Percentage Calculation Guide — Formulas & Examples

Percentages appear everywhere — discounts, exam scores, growth reports and tax math. This guide covers the four calculations people reach for most often.

X% of Y

Formula: (X ÷ 100) × Y

Example: 15% of 200 = 0.15 × 200 = 30

Percentage increase

Formula: ((New − Old) ÷ Old) × 100

Example: Price rises from ₹100 to ₹125 → ((125−100)÷100)×100 = 25% increase

Percentage decrease

Formula: ((Old − New) ÷ Old) × 100

Example: Price drops from ₹100 to ₹80 → 20% decrease

Percentage difference

Compares two values relative to their average:

Formula: |A − B| ÷ ((A + B) ÷ 2) × 100

Useful when neither value is clearly the “original” baseline.

Real-world applications

• Shopping — compute discount on MRP.
• Business — month-on-month sales change.
• Education — exam marks and grade boundaries.
• Finance — return on investment approximations.

Quick tips

1. Always identify your base (the value you divide by).
2. “Increase by 10%” then “decrease by 10%” does not return to the start.
3. Round only at the end for multi-step problems.

Calculate instantly

Use MerQPrime’s percentage calculator for all four modes with step-by-step explanations — free and private in your browser.