Percentage Calculator Guide

A clear, no-nonsense guide to percentages. Short formulas, small examples, and a free calculator you can use right away.

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Last updated: Sep 08, 2025

Basic percentage calculations

What is X% of Y?

The go-to operation for discounts, tips and taxes.

Formula: Result = (Percentage × Number) ÷ 100
  • 15% of 200 ⇒ 15 × 200 ÷ 100 = 30
  • 25% of 80 ⇒ 25 × 80 ÷ 100 = 20

X is what percent of Y?

Useful for grades, progress, and ratios.

Formula: Percentage = (Number ÷ Total) × 100
  • 50 of 200 ⇒ (50 ÷ 200) × 100 = 25%
  • 30 of 150 ⇒ (30 ÷ 150) × 100 = 20%

Percentage change (increase / decrease)

Handy for prices, KPIs, and any before/after comparison.

Formula: ((New − Old) ÷ Old) × 100

Percentage increase

100 → 120 ⇒ ((120 − 100) ÷ 100) × 100 = 20%
500 → 750 ⇒ ((750 − 500) ÷ 500) × 100 = 50%

Percentage decrease

200 → 160 ⇒ ((160 − 200) ÷ 200) × 100 = −20%
80 → 60 ⇒ ((60 − 80) ÷ 80) × 100 = −25%

Markup, discount & margin

Markup (based on cost)

Final Price = Cost × (1 + Markup). Example: Cost $50, markup 30% ⇒ $50 × 1.30 = $65.

Discount (based on price)

Final Price = Price × (1 − Discount). Example: $200 with 15% off ⇒ $200 × 0.85 = $170.

Margin is different: Margin = (SP − Cost) ÷ SP. If Cost = $80 and SP = $100, Margin = 20%, Markup = 25%.

Compound interest, made simple

Savings and investments grow faster when interest compounds.

Formula: Final = Principal × (1 + Rate)^Time
  • $1,000 at 5% for 10 years ⇒ $1,000 × 1.05^10 = $1,628.89
  • $5,000 at 8% for 5 years ⇒ $5,000 × 1.08^5 = $7,346.64

Try our Finance Calculator for amortization and ROI.

Percentages in Excel / Google Sheets

X% of Y: =X%*Y or =X/100*Y (e.g., =15%*200 → 30)
What percent is X of Y: =X/Y then format as % (e.g., =50/200 → 25%)
Increase/Decrease: =(New-Old)/Old then format as %

Real-world use cases

Everyday:

  • Shop discounts and tips
  • Battery or grade percentages
  • Splitting bills fairly

Business:

  • Pricing (markup/margin), promotions
  • KPI growth vs last month
  • Taxes/VAT checks and quick ROI

Quick tips & common mistakes

Check units: keep rate and time aligned (monthly with months).
Don’t mix margin with markup: they aren’t interchangeable.
Round only at the end for financial work.
Practice: A price goes from $80 to $92. What’s the increase %? (Answer: 15%)
Practice: Cost $40, margin 20%. What selling price? (Answer: $50)

Frequently asked questions

How do I calculate percentage increase?

Use ((New − Old) ÷ Old) × 100. Example: 120 vs 100 → ((120 − 100) ÷ 100) × 100 = 20%.

What’s the difference between markup and margin?

Markup is based on cost; margin is based on selling price. Markup = (SP − Cost) ÷ Cost. Margin = (SP − Cost) ÷ SP.

How does compound interest work?

You earn interest on the principal and on previous interest: Final = Principal × (1 + Rate)^Time.

Can I use this calculator for business?

Yes—pricing, profit checks, discounts, taxes, ROI, and quick what-if scenarios.

Ready to run the numbers?

Use the calculator below, then explore our other free tools.

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