Percentage Calculator — Why a 20% + 10% Discount Isn't 30% Off
Calculate "X% of Y," percent increase, and percent decrease in one tool. See why stacking a 20% discount with an extra 10% off gives 28% off, not 30% — with a lookup table for common combinations. Also handy for tax and interest math.
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Discount Amount Quick Reference (%OFF)
| Original Price | 5%OFF | 10%OFF | 15%OFF | 20%OFF | 25%OFF | 30%OFF | 50%OFF |
|---|---|---|---|---|---|---|---|
| 1,000 | 950 | 900 | 850 | 800 | 750 | 700 | 500 |
| 2,000 | 1,900 | 1,800 | 1,700 | 1,600 | 1,500 | 1,400 | 1,000 |
| 3,000 | 2,850 | 2,700 | 2,550 | 2,400 | 2,250 | 2,100 | 1,500 |
| 5,000 | 4,750 | 4,500 | 4,250 | 4,000 | 3,750 | 3,500 | 2,500 |
| 10,000 | 9,500 | 9,000 | 8,500 | 8,000 | 7,500 | 7,000 | 5,000 |
| 20,000 | 19,000 | 18,000 | 17,000 | 16,000 | 15,000 | 14,000 | 10,000 |
| 30,000 | 28,500 | 27,000 | 25,500 | 24,000 | 22,500 | 21,000 | 15,000 |
| 50,000 | 47,500 | 45,000 | 42,500 | 40,000 | 37,500 | 35,000 | 25,000 |
| 100,000 | 95,000 | 90,000 | 85,000 | 80,000 | 75,000 | 70,000 | 50,000 |
Formula: Discounted price = Original price × (1 − discount rate). Example: 10,000 × (1 − 0.20) = 8,000 (20% off).
What Is Percentage Calculation?
Percentage calculation means figuring out what share one quantity represents of another, or working out a specific amount once a percentage is known. It comes up constantly in everyday life — the discounted price at checkout, the tax added to a bill, a test score expressed out of 100, or how much sales grew compared with last year — yet it is surprisingly easy to second-guess whether you should multiply or divide, or whether increases and decreases use different formulas. This tool removes that guesswork: just switch between four modes — "What is X% of Y?", "What percent is A of B?", "Increase by X%", and "Decrease by X%" — and the result appears the moment you type a number.
All the math runs locally in your browser using JavaScript, so nothing you enter is ever sent to a server. Decimal percentages (such as 3.5%) are fully supported, which makes this handy for situations where round numbers rarely apply, like reduced VAT rates or mortgage interest calculations — you get an answer faster and more reliably than working it out by hand or with a basic calculator.
How to Use the Percentage Calculator
- Choose the calculation mode you need Pick from the four cards — "What is X% of Y?", "What percent is A of B?", "Increase by X%", or "Decrease by X%" — based on the question you are trying to answer.
- Enter the base value Type the starting amount or quantity into the "Value" field. Negative numbers and decimals are both accepted.
- Enter the percentage Type the percentage figure into the "Percentage" field. The result recalculates automatically as you type — there is no button to press.
- Read the result and the difference The "Result" and "Difference" both appear at once. For the increase and decrease modes, you can immediately see how much the value changed in absolute terms as well.
Tips for getting more out of it
- To find what percent A is of B: A ÷ B × 100. Example: 30 is what percent of 120? → 30 ÷ 120 × 100 = 25%.
- "Increase by X%" means value × (1 + X/100). Adding 10% sales tax on 10,000 → 10,000 × 1.1 = 11,000.
- "Decrease by X%" means value × (1 − X/100). A 20% discount on 10,000 → 10,000 × 0.8 = 8,000.
- Stacking discounts: a 20% discount followed by an additional 10% off is not 30% off — it's 28% off (0.8 × 0.9 = 0.72 of original).
- The same formula works for interest rates. A principal of 10,000 at 3% annual interest for one year yields 10,000 × 1.03 = 10,300.
When This Tool Comes in Handy
Checking a sale price on the spot
Quickly confirm what a discounted price actually is as a percentage of the original — handy when you want a fast, calculator-style answer for a specific item rather than a general estimate.
Working out test scores or survey response rates
Drop a score and a maximum, or a response count and a total, into "What percent is A of B?" to instantly turn raw numbers into a percentage.
Tracking year-over-year or month-over-month change
Feed sales figures, visitor counts, or other results into "Increase by X%" or "Decrease by X%" to see both the amount and the percentage of change from a target or baseline figure.
Sanity-checking sales tax and interest calculations
Confirm the basic formula behind tax and interest (value × (1 ± percentage/100)) here first. For a full breakdown of the tax amount itself, the sales tax calculator is a useful next stop.
Estimating the real price after stacked discounts or coupons
Use the quick-reference table to see the true combined discount when multiple markdowns are applied in sequence. For comparing discount scenarios in more depth, try the discount calculator.
Percentage Terms Explained
- Percent (%)
- A unit expressing a share of a whole treated as 100 parts. The word derives from the Latin "per centum" ("per hundred") — 50% is half of the whole, and 100% is the whole itself.
- Ratio / Proportion
- The general idea of how much one quantity is relative to a reference quantity. A percentage is simply one way of expressing a ratio, scaled by 100 — the same relationship can also be written as a fraction or a decimal.
- Base value
- The reference quantity a percentage is calculated against — what this tool calls the "Value" field. Choosing a different quantity as the base can produce a very different percentage from the same pair of numbers.
- Percentage point (pp)
- A unit for the absolute difference between two percentages. If an interest rate rises "from 2% to 3%," that is a 1 percentage point increase, but expressed as a relative change (a percent increase), it is a 50% increase.
- Percent increase / decrease
- How much a value has grown or shrunk relative to its original amount, expressed as a percentage. It is calculated as (new value − original value) ÷ original value × 100 — exactly what this tool's "Increase by X%" and "Decrease by X%" modes solve for.
- Compounding (stacked) discounts
- What happens when two or more discounts are applied one after another rather than added together. Each additional discount is calculated on the already-reduced price, so a 20% discount followed by 10% off works out to 28% off overall, not 30%.
Frequently Asked Questions
Side Note — Why "percent" and "percentage points" are different
When a news headline says "interest rates rose by 1%," it's easy to assume they mean a 1% relative increase. But in practice, this often means 1 percentage point — an absolute change from, say, 2% to 3%. That's a 50% relative increase, not 1%. Journalists mix up these terms regularly, which can mislead readers about the true magnitude of changes.
The word "percent" comes from the Latin per centum, meaning "per hundred." The % symbol itself likely evolved from a scribal shorthand for "p. cento" compressed over centuries of handwriting into "pc" → "%" — a typographic fossil from Renaissance commercial manuscripts.
Percentage calculations appear everywhere: store discounts, nutrition labels, election results, financial returns, and scientific measurements. Mastering the three basic operations — finding a percent of a value, finding what percent one number is of another, and calculating percent change — covers the vast majority of real-world needs.