Abstract: For the length calculation of stirrup, it is not formed the unified standard in engineering.Based on the definition of thickness of protection layer, diameter of flexural center, center length of inner package, length of outer package, length of rebar hook, the measure of the difference, anti-seismic and non-seismic, The stirrup' calculation formula under the assumption of common measurement software was analyzed, the theoretical calculation formula for the center line of common stirrup was derived, and then compared the calculated deviation between common measurement software and the center line theory.The center line theory was more accurate, because it can avoid the steel processing measure when internal pressure external pull difference effect.The theoretical calculation formula for the center line of common stirrup was recommended in engineering.