percentage increase and decrease calculator

How to use this percentage increase and decrease calculator

This page gives you three practical calculators in one place. If you are comparing two values and want to know whether the result is an increase or a decrease, use the first tool. If you already know the percentage and want to apply it to a starting number, use the second or third tool.

• Tool 1: Compare old value vs. new value to find percentage change.
• Tool 2: Add a percentage increase to a starting value.
• Tool 3: Subtract a percentage decrease from a starting value.

Percentage increase formula

When a value goes up, use this formula:

Percentage Increase = ((New Value - Old Value) / Old Value) × 100

Example: Your monthly sales rise from 400 to 500.

• Difference = 500 - 400 = 100
• Divide by old value = 100 / 400 = 0.25
• Convert to percent = 0.25 × 100 = 25% increase

Percentage decrease formula

When a value goes down, the core formula is the same, but the result is negative and interpreted as a decrease:

Percentage Change = ((New Value - Old Value) / Old Value) × 100

Example: A product drops from 120 to 90.

• Difference = 90 - 120 = -30
• Divide by old value = -30 / 120 = -0.25
• Convert to percent = -25% = 25% decrease

Applying a known percentage to a number

Increase a number by a percentage

Use:

New Value = Original × (1 + Percentage/100)

If your rent is 1,200 and it rises by 5%:

1,200 × 1.05 = 1,260

Decrease a number by a percentage

Use:

New Value = Original × (1 - Percentage/100)

If a jacket costs 80 and has a 30% discount:

80 × 0.70 = 56

Real-life use cases

• Personal finance: salary raises, inflation impact, monthly expense changes.
• Shopping: sale prices, markups, coupon stacking checks.
• Investing: portfolio growth rates, drawdowns, year-over-year comparisons.
• Business: revenue growth, cost reduction tracking, conversion-rate changes.
• School: exam score improvements and grade trend analysis.

Common mistakes to avoid

1) Using the new value as the base

Percentage change is measured relative to the old/original value. Using the new value changes the result and leads to incorrect conclusions.

2) Confusing percentage points with percent change

If a rate moves from 10% to 12%, that is a 2 percentage-point increase, but a 20% relative increase.

3) Forgetting that a drop and rebound are not symmetric

A 50% decrease requires a 100% increase to get back to the original value. This is one of the most common misunderstandings in budgeting and investing.

Quick reference examples

• From 50 to 65 → 30% increase
• From 65 to 50 → 23.08% decrease
• 200 increased by 15% → 230
• 200 decreased by 15% → 170

Final thoughts

A reliable percentage increase and decrease calculator saves time and reduces errors. Whether you are checking a raise, validating a discount, or analyzing business performance, these formulas give you clear, decision-ready numbers in seconds.