Abstract Background Endoscopic measurement of lesion size is important for the assessment of the healing process, treatment strategies, and cancer staging. Consequently, accurate measurement of the size of the lesion is critical for proper candidate selection for endoscopic resection of early gastric cancer. A visual estimation of the size of the lesion has been widely used, but inaccuracy has been found. Hence, several endoscopic measurement methods have been proposed, but no reliable method has been used thus far. Therefore, we developed a guidewire scale for measuring the size of the lesion. This study focused on improving the accuracy on a practical level. Methods The guidewire scale consisted of green and yellow bands alternating on the wire, which were 5 mm for each band and a total of 30 mm in length. To assess the procedure’s validity and reliability, endoscopists estimated the size of the artificial lesions in the model by visually comparing them with the guidewire measurements. We found measurement error of the guidewire by 0.036±0.25 mm. The endoscopists determined the size of the lesion by the guidewire scale and used a numeral rating scale (NRS) to assess the satisfaction after the procedure. Results We enrolled 52 patients who were evaluated for the size of the lesion by the guidewire scale. The rate of completion of the procedure was 100% without any complications. Overall, the endoscopists’ satisfaction was good (NRS ≥ 8), but satisfaction was significantly higher in the dyspepsia group (P = 0.03). The mean of the size of the lesion was 10.7±8.9 mm, and small ulcer size (size ≤ 1 cm) displayed more satisfaction. The average time of measuring the lesion was 39.4±13.6 seconds, and the procedure time ≤ 30 seconds was highly satisfactory. Conclusions Our guidewire scale enhanced the accuracy of the endoscopic measurement of the size of the lesion. Thus, this guidewire scale could be used to estimate the sizes easily and safely in a clinical study.