WebToolArcFormula and interpretation
increase % = (new − old) ÷ |old| × 100.
Use only when the new value is above the old value; otherwise use percentage change/decrease.
Worked check
Going from 80 to 100 is an increase of 20 on a base of 80, so the increase is 25%. The denominator is the old value. This is why an increase from 80 to 100 is not described as 20% simply because the raw difference is 20.
Rule: increase % = (new − old) ÷ |old| × 100. Use only when the new value is above the old value; otherwise use percentage change/decrease. 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.