How to Calculate Percentage Increase
The formula, the common mistakes, and when to use decrease instead.
(new − old) ÷ old × 100. That is the whole thing.
Percentage increase describes how much a value grew, relative to the original. One formula, works every time: (new − old) ÷ old × 100.
Part of: Everyday Calculators
Quick answer
(new − old) ÷ old × 100. That is the whole thing.
Key points
- ▸ Formula: (new − old) ÷ old × 100 = percent increase.
- ▸ Use old value as the denominator, not new — this is the single most common mistake.
- ▸ If new is less than old, the result is negative → that is a percentage decrease.
- ▸ A 100% increase means the value doubled. A 200% increase means it tripled.
- ▸ "Percent of" and "percent change" are different operations — do not confuse them.
Examples
- Price up from £50 to £60(60 − 50) ÷ 50 × 100 = 20% increase.
- Headcount 120 → 150(150 − 120) ÷ 120 × 100 = 25% increase.
- Common mistakeDividing by the NEW value: (60 − 50) ÷ 60 = 16.7%. Wrong denominator — always use the OLD value.
When to use which tool
- Percent Increase / Decrease CalculatorThe dedicated tool — handles both increase and decrease, shows the working.Calculate the percent change from one value to another — increase or decrease — with exact delta and ratio.
- Percentage CalculatorGeneral-purpose percentage tool. Pick this if you also need "what is X% of Y" style questions.Calculate percentages: what is X% of Y, X is what % of Y, and % change.
Related
- Percent Increase / Decrease CalculatorCalculate the percent change from one value to another — increase or decrease — with exact delta and ratio.
- Percentage CalculatorCalculate percentages: what is X% of Y, X is what % of Y, and % change.
- How to Calculate a Percent of a NumberMultiply by the decimal — that is the whole trick.
- Percentage Mental Math TricksFive shortcuts that cover tips, discounts, tax, and sale prices.
Frequently asked questions
› Is a 50% increase the same as a 50% decrease back to original? Trust & accuracy
No. 100 → 150 is +50%. 150 → 100 is −33.3% (back to old value). The asymmetry trips people up constantly.
› What if the old value is zero?
Percentage increase is undefined when the starting value is zero — you cannot divide by zero. Report the absolute change instead.
› How accurate are online calculators and converters? Trust & accuracy
Online calculators are only as accurate as the numbers, units, assumptions, and rounding choices you enter. Recheck the input values first, then compare the formula against your real situation. For legal, tax, medical, financial, or professional decisions, treat the result as a planning estimate, not advice.
› What inputs should I double-check first? Troubleshooting
Double-check units, dates, percentages, decimal placement, and whether the input is before-tax, after-tax, gross, net, original, or final. Most calculator mistakes come from feeding the right formula the wrong base. If the result feels off, rebuild it from a simple worked example.
› Why do two calculators sometimes give different answers? Comparison
Two calculators may round at different steps, use different defaults, or interpret the same label differently. Percent, time, finance, and unit tools are especially sensitive to basis and rounding rules. Compare the formula, not just the final number, before deciding which result to trust.