Percentage Calculation Examples

Percentage calculations are among the most commonly needed arithmetic operations in business applications: discounts in e-commerce checkout flows, tax calculations in invoicing systems, conversion rates in analytics dashboards, interest rates in financial modeling, and progress indicators in project management tools. Having clear formulas for the most common percentage patterns prevents the off-by-one errors and formula inversions that cause subtle calculation bugs. This collection covers six percentage scenarios that appear most frequently in real applications. "What is 15% of 250?" (= 37.5) is the foundational calculation: multiply the percentage by the whole and divide by 100, or equivalently multiply by the decimal form (0.15 × 250). This is used for discount amounts, tip calculations, and tax amounts. "75 is what percent of 300?" (= 25%) inverts the calculation: divide the part by the whole and multiply by 100. This is used for progress percentages, completion rates, and ratio displays. A common implementation mistake is dividing the wrong way (300/75 = 4, not 25%) — always divide the part by the whole. Percentage change from 80 to 100 (= 25% increase) is the formula used for growth metrics everywhere: monthly revenue growth, week-over-week user signups, year-over-year comparison. The formula is always ((new - old) / old) × 100. A positive result is an increase, a negative result is a decrease. The denominator is always the old value — dividing by the new value instead gives the wrong answer. Discount price calculation ($120 × (1 - 0.20) = $96) is the clean implementation pattern for applying percentage discounts in code. Rather than calculating the discount amount and subtracting it (120 - 24 = 96), the single multiplication is less error-prone and requires fewer variables. In code: const discountedPrice = originalPrice * (1 - discountRate). Reverse discount calculation (finding the original price from a discounted price) is the formula developers most often get wrong. If the sale price is $70 after a 30% discount, the original is $70 / 0.70 = $100, not $70 + 30% of $70 (which gives $91, not $100). This matters for displaying the "was" price next to a sale price. Tax calculation ($45.99 × 0.085 = $3.91) is straightforward but needs care with rounding: always round to two decimal places for currency using Math.round(amount * 100) / 100 or a currency library like Dinero.js. Floating-point arithmetic in JavaScript (0.1 + 0.2 = 0.30000000000000004) can produce surprising results in tax calculations. Percentage points vs percent: an increase from 10% conversion rate to 15% is a 5 percentage point increase but a 50% relative increase. These terms are not interchangeable — use "percentage points" for absolute changes in a rate, and "percent" for relative changes. Mixing them up in business dashboards causes significant misinterpretation.

# What is 15% of 250?
15% of 250 = 37.5

# 75 is what percent of 300?
75 / 300 × 100 = 25%

# Percentage change from 80 to 100?
(100 - 80) / 80 × 100 = 25% increase

# Price after 20% discount on $120
$120 × (1 - 0.20) = $96.00

# Original price before 30% discount if sale price is $70
$70 / (1 - 0.30) = $100.00

# 8.5% sales tax on $45.99
$45.99 × 0.085 = $3.91