You get a pay rise, a discount code, or a sales report, and suddenly you need to work out a percentage increase and decrease — and it’s easy to mix up which number goes on top. Type the wrong formula into a spreadsheet and a 20% jump can quietly turn into a 16.7% one, throwing off a budget or a report. This guide walks through the correct formulas, the mistake almost everyone makes at least once, and how to check your work with CheckMatter’s free Percentage Calculator.
The Percentage Increase and Decrease Formula
Both calculations start the same way: find the difference between the new value and the old value, then divide by the old value, then multiply by 100. The only thing that changes is whether the difference is positive or negative.
Percentage increase: ((New Value − Old Value) ÷ Old Value) × 100
Percentage decrease: ((Old Value − New Value) ÷ Old Value) × 100
The critical detail — the one that trips up most people — is that you always divide by the original value, never the new one. If a product goes from ₹500 to ₹600, the increase is (600 − 500) ÷ 500 × 100 = 20%. If it then drops back from ₹600 to ₹500, the decrease is (600 − 500) ÷ 600 × 100 = 16.7%, not 20%. Same ₹100 swing, two different percentages, because the base changed.
Why the Base Value Trips People Up
This asymmetry feels counterintuitive, but it’s simple arithmetic once you see it laid out. A percentage increase and decrease of the same dollar or rupee amount will never be equal in percentage terms unless the change is zero, because the denominator (the base) is different each time. Retailers sometimes exploit this: a “50% off” sale followed by a “50% increase” back to the original price doesn’t get you back to where you started in absolute terms — the math only works out because the base shifted.
A quick way to sanity-check yourself: percentage decreases from a larger base will always look smaller than the percentage increase needed to reverse them. If something drops 50%, it needs a 100% increase — not 50% — to get back to its original value.
Worked Examples
Salary example: if your monthly salary rises from ₹45,000 to ₹51,750, the percentage increase and decrease formula gives you (51,750 − 45,000) ÷ 45,000 × 100 = 15%.
Sales example: if a product’s price drops from ₹2,000 to ₹1,700, that’s (2,000 − 1,700) ÷ 2,000 × 100 = 15% decrease.
Notice both examples use 15% but represent very different absolute changes (₹6,750 versus ₹300) — the percentage only tells you the relative change against that specific base, which is exactly why comparing percentages across different starting values needs a bit of care.
Common Mistakes to Avoid
The most frequent error is dividing by the new value instead of the old one, which quietly understates increases and overstates decreases. A second common mistake is forgetting to multiply by 100, leaving you with a decimal (0.15) instead of a percentage (15%). A third is confusing percentage change with percentage points — going from 20% to 25% is a 5 percentage point increase, but a 25% percentage increase relative to the original 20%.
Government statistical agencies are careful about this distinction for exactly this reason. The U.S. Bureau of Labor Statistics, for instance, publishes explicit guidance on calculating percent change in economic indicators like the Consumer Price Index, precisely because small errors in base value compound into meaningful reporting mistakes. You can review their methodology on the BLS CPI calculation guide.
Key Takeaways
- Always divide by the original (old) value, not the new one.
- A percentage increase and decrease of the same absolute amount are rarely equal, because the base changes.
- Reversing a percentage decrease always requires a larger percentage increase.
- Don’t confuse percentage change with percentage points — they measure different things.
FAQ
Is percentage increase and decrease always calculated the same way?
The formula structure is the same, but you always divide by the starting value in both cases — that’s what makes an increase and a decrease from the same numbers produce different percentages.
Why does a 50% decrease need a 100% increase to reverse?
Because the base for the decrease was the original (larger) number, while the base for the increase back is the new (smaller) number — a smaller base means the same absolute change is a bigger percentage.
How can I double-check my calculation?
Run your numbers through the Percentage Calculator to confirm your manual math before using it in a report or budget.
For more everyday math guides like this one, browse the CheckMatter blog.