~/tools/percentage-increase-decrease $ run calculator --apply-percentage
Percentage Increase/Decrease Calculator
Apply a percentage growth or reduction to a number. (e.g. Increase or decrease 100 by 20%).
A percentage increase/decrease calculator applies a change rate to a number. An increase multiplies the value by (1 + rate/100), while a decrease multiplies it by (1 - rate/100).
120
Step-by-step: 100 × (1 + 20 / 100) = 120
// How to Calculate Percentage Increase or Decrease
Adding or subtracting a percentage to a base amount is a common daily calculation. Whether you are dealing with price markups, inflation, population growth, weight loss, or battery drain, you can easily find the final number using decimals.
Mathematical Formulas:
Percentage Increase: New = Old × (1 + Percent / 100)
Percentage Decrease: New = Old × (1 - Percent / 100)
Worked Example 1: Increasing 120 by 15%
Suppose you want to apply a 15% increase to 120.
1. Express the rate as a decimal multiplier: 1 + (15 / 100) = 1.15.
2. Multiply by the original value: 120 × 1.15 = 138.
Therefore, 120 increased by 15% is 138.
Worked Example 2: Decreasing 80 by 25%
Suppose you want to apply a 25% reduction to 80.
1. Express the reduction factor: 1 - (25 / 100) = 0.75.
2. Multiply by the original value: 80 × 0.75 = 60.
Therefore, 80 decreased by 25% is 60.
// Frequently Asked Questions
▸Can you decrease a number by more than 100%?
Mathematically, yes. If you decrease a positive number by more than 100%, the result will cross zero and become a negative number. For instance, 50 decreased by 150% is 50 * (1 - 1.5) = -25.
▸What does it mean to increase a rate by 100%?
Increasing any value by 100% is equivalent to doubling it. For example, 10 increased by 100% is 10 * (1 + 1) = 20.
▸What is a practical use of percentage decrease?
Percentage decrease is commonly used when shopping. For instance, when an item is on sale, or during seasonal retail markdowns. It is also used to calculate mass loss, depreciation, and chemical decays.
▸Why are we asked to verify formulas using unit tests?
Verifying calculations programmatically ensures that standard mathematical formulas are absolutely correct under all edge cases. This protects small business owners and students from off-by-one and division-by-zero errors.