ABSTRACT Aim To compute the uncertainty of time‐in‐ranges, such as time in range (TIR), time in tight range (TITR), time below range (TBR) and time above range (TAR), to evaluate glucose control and to determine the minimum duration of a trial to achieve the desired precision. Materials and Methods Four formulas for the aforementioned time‐in‐ranges were obtained by estimating the equation's parameters on a training set extracted from study A (226 subjects, ~180 days, 5‐minute Dexcom G4 Platinum sensor). The formulas were then validated on the remaining data. We also illustrate how to adjust the parameters for sensors with different sampling rates. Finally, we used study B (45 subjects, ~365 days, 15‐minute Abbott Freestyle Libre sensor) to further validate our results. Results Our approach was effective in predicting the uncertainty when time‐in‐ranges are estimated using n days of continuous glucose monitoring (CGM), matching the variability observed in the data. As an example, monitoring a population with TIR = 70%, TITR = 50%, TBR = 5% and TAR = 25% for 30 days warrants a precision of ±3.50%, ±3.68%, ±1.33% and ±3.66%, respectively. Conclusions The presented approach can be used to both compute the uncertainty of time‐in‐ranges and determine the minimum duration of a trial to achieve the desired precision. An online tool to facilitate its implementation is made freely available to the clinical investigator.