Percentage Formula & Tricks: Calculate Percentages Fast

Percentages show up everywhere — discounts, exam marks, interest rates, tax slabs, tips — yet most people only remember one version of the percentage formula and get stuck when a question is phrased slightly differently. Here are all three common percentage calculations, explained with examples.

1. Finding a Percentage of a Number

This is the most common case — "what is X% of Y?"

Result = (Percentage / 100) × Number

Example: What is 20% of ₹1,500? → (20 / 100) × 1,500 = ₹300

This is exactly the calculation used for discounts, tips, and tax additions.

2. Finding What Percentage One Number Is of Another

This answers "X is what percent of Y?" — common in exam-score questions.

Percentage = (X / Y) × 100

Example: You scored 78 out of 90 marks. What percentage is that? → (78 / 90) × 100 = 86.67%

3. Percentage Increase or Decrease (Percentage Change)

This is the one people mix up most often — it compares an old value to a new value.

% Change = [(New Value − Old Value) / Old Value] × 100

Example: A product's price increased from ₹800 to ₹920.

• Change = 920 − 800 = 120
• % Change = (120 / 800) × 100 = 15% increase

If the new value is lower than the old value, the result is negative — that negative sign represents a percentage decrease, not an error.

A Common Mistake: Reversing a Percentage Increase

If a price increased by 20% to reach ₹1,200, you cannot simply subtract 20% of ₹1,200 to find the original price — that gives the wrong answer, similar to the GST-inclusive mistake many people make. Since ₹1,200 represents 120% of the original price, the correct method is:

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

Original Price = 1,200 / 1.20 = ₹1,000

Quick Mental Math Tricks

• 10% of any number — just move the decimal point one place left (10% of 450 = 45).
• 5% of any number — find 10% and halve it (5% of 450 = 22.5).
• 15% of any number — find 10%, then add half of that 10% (15% of 450 = 45 + 22.5 = 67.5). Useful for quick tip calculations.
• 1% of any number — move the decimal point two places left (1% of 450 = 4.5), useful as a building block for other percentages.

Calculate Any Percentage Instantly

For anything more complex — or when you just want a guaranteed-accurate answer without doing mental math — use our free Percentage Calculator, which handles all three calculation types instantly.

Frequently Asked Questions