A model of an eddy current probe in the presence of an open defect, such as an elongated crevice corrosion pit, is developed. Assumptions are made in respect to field shape, field width, and lift-off function, which are then justified using experimental data. The technique generates estimates of both crevice depth and width without resorting to microminiature probes; other methods tend to generate a response to “volume lost” rather than the more important geometrical parameters. The model is tested against experimental data from artificial pits in a zirconium-niobium material in the range 50 to 800 μm deep, 500 to 2000 μm wide, and 5 to 20 mm long. The technique is also useful in assessing eddy current probe performance.