Percentage Calculator: Formula, GST & Marks Percentage Guide (2026)
📅 July 2026⏱ 8 min read✍️ ToolLoom Editorial
Percentages appear everywhere in Indian daily life — GST slabs, exam scores, salary hikes, discount calculations, and bank interest rates. Yet most people guess or use a calculator without understanding which formula to apply. This guide covers every core percentage type with worked ₹ and marks examples.
Final ÷ (1 ± X/100) = Original Example: ₹1,180 after 18% GST → 1180÷1.18 = ₹1,000
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ToolLoom's Percentage Calculator handles all four types — just select the type you need, enter your numbers, and get the result instantly without needing to remember which formula to use.
Percentage Increase and Decrease
Percentage Change Formula
% Change = [(New Value − Old Value) ÷ Old Value] × 100
Situation
Old Value
New Value
Calculation
Result
Salary hike
₹50,000
₹60,000
[(60K−50K)÷50K]×100
+20% increase
Price drop
₹800
₹640
[(640−800)÷800]×100
−20% decrease
Exam score change
340/500
420/500
[(420−340)÷340]×100
+23.5% increase
Petrol price rise
₹96/litre
₹104/litre
[(104−96)÷96]×100
+8.3% increase
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Percentage increase and decrease are not symmetrical. A 20% increase followed by a 20% decrease does not return to the original value — it results in a net −4% change. ₹100 → +20% → ₹120 → −20% → ₹96, not ₹100.
Reverse Percentage — Finding the Original Value
Reverse percentage is used when you know the final value after a percentage change and need to find the original. Common uses: finding the base price before GST was added, or the original price before a discount was applied.
Reverse Percentage Formulas
Original (before increase): Final ÷ (1 + %/100)
Original (before decrease): Final ÷ (1 − %/100)
Scenario
Final Value
%
Formula
Original
Price before 18% GST
₹1,180
18%
1180 ÷ 1.18
₹1,000
Price before 20% discount
₹800
20%
800 ÷ (1−0.20)
₹1,000
Salary before 10% hike
₹55,000
10%
55,000 ÷ 1.10
₹50,000
Percentage in GST Calculations
GST is the most common place where percentage calculation arises in Indian everyday life. The key formulas are:
Add GST: Price with GST = Base Price × (1 + GST%/100). For 18% GST on ₹2,500: 2,500 × 1.18 = ₹2,950
Remove GST (reverse): Base Price = GST-inclusive price ÷ (1 + GST%/100). For ₹2,950 at 18%: 2,950 ÷ 1.18 = ₹2,500
GST amount only: GST Amount = Base Price × (GST%/100). 18% of ₹2,500 = ₹450
GST Slab
Common Items
On ₹1,000 base
5%
Essential foods, transport, books
₹1,050 total
12%
Processed foods, construction services
₹1,120 total
18%
Most services, electronics, clothing above ₹1,000
₹1,180 total
28%
Luxury items, automobiles, tobacco
₹1,280 total
Marks Percentage for Indian Exams
Marks Percentage
Percentage = (Total Marks Obtained ÷ Maximum Marks) × 100
Exam / Board
Special Rule
Example
CBSE Class 10
Best-of-five subjects rule (add best 5 out of all subjects)
Best 5 total: 460/500 = 92%
CBSE Class 12
All 5 subjects counted (theory + practical combined)
480/500 = 96%
State boards
Varies — some include sports/extra subjects
Check board notification
JEE / NEET rank
Percentile, not percentage — based on rank position
99.5 percentile ≠ 99.5%
University CGPA to %
CGPA × 9.5 (Delhi University); others vary by university
8.0 CGPA × 9.5 = 76%
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Percentile vs Percentage: Percentile shows your rank relative to all test-takers — 99th percentile means you scored higher than 99% of candidates. Percentage is your raw score as a fraction of total marks. In competitive entrance exams, percentile is what matters for rank lists.
Percentage in Salary Hikes
Two common salary percentage questions:
What is my hike percentage? → [(New CTC − Old CTC) ÷ Old CTC] × 100. A jump from ₹8L to ₹10L CTC is [(10−8)÷8]×100 = 25% hike
What will my salary be after a 30% hike? → New CTC = Old CTC × (1 + 30/100). ₹8L × 1.30 = ₹10.4L
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Always negotiate on the base CTC, not the hike percentage. A 30% hike from ₹5L (= ₹6.5L) is less than a 20% hike from ₹7L (= ₹8.4L). The absolute number matters more than the percentage.
Percentage in Discounts
What You Want to Find
Formula
Example
Sale price after X% discount
MRP × (1 − X/100)
₹2,000 at 25% off → ₹2,000 × 0.75 = ₹1,500
Discount amount
MRP × (X/100)
25% of ₹2,000 = ₹500
Discount % from MRP and sale price
[(MRP−Sale)÷MRP]×100
MRP ₹2,000, Sale ₹1,500 → [(2000−1500)÷2000]×100 = 25%
Original price before discount
Sale Price ÷ (1 − X/100)
₹1,500 at 25% off → 1500÷0.75 = ₹2,000
% Calculate Any Percentage Instantly — Free
Percentage of a number, percentage change, reverse percentage, marks percentage, and GST calculations — all in one free tool.
To find what percentage X is of Y: Percentage = (X ÷ Y) × 100. For example, if you scored 450 marks out of 600, your percentage is (450 ÷ 600) × 100 = 75%. To find X% of a number Y: Result = (X ÷ 100) × Y. For example, 18% of ₹5,000 = (18 ÷ 100) × 5,000 = ₹900.
Percentage Change = [(New Value − Old Value) ÷ Old Value] × 100. If the result is positive, it's an increase. If negative, it's a decrease. Example: A salary that goes from ₹50,000 to ₹60,000 is an increase of [(60,000 − 50,000) ÷ 50,000] × 100 = 20%.
To add GST: Price with GST = Original Price × (1 + GST rate/100). For 18% GST: ₹1,000 × 1.18 = ₹1,180. To find original price from GST-inclusive price: Original = GST-inclusive price ÷ (1 + GST rate/100). So ₹1,180 ÷ 1.18 = ₹1,000 original price.
Marks Percentage = (Total Marks Obtained ÷ Maximum Marks) × 100. For CBSE Class 10, the best-of-five rule applies — add your best 5 subject marks and divide by the maximum possible (typically 500). For board exams with practical marks, add theory and practical marks before calculating.
Reverse percentage finds the original value when you know the final value and the percentage applied. For a percentage increase: Original = Final Value ÷ (1 + percentage/100). For a decrease: Original = Final Value ÷ (1 − percentage/100). This is used to find the base price before GST or the MRP before a discount.
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