A method of determining the length of a polynucleotide target is provided. With this method, a target is first hybridized to an array of first probes having different, determined lengths, resulting in the formation of duplexes between the polynucleotide target and the first probes. These duplexes have a single stranded section of target if the target is longer than the first probe it is in a duplex with. Next, a second probe having a determined length is hybridized to these duplexes. If the length of the target is greater than the length of the first probe it is displaced during this hybridization step by the process of branch migration. In contrast, if the length of the target is less than or equal to the length of the first probe, it is not displaced. Thus, the length of the polynucleotide target can be determined.