Free online tool
Percentage Calculator
Three quick calculators to cover the most common percentage questions.
Percentage basics
A percentage is a way of expressing a number as a fraction of 100. It's used everywhere from taxes to tip jars, sale prices to interest rates. The three most common questions people ask are: "what is X% of a value?", "one number is what percent of another?", and "how much did something change in percent terms?" This tool covers all three.
Formulas used
Percent of a number: value × (percent ÷ 100). Percent one number is of another: (part ÷ whole) × 100. Percent change: ((new − old) ÷ old) × 100. A negative percent change is a decrease.
Real-world uses
Figuring out a 15% tip on a $60 bill, checking that a 20% off sale really beats a coupon, comparing year-over-year revenue growth, calculating grade weights, or converting a survey response into a share of the total.
+Can I use decimals?
Yes — every input accepts decimal values.
+Does % change go above 100?
Yes. Doubling is +100%, tripling is +200%, and so on.
Percentages, and why up and down aren't mirror images
A percentage is just a fraction scaled to 100, but the arithmetic around increases and decreases trips people up constantly because raising a number by X% and then lowering it by X% does not bring you back to where you started.
Percentages show up everywhere from grocery discounts to mortgage rates, and most of the confusion comes from not being clear about which number is the base.
Increase then decrease is not a round trip
Take 200 and increase it by 50%: you get 300. Now decrease 300 by 50%: you get 150, not 200. That's because the second percentage is calculated on the new, larger base, not the original one. This asymmetry is exactly why 'prices went up 20% then down 20%' still leaves you paying more than before.
Three different questions, three formulas
'What is 15% of 80' is 0.15 x 80 = 12. 'What percent is 12 of 80' is (12/80) x 100 = 15%. 'What number, increased by 15%, equals 92' requires dividing: 92 / 1.15 = 80. These look similar but use the value as a multiplier, a ratio, or a divisor respectively, so plugging numbers into the wrong slot gives a wrong answer that still looks plausible.
A common trap
When comparing two percentages, like a rate moving from 4% to 6%, that's a 2 percentage-point rise but a 50% relative increase (2/4). Mixing up 'percentage points' with 'percent change' is one of the most common reporting errors in news and finance, so state which one you mean.
Stacked discounts don't simply add
A store advertising '30% off, then an extra 20% off' isn't giving you 50% off total. The second discount applies to the already-reduced price: $100 becomes $70 after the first cut, then $56 after the second (20% of 70 is 14). The combined effect is 44% off, noticeably less generous than adding the two percentages together, which is a distinction worth checking before assuming a deal is bigger than it is.
Where this shows up day to day
Tip calculations, sales tax, salary raises expressed as a percentage of current pay, interest accrued on a loan balance, and body-fat or grade-point figures all rely on exactly this arithmetic. Restaurant tipping is the simplest case — 18% of a $64 bill is 0.18 x 64 = $11.52 — while a raise 'from $55,000 to $60,500' is a 10% increase because (60500-55000)/55000 = 0.10.
Percentages in interest and growth calculations
Compound interest relies on repeated percentage growth applied to an ever-changing base, which is exactly the same mechanic as the stacked-discount example above but running in the opposite direction: a savings balance earning 5% annually grows by more dollars each year even at a constant rate, because 5% of a larger balance is a larger amount. This is also why credit card debt at 20% APR balloons faster than most people expect if only minimum payments are made.
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