How do I calculate 18% GST?

Multiply the base price by 0.18 to get the GST amount, then add it to the base price. Example: ₹5,000 × 0.18 = ₹900 GST. Total = ₹5,900. To find the base price from a GST-inclusive price, divide by 1.18.

How do I calculate a percentage increase?

Percentage increase = ((New value - Old value) / Old value) × 100. Example: salary from ₹60,000 to ₹69,000 is a ((69,000-60,000)/60,000)×100 = 15% increase.

What is the quick way to calculate 10% of a number?

Move the decimal point one place to the left. 10% of ₹4,750 = ₹475. 10% of ₹12,600 = ₹1,260. For 20%, double the 10% amount. For 5%, halve the 10% amount.

Percentage Calculations for Everyday Life: Discounts, Tax, Interest, and More

Why Percentages Are So Important

Percentages appear everywhere in daily life: discount tags at shops, interest rates on loans and FDs, salary hike letters, GST invoices, exam scores, and investment returns. Not understanding percentages means being at a disadvantage in all of these.

The Fundamental Percentage Formula

Percentage = (Part / Whole) × 100

This formula rearranges to solve three types of problems:

Finding the percentage: What percentage is 45 out of 120?

Answer: (45 / 120) × 100 = 37.5%

Finding the part when given percentage: What is 35% of ₹4,800?

Answer: 4,800 × (35/100) = ₹1,680

Finding the whole when given percentage and part: ₹1,680 is 35% of what?

Answer: 1,680 / (35/100) = 1,680 × (100/35) = ₹4,800

Calculating Discounts at Shops

Sale price = Original price × (1 - discount/100)

Discount amount = Original price × (discount/100)

Example: Shirt marked ₹2,400 with 30% discount:

Discount = 2,400 × 0.30 = ₹720

Sale price = 2,400 - 720 = ₹1,680

Or: Sale price = 2,400 × 0.70 = ₹1,680

Quick mental calculation trick: For 10% off, move decimal one place left. For 20% off, double the 10% amount.

GST Calculation in Both Directions

Adding GST:

GST amount = Base price × (rate/100)

Total = Base price + GST amount

Example: Item at ₹5,000 + 18% GST:

GST = 5,000 × 0.18 = ₹900

Total = ₹5,900

Removing GST (finding pre-GST price from GST-inclusive price):

Base price = Total ÷ (1 + rate/100)

GST amount = Total - Base price

Example: You paid ₹5,900 inclusive of 18% GST:

Base price = 5,900 ÷ 1.18 = ₹5,000

GST paid = 5,900 - 5,000 = ₹900

Salary and Income Calculations

Percentage increase (salary hike):

New salary = Old salary × (1 + percentage/100)

Example: ₹60,000 salary with 15% hike:

New salary = 60,000 × 1.15 = ₹69,000

Increase = ₹9,000

Percentage decrease:

Example: Salary cut of 10% from ₹60,000:

New salary = 60,000 × 0.90 = ₹54,000

Finding percentage change:

% change = [(New value - Old value) / Old value] × 100

Example: Sales went from ₹8,00,000 to ₹9,20,000:

% change = [(9,20,000 - 8,00,000) / 8,00,000] × 100 = 15% increase

Interest Rate Calculations

Simple interest: SI = Principal × Rate × Time / 100

Example: ₹50,000 at 7% for 3 years:

SI = 50,000 × 7 × 3 / 100 = ₹10,500

Total = ₹60,500

Compound interest (annual):

A = P × (1 + r/100)^t

Example: ₹50,000 at 7% compounded annually for 3 years:

A = 50,000 × (1.07)^3 = 50,000 × 1.2250 = ₹61,258

Interest = ₹11,258 (₹758 more than simple interest)

Quick Mental Math Tricks for Percentages

10% of any number: Move decimal one place left (10% of ₹4,750 = ₹475)

5% of any number: Half of 10% (5% of ₹4,750 = ₹237.50)

15%: Add 10% and 5%

20%: Double the 10%

25%: Divide by 4

50%: Divide by 2

1%: Move decimal two places left

Example: 18% GST on ₹3,000:

10% = ₹300

8% = (10% × 0.8) = ₹240

18% = ₹300 + ₹240 = ₹540

Using Lazyblink for Percentage Calculations

lazyblink.com/tools/calculators/percentage-calculator handles all percentage calculation types: find percentage, find the whole from a percentage and part, calculate percentage increase/decrease, and multi-step calculations like GST and compound interest.