WebToolArcFormula and interpretation
decrease % = (old − new) ÷ |old| × 100.
A 50% decrease needs a 100% increase to return to the starting value.
Worked check
Going from 100 to 80 is a decrease of 20 on a base of 100, so the decrease is 20%. Returning from 80 to 100 requires a 25% increase because the reference base has changed.
Rule: decrease % = (old − new) ÷ |old| × 100. A 50% decrease needs a 100% increase to return to the starting value. For a final check, estimate the expected size of the answer before copying it, keep units consistent, and round only the displayed result rather than intermediate values. Re-enter one simple case by hand when possible; a result with the wrong sign or order of magnitude usually points to an input or reference-base mistake.