A common fat quarter is about 18 × 21 inches
It is typically half the fabric width by half a yard, but selvages, squaring, manufacturer width, and previous trimming can change the usable rectangle.
The label is not the geometry
Measure crosswise width and lengthwise length. Those edited physical dimensions must control whether pieces fit, not the preset name.
Test the actual rectangle
Use the Pieces from Fabric calculator for one stock piece, then move to the project planner for multiple remnants and purchase shortfall.