coefficient as a function of tube voltage and penetration thickness, establishing a mathematical model that links penetration thickness, tube voltage, and image gray levels. We then design experiments using a steel plate to determine the unknown coefficients in the model, leading to an accurate calculation model for defect size in the direction of X-ray penetration. The model's accuracy is validated through digital radiography and CT experiments conducted on small-diameter pipe welds, with measurement errors ranging from 8.16% to 8.73%</p>