How to calculate a percentage

A percentage is a fraction with denominator 100. 17% means 17 out of every 100. 6.5% GST on a restaurant bill, 85% exam marks, 15% Diwali discount on Flipkart — the same idea, different contexts. Once you can move between parts, wholes, and percents, school sums and shop math stop feeling like different subjects.

Core formulae

What percent is A of B? (A ÷ B) × 100
What is P% of B? (P ÷ 100) × B
Percentage change (increase or decrease): ((New − Old) ÷ Old) × 100
Reverse — find the whole when you know the part and percent: Part ÷ (P ÷ 100)

Example — exam marks

You scored 72 out of 80 in a CBSE unit test. Percentage = (72 ÷ 80) × 100 = 90%. Scored 340 out of 500 in a competitive mock? (340 ÷ 500) × 100 = 68%. Quick check on the percentage calculator.

To pass you need 40%. On a 75-mark paper, the pass mark = 40% of 75 = 0.40 × 75 = 30 marks. Reverse calculation: you need at least 30.

Example — shop discount

MRP 1,999 on Myntra, discount 15%. Discount amount = 0.15 × 1999 = 299.85 ≈ 300. Sale price ≈ 1,699.

Stacked offers need care. "15% off, then extra 10% off" — the second 10% usually applies on the already-reduced price, not on MRP. 1,999 → 15% off → 1,699 → 10% off → 1,529.10, not 1,529 from 25% off MRP (which would be 1,499.25). Shop policies differ; read the cart summary.

The discount calculator helps for single-rate cases.

Example — salary hike

Old CTC 6,00,000, new 6,60,000 after appraisal. Rise = 60,000. Percentage increase = (60000 ÷ 600000) × 100 = 10%. Your take-home may not rise 10% if tax slabs and PF changed — CTC percentage and in-hand percentage differ.

A job offer of 8,00,000 vs current 7,00,000: (100000 ÷ 700000) × 100 ≈ 14.3% hike. Useful for comparing offers, not sufficient alone — consider bonus, ESOP, and commute cost.

Example — GST cross-check

GST is a percentage of taxable value. A 500 item at 5% GST → tax = 25. At 18% → tax = 90. If the MRP 590 is inclusive of 18% GST, the tax inside is 590 × 18/118 = 90, not 18% of 590 (= 106.20). Inclusive vs exclusive matters — use the GST calculator so the mode stays honest.

Percentage skills and GST tools complement each other. See how GST is calculated for the full formula walkthrough.

Example — tip at a restaurant

Bill 1,200 before tip. You want to leave 10%. Tip = 0.10 × 1200 = 120. Total outflow 1,320. For 15%: 180 tip. Mental shortcut: 10% = move decimal one place left (120). 5% = half of that (60). 15% = 10% + 5% = 180.

Example — profit and loss (shopkeeper)

Bought 100 kg rice at 40/kg = 4,000. Sold at 50/kg = 5,000. Profit = 1,000. Profit percentage on cost = (1000 ÷ 4000) × 100 = 25%. On selling price it would be (1000 ÷ 5000) × 100 = 20%. school maths usually uses cost price as the base unless stated otherwise.

Example — population growth (census context)

A town's population rose from 2,40,000 to 2,76,000 in a decade. Increase = 36,000. Percentage growth = (36000 ÷ 240000) × 100 = 15% over 10 years, not per year. Annualised growth is a different calculation (compound), not simple percentage.

Common slips

Wrong base: Dividing by the new value instead of the old when calculating percentage change. A rise from 100 to 150 is 50% increase (50/100), not 33.3% (50/150).
Adding percentages on different bases: 10% off MRP plus 5% cashback on card is not "15% off" — the bases differ.
"Increased by 100%" means doubled, not "became 100". Increased by 100% → 2× original.
Confusing percentage points with percent change: Interest rate rose from 8% to 10% = 2 percentage points, not a 2% increase. The percent change in the rate itself is (2/8) × 100 = 25%.

When in doubt, write the fraction first (part over whole), then multiply by 100.

Example — finding the whole from part and percent

450 is 15% of what number? Whole = 450 ÷ 0.15 = 3,000. Useful when a GST-inclusive bill shows only the tax line and you need the taxable value without the calculator.

Example — successive percentage changes

Salary 50,000. Hike 10% → 55,000. Cut 10% next year → 49,500, not 50,000. Percentage changes compound; they do not cancel. Always apply each change on the current base.

Example — marks in boards

CBSE: 78/100 = 78%. But internal assessments sometimes weight sections — 80 marks exam (70% weight) + 20 marks project (30% weight). Weighted percent = (exam% × 0.7) + (project% × 0.3). Do not average percentages with different weights.

Example — mobile data usage

2.4 GB used of 3 GB pack. Usage percent = (2.4 ÷ 3) × 100 = 80%. Remaining = 20% = 0.6 GB. Same formula structure as exam marks — part over whole.

Example — fuel price change

Petrol was 96/litre, now 104. Increase = (8 ÷ 96) × 100 ≈ 8.33%. News headlines sometimes round differently — verify with the formula when budgeting monthly fuel spend for a 30-litre tank (≈ 240 more per fill).

Example — voting and turnout

1,20,000 votes cast of 2,00,000 registered voters. Turnout = (120000 ÷ 200000) × 100 = 60%. Same structure as exam marks — part divided by whole, times 100.

Example — recipe scaling

Recipe serves 4 with 200 g flour. Cooking for 6? Scale factor = 6/4 = 1.5. Flour needed = 200 × 1.5 = 300 g. Percentage thinking: 50% more people → 50% more flour. Same ratio logic as marks and discounts.

Example — cricket strike rate style thinking

Scored 45 runs off 30 balls. Runs per ball = 45/30 = 1.5. As "percentage of a run per ball" that is 150% — cricket uses different notation but the part-over-whole idea matches school percentage.

Practical takeaway

Run the numbers twice with conservative assumptions before you commit — whether that is a tax line on an invoice, a monthly SIP amount, a BMI trend over months, an age cut-off for a form, or a discount on a sale. Paperwork and family budgets both punish rushed mental maths. Bookmark the relevant Kalkulator.in tool, write down inputs on paper, and keep the screenshot or printout until the transaction is done. Small habits at the calculator stage prevent expensive corrections later — tax notices, broken SIP goals, wrong admission forms, or loan EMI surprises you could have caught with one careful pass.

Disclaimer: Educational maths. Not financial or tax advice.