How do I calculate a percentage of a number?

Multiply the number by the percentage and divide by 100. For example, 20% of 500 is 500 times 20 divided by 100, which equals 100.

How do I calculate percentage increase or decrease?

Subtract the old value from the new value, divide by the old value, and multiply by 100. A positive result is an increase; a negative result is a decrease.

Can the calculator handle GST percentages?

Yes. Enter the base amount and the GST rate to calculate the tax amount and total. This works for 5%, 12%, 18%, and 28% GST slabs.

Percentage Calculator Guide — How to Calculate Percentages Easily

ToolsOfIndia.com Team · October 2026 · 6 min read

Percentages are everywhere — discounts at the market, exam results, tax rates, interest calculations, and business metrics. Despite being a fundamental math concept, many people still struggle with percentage calculations, especially when reverse percentages or percentage changes are involved. A percentage calculator eliminates the guesswork.

The Three Core Percentage Calculations

1. Finding a Percentage of a Number

This is the most basic percentage task. The formula is:

Result = (Number × Percentage) ÷ 100

For example, what is 15% of 2,000? The calculation is 2,000 × 15 ÷ 100 = 300. This is useful for calculating discounts, tips, tax amounts, and commissions.

2. Finding What Percentage One Number Is of Another

The formula is:

Percentage = (Part ÷ Total) × 100

For example, if you scored 85 marks out of 100, your percentage is 85 ÷ 100 × 100 = 85%. If the total was 120 marks and you scored 96, the calculation is 96 ÷ 120 × 100 = 80%.

3. Percentage Increase or Decrease

The formula is:

Change = ((New Value − Old Value) ÷ Old Value) × 100

For example, if a product's price increased from ₹400 to ₹500, the percentage increase is (500 − 400) ÷ 400 × 100 = 25%. If it dropped from ₹500 to ₹400, the decrease is (400 − 500) ÷ 500 × 100 = −20%, meaning a 20% decrease.

Practical Examples in Daily Life

Shopping Discounts

A shop advertises 35% off on a jacket priced at ₹3,200. The discount amount is 3,200 × 35 ÷ 100 = ₹1,120. The final price is 3,200 − 1,120 = ₹2,080. The calculator does both steps instantly.

Exam Marks

A student scores 425 out of 600 across six subjects. The percentage is 425 ÷ 600 × 100 = 70.83%. This determines class ranking and eligibility for many college admissions.

GST Calculation

A product is priced at ₹5,000 before GST, and the applicable rate is 18%. The GST amount is 5,000 × 18 ÷ 100 = ₹900. The total including GST is ₹5,900. Use the invoice generator to create a GST-compliant invoice with this calculation built in.

Salary Hike

If your salary increased from ₹40,000 to ₹46,000, the percentage hike is (46,000 − 40,000) ÷ 40,000 × 100 = 15%. This helps you evaluate whether a raise keeps pace with inflation.

How to Use the ToolsOfIndia Percentage Calculator

The calculator supports all three calculation types in a single interface:

Open the percentage calculator.
Choose the calculation type: "X% of Y," "X is what % of Y," or "percentage change."
Enter the two values in the input fields.
The result appears instantly — no need to click calculate.

Common Percentage Mistakes

Confusing percentage points with percentages: If an interest rate rises from 8% to 10%, that is a 2 percentage point increase, but a 25% relative increase. These are different statements.
Wrong base for percentage change: Always divide by the old (original) value, not the new one. Dividing by the new value gives an incorrect result.
Adding percentages directly: Two successive 10% discounts do not equal 20% off. They equal 19% off, because the second discount applies to the already-reduced price.

Conclusion

Percentage calculations are essential for shopping, academics, finance, and business. The percentage calculator handles all three core percentage operations instantly and accurately, saving you from mental math errors. Whether you are calculating a discount, evaluating an exam score, or working out GST, the tool gives you the answer in real time.