A flow measuring device for measuring flow of a fluid. A housing has a passage which extends between an inlet and an outlet. A shaft is disposed within the housing and is rotatable about a central axis. A flow detector is mounted on the shaft and disposed at least partially within the passage. The flow detector includes a rotatable, generally cylindrical body that rotates about a central axis as the fluid flows. A magnet is disposed within the housing such that the central axis extends through it, and is configured to rotate simultaneously with the body about the central axis. A magnetic encoder is disposed within the housing and configured to sense rotation of the magnet as the flow detector angularly displaces about the central axis so as to detect angular displacement of the flow detector.