Mark both ends with a dated pencil line, measure the width at a marked point with a feeler gauge or crack card, photograph it, and repeat in three months. Movement across a full wet and dry cycle is the answer.
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Monitoring is free, it takes ten minutes, and it converts the most anxious question in this niche into a measurement. It is also the single most useful thing a homeowner can do before spending money. The method. Draw a short pencil line across the crack at each end and date it, so any lengthening is visible. Pick a point along the crack, mark it, and measure the width there with a crack comparator card or a feeler gauge; a card costs a few dollars. Photograph the crack with a tape measure or a coin in frame for scale, and note the date. Repeat every three months. Why three months and why a full year. Soil moisture in this region swings seasonally, and a crack that opens in a dry late summer and closes in a wet spring is responding to that cycle rather than progressing. Only a full wet and dry cycle distinguishes cyclic movement from genuine progression, which is why a contractor evaluating a crack on a single visit has less information than a homeowner with four dated measurements. Crack monitors, the small plastic gauges with overlapping calibrated plates, do the same job with less effort and mount with epoxy on either side of the crack. Worth using where you want a clean record. What the readings mean. No measurable change across a year is strong evidence of a stable, non-structural crack. Steady widening in one direction is progression and warrants evaluation. Seasonal opening and closing without net change is soil cycling, which is worth addressing through drainage rather than structural repair. What to do in parallel while you monitor: correct the moisture conditions. Greater Cincinnati sits on glacial till over Ordovician limestone and shale, with clay-rich soils that hold water and hillside slopes that concentrate it. Extending downspouts and correcting grading is the cheapest structural intervention available and it addresses the cause rather than the crack.