X-ray detection limit and sensitivity are important figure of merits for perovskite X-ray detectors. The well-known Currie method and the followed definitions in the International Union of Pure and Applied Chemistry deal with the time-accumulated pulse counting of atomic decay for the detection limit determination of mass or concentration. Literatures lack a validated mathematic expression for determining the lowest detectable dose rate of perovskite X-ray detector working in continuous current mode. We propose new methods based on a statistical model with a mathematic expression for detection limit determination of perovskite X-ray detector, where a prior detection limit can be calculated through only dark current measurement, and a posterior check of the detectability can be performed after measurement of the signal current. With X-ray detectors fabricated of methylammonium lead triiodide (MAPbI3) single crystals from inverse temperature growth, we validated our methods with the lowest detection limit of ~ 2.4 nGyair/s achieved by disabling charge injection of a device architecture.