Math · 5 min read · November 28, 2024

8 Percentage Calculations You Need Every Day

From working out a restaurant tip to understanding your salary raise to spotting whether a "50% off" deal is actually good — percentages appear constantly in everyday life. Here are the eight calculations worth knowing cold.

1. Finding a Percentage of a Number

The most basic percentage calculation. Used for tips, tax, and discounts.

Formula: Answer = (Percentage ÷ 100) × Number

20% of $85 restaurant bill $85 × 0.20 = $17.00 tip

12% sales tax on $49.99 $49.99 × 0.12 = $6.00 tax

15% off $120 jacket $120 × 0.15 = $18 discount → $102 final price

2. Calculating Percentage Change

Essential for salary negotiation, investment returns, and price comparisons.

Formula: % Change = ((New − Old) ÷ Old) × 100

Salary from $45,000 to $52,000 ((52000−45000)÷45000)×100 = +15.6% raise

Price from $80 to $60 ((60−80)÷80)×100 = −25% decrease

Portfolio from $10K to $8.5K ((8500−10000)÷10000)×100 = −15% loss

3. Finding the Original Price Before Discount

Common when shopping: the price tag shows "sale price" and you want to know the original.

Formula: Original = Sale Price ÷ (1 − Discount%/100)

Jacket $85 after 30% off $85 ÷ 0.70 = $121.43 original

Phone $399 after 25% off $399 ÷ 0.75 = $532 original

4. Calculating a Percentage Increase (Adding VAT or Tax)

For adding GST/VAT to prices or calculating total cost with taxes.

Formula: New Price = Original × (1 + Rate/100)

$100 item + 15% VAT $100 × 1.15 = $115.00

$250 service + 8% GST $250 × 1.08 = $270.00

5. What Percentage Is X of Y?

Used to understand market share, test scores, and ratios.

Formula: Percentage = (Part ÷ Whole) × 100

You scored 38/45 on a test (38÷45)×100 = 84.4%

$320 spent of $1,200 budget (320÷1200)×100 = 26.7% used

6. Calculating a Tip Quickly in Your Head

You should not need a calculator for a basic tip.

20% trick: Move decimal left one place (=10%), then double it

20% on $63 10% = $6.30, doubled = $12.60

15% on $45 10% = $4.50 + half ($2.25) = $6.75

18% on $80 10% = $8, add 8% (=$6.40) → $14.40

7. Expressing a Debt or Savings as % of Income

Critical for budgeting — the 50/30/20 rule and debt-to-income ratio.

Formula: % of Income = (Amount ÷ Monthly Income) × 100

$1,400 rent on $4,800 income (1400÷4800)×100 = 29.2% — good

$2,800 rent on $4,800 income (2800÷4800)×100 = 58.3% — danger zone

8. Percentage Points vs Percentage Change

A common source of confusion in news and finance.

These are different: going from 2% to 3% is +1 percentage point but +50% change in rate

Interest rate 2% → 3% Rose by 1 percentage point, or 50% relative increase

Pass rate 60% → 72% Rose by 12 percentage points, or 20% relative increase

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