When I cut using a ruler and rotary cutter - my ruler would be on x measurement, but my actual cut is x plus a thread or two more .
So, after several cuts, the theoretical amount of fabric used should be exactly strips cut times the width of the strip.
However, in my experience, I ended up short with that thinking
Example :ten strips cut 2.5 inches wide
Theoretical :. 10 * 2.5 = 25
Actual: 10 * 2.52 = 25.2
This only really matters when one has an "iffy" amount of material available.