Background Fecal calprotectin (FC) has considered as a useful surrogate marker to predict which patients with ulcerative colitis (UC) are in endoscopic activity. FC has to be collected at the time of endoscopy for more accurate correlation between FC and endoscopic disease activity. In real practice, however, both tests cannot be easy to perform at the same time. Therefore, the aim of this study was to evaluate the optimal time interval for the correlation between FC and endoscopic disease activity in UC patients. Methods We analysed retrospectively 103 cases (79 patients) that was performed FC measurement and endoscopy within 3 months. FC quantitative tests was defined as normal (<100 μg/g), mild (100–200, 100–250 or 100–300 μg/g), and moderate to severe activity (>200, > 250, or > 300 μg/g). Endoscopic activity was graded using the Mayo endoscopic subscore (MES). After assessing concordance between FC level and MES, we use the youden index method to estimate the optimal time interval between both tests using the receiver operator curves (ROCs). Results Among 79 UC patients, 64.5% (51/79) were male. The mean FC level was 673.6 ± 1054 μg/g. The mean interval between FC measurement and endoscopy was 13.8 ± 22.7 days. FC levels and MES were positively correlated (r = 0.473, p < 0.001). Using a ROC curve with a cut off value of FC 200 μg/g, the optimal cut-off of the time interval for separation of non-correlation and correlation between FC level and MES was 7 days with a sensitivity of 74.4% and a specificity of 50.0% (AUC 0.6032; 95% CI 0.4779–0.6896). Similarly, using ROC curves with a cut off value of FC 250 or 300 μg/g as a standard for dividing mild and moderate, the optimal cut-off of the time interval were all 5.5 days (FC 250, sensitivity 71.7%, specificity 49.1, AUC 0.5862, 95% CI 0.4728–0.6884; FC 300, sensitivity 69.6%, specificity 47.4, AUC 0.5549, 95% CI 0.4668–0.6513). Conclusion FC level within 7 days could reflect endoscopic disease activity in patients with UC. In patients with high-level FC more than 200 μg/g, endoscopic evaluation for therapeutic decision making should be performed within a maximum of 7 days.