Flow straightener for a flowmeter, in particular an ultrasonic metering device, which can be fitted into a flow pipe which is upstream of or in the flowmeter and through which the fluid to be measured flows with a main direction of flow (H), comprising a number of vanes (11, 12, 16, 17), which each have surfaces (13) that are parallel to one another and, in the fitted position, run parallel to the main direction of flow (H) of the fluid to be measured, the vanes (11, 12, 16, 17) being arranged in such a way that, in the fitted position, the flow cross section (Q) of the fluid-carrying flow pipe is subdivided into a number of partial cross sections (TQ1,2,3,4,5), at least some of the partial cross sections (TQ1,2,3,4,5) being of different sizes.