📊 Percentage Increase Calculator: Formula, Examples, and Common Mistakes

Revenue grew from $42,000 to $58,800. What's the growth rate? If you said 28.6%, you're wrong — it's 40%. If you said 40%, you probably used the formula correctly. If you're unsure, you're in the company of many spreadsheet users who've made this mistake in quarterly reports. Here's the correct approach, with real-world examples and the psychology behind why people get it wrong.

The Formula (Memorize This)

Percentage Increase = ((New Value − Old Value) ÷ |Old Value|) × 100

The denominator is always the original (old) value. The absolute value in the denominator handles cases where the old value is negative (e.g., going from a loss to a profit). Walk through it step by step: (1) subtract old from new to get the absolute change, (2) divide by old to get the proportional change, (3) multiply by 100 to convert to percentage.

The Most Common Mistake: Dividing by the Wrong Value

The #1 error: people divide by the new value instead of the old value. Using the example above: (58800 - 42000) / 58800 × 100 = 28.6%. This is wrong. The question is "how much did it grow relative to where it started?" — not "what portion of the new total is the growth?" The denominator represents the reference point, and the reference point is always the starting value.

Why do people make this mistake? Psychological research on numeracy suggests that when people are given "before" and "after" numbers, the "after" number is more salient (it's the current state, the result), and people naturally anchor on it. The percentage increase formula requires anchoring on the "before" — which is counterintuitive. This is why spreadsheets with built-in percentage change functions are safer than manual calculation.

Real-World Examples (Test Yourself)

Percentage Decrease (Same Formula)

The same formula works for decreases — the result is simply negative. Revenue dropped from $100,000 to $85,000: ((85-100)/100)×100 = -15%. Report this as "15% decrease" or "-15% growth" — both are technically correct, but "15% decrease" is clearer in plain language.

Edge Cases to Watch For

• Old value is 0: The percentage increase from 0 to any positive number is mathematically undefined (division by zero). In business reporting, this is typically described as "from zero to X" or "100% growth" (using a different formula). Some finance conventions use "N/A" or "N/M" (not meaningful).
• Old value is negative: Going from -$5,000 (loss) to +$3,000 (profit): ((3000 - (-5000)) / |-5000|) × 100 = 160% increase. The absolute value in the denominator generalizes the formula to handle negative old values.
• Very small old value: An increase from 1 to 10 is a 900% increase — technically correct but potentially misleading. In reporting, note the absolute numbers alongside the percentage for context.

For quick, error-free calculations, use the Percentage Calculator. It handles percentage increase, decrease, "X% of Y," and "X is what percent of Y" — no manual formula required.