People often think percentage increase and percentage decrease are two separate calculations with different rules. In reality, they are two possible outcomes of a single formula. The sign of the result — positive or negative — tells you which direction the change went.
This article breaks down the distinction, explains why one formula handles both, and shows you exactly when to say "increase" versus "decrease" — with a comparison table and clear examples.
The Core Formula (Works for Both)
That's it. You don't change anything depending on whether you expect a rise or fall — you just do the calculation and let the result tell you.
What Makes It an "Increase"?
A percentage increase occurs when the new value is greater than the original value. The subtraction in step one (New − Original) produces a positive number, which stays positive through the rest of the calculation.
Example: Rent rises from $1,200 to $1,380.
$1,380 − $1,200 = $180 → $180 ÷ $1,200 = 0.15 → 0.15 × 100 = +15%
Positive result → percentage increase of 15%.
What Makes It a "Decrease"?
A percentage decrease occurs when the new value is less than the original. The subtraction produces a negative number, which carries through to give a negative percentage.
Example: A subscription drops from $29.99 to $19.99.
$19.99 − $29.99 = −$10.00 → −$10.00 ÷ $29.99 ≈ −0.3334 → × 100 ≈ −33.34%
Negative result → percentage decrease of about 33%.
Side-by-Side Comparison
| Feature | Percentage Increase | Percentage Decrease |
|---|---|---|
| New vs. Original | New > Original | New < Original |
| Result sign | Positive (+) | Negative (−) |
| Formula used | Same formula | Same formula |
| Maximum value | Unlimited (can exceed 100%) | −100% (value reaches zero) |
| Typical use | Growth, gains, raises | Drops, discounts, losses |
The "Percentage Off" Confusion
Retail and e-commerce sometimes describe discounts as a positive number: "30% off!" This is a percentage decrease expressed as a positive magnitude for readability. Mathematically, the price change is −30%. The formula still gives you a negative result — the positive phrasing is just a communication convention.
Key rule: If you use the standard formula and the result is negative, always say "percentage decrease" — never "negative percentage increase." The two terms are not interchangeable.
Symmetry Trap: Increase Then Decrease
A common misconception: if something increases by 50% and then decreases by 50%, you'd expect to be back where you started. You're not. Starting at $100:
- +50% → $150
- −50% of $150 → $75
You're 25% below the original. This is because the base changes between the two calculations. Percentage changes are not symmetrical when applied sequentially — a critically important nuance in finance and investing.
For more on where this distinction matters, see 10 real-world uses of percentage increase. And if you want to make sure your calculations are correct, use our free percentage change calculator — it handles increases and decreases equally.
When You're Not Sure Which Direction
If you're unsure whether you're dealing with an increase or decrease before calculating, that's fine — just apply the formula. The sign in the output will tell you. If the result is +, it's an increase. If it's −, it's a decrease. Our calculator colour-codes the result in green or red for immediate clarity.
Also worth reading: our step-by-step guide to calculating percentage increase covers the formula in depth, including how to handle negative starting values.