How to Calculate Percentage Change the Right Way

Percentage change measures how much a value has increased or decreased relative to its original value. Getting the formula right is critical — using the wrong denominator is the most common mistake.

The formula: Percentage Change = ((New Value − Old Value) / Old Value) × 100. Always divide by the old (original) value, never the new value. If the result is positive, the value increased. If negative, it decreased.

Example 1 — Increase: Monthly users grew from 10,000 to 13,500. Change = (13,500 − 10,000) / 10,000 × 100 = +35%. Users increased by 35%.

Example 2 — Decrease: Website traffic dropped from 50,000 to 42,500 visits. Change = (42,500 − 50,000) / 50,000 × 100 = −15%. Traffic decreased by 15%.

The wrong denominator mistake: If you divide by the new value instead of the old, you get a different (incorrect) result. In Example 1, dividing by 13,500 (new) gives 25.9%, not 35%. Always use the starting value as the denominator.

When the starting value is zero: Percentage change from zero is mathematically undefined (division by zero). In business contexts, say "increase from 0 to X" rather than attempting a percentage. Tooler's Percentage Calculator handles this edge case gracefully.

Compounding changes: Multiple percentage changes do not simply add up. A 20% increase followed by a 20% decrease does not return to the original. Starting from 100: +20% → 120, then −20% → 96. The net change is −4%, not 0%.