A percentage-of calculator answers the most common everyday percent question: X percent of Y equals (X × Y) ÷ 100. This calculator estimates a percent of a number using the standard percent-of-a-number formula, expressed as the resulting value plus the original value increased by that percent and decreased by that percent. Type the percent, type the number, read three answers at once.
The formula itself is grade-school arithmetic, codified by the National Council of Teachers of Mathematics' percent-as-a-ratio standard (per the NCTM Principles and Standards for School Mathematics, 2000): a percent is just a ratio with a denominator of 100, so 18 percent of $43.50 means (18 × 43.50) ÷ 100, which equals $7.83. Move the decimal two places left on the percent, then multiply.
The plus-and-minus outputs cover the next-step questions. Y plus X percent is the total after a tip, markup, or sales tax: a $43.50 restaurant bill plus a 20 percent tip is $52.20. Y minus X percent is the price after a discount: a $79 sticker minus 30 percent off is $55.30. One subtle pitfall worth flagging: stacked percent changes do not add. A 20 percent increase followed by a 20 percent decrease leaves you at 96 percent of the original, not 100. Each percent operates on its own base, so multiply growth and shrink factors instead of summing the percents when applying several in sequence.
Key takeaway
Percent-of is just multiplication in disguise: X% of Y = X × Y ÷ 100. Move the decimal point two places left on the percent and multiply. Once you internalize that, mental math gets easier, 10% of any number is just shifting the decimal one place left, and most everyday percents (15%, 20%, 25%) can be built from 10% by halving, doubling, or splitting.
How it's calculated
Three formulas for three outputs:
- X% of Y =
value × percent ÷ 100 - Y plus X% =
value × (1 + percent ÷ 100) - Y minus X% =
value × (1 − percent ÷ 100)
The "plus" and "minus" forms are convenient shortcuts for common follow-up questions. They avoid the two-step "find the percent, then add/subtract from the value" calculation: instead, multiply once by a growth factor (1.20 for +20%) or shrink factor (0.70 for −30%).
A common pitfall: stacking percent changes is not additive. A 20% increase followed by a 20% decrease leaves you at 96% of the original, not 100%. Each percent operates on its own base. If you need to apply multiple percents in sequence (markup then discount, tax then tip), multiply the growth/shrink factors together rather than summing the percents.
Source: Percent-of-a-number, result = value × percent ÷ 100
Examples
20% tip on a $43.50 dinner bill
- Percent 20%
- Value 43.5
20% of $43.50 is $8.70, the tip itself. Y plus X% gives you $52.20, the total to write on the receipt or charge to the card. Y minus X% ($34.80) is less useful here, but would be the answer if you were splitting a bill where one diner is contributing 20% less than equal share.
30% off a $79 sticker price
- Percent 30%
- Value 79
30% of $79 is $23.70, the dollar discount you save. Y minus X% gives you $55.30, the actual price you'll pay at the register (before any sales tax). Y plus X% ($102.70) is the answer to "what would this cost if it were 30% more expensive", useful for checking whether a discount is real against a recently-marked-up price.
8.875% sales tax on a $250 purchase
- Percent 8.875%
- Value 250
8.875% of $250 is $22.19 in tax. Y plus X%, the total you'll actually pay, is $272.19. NYC's combined sales tax rate is 8.875%; this is a realistic everyday calculation for anyone shopping there.
Frequently asked questions
What's the difference between "% of" and "% off"?
"X% of Y" is the percent itself, the dollar amount that the percent represents. 30% of $80 is $24. "X% off Y" is the value after subtracting that percent, what you actually pay. 30% off $80 is $56 ($80 − $24). This calculator gives you both: the result output is "% of" (the $24), and the Y minus X% output is "% off" (the $56). Confusing the two is one of the most common everyday percent mistakes, a "30% off" sticker means you pay 70% of the original, not 30%.
How do I calculate this in my head without a calculator?
Anchor on 10%: shift the decimal one place left. 10% of $45 is $4.50. From there, build the percent you want: 20% is 10% doubled ($9). 5% is 10% halved ($2.25). 15% is 10% + 5% ($6.75). 25% is a quarter (divide by 4, $11.25). For weird percents, flip them: 36% of 25 is the same as 25% of 36 (a quarter of 36 = 9). For tax-style percents (7-9%), round to 10% then scale down slightly. Almost every everyday percent question can be decomposed this way faster than reaching for a phone.
What does it mean when a percent is over 100%?
More than 100% of Y means a number larger than Y. 150% of $80 is $120, half again as much. 200% of $80 is $160 (twice the original). This is common in markup calculations, a retailer might mark up wholesale cost by 150% to set a sticker price, and in growth statistics, a startup growing 300% year-over-year ended at 4× its starting size. There's no upper bound. The calculator handles any positive percent input; just enter 150 for 150%, not 1.5.
What about calculating percent **of** something when I know the result?
That's the inverse problem, "the tax was $22.19 on a $250 purchase, what's the rate?", and it's actually the percentage calculator (not this one). Use a separate calculator: divide the part by the whole and multiply by 100. The formula is (part ÷ whole) × 100. So $22.19 ÷ $250 × 100 = 8.876%, confirming an 8.875%-ish sales tax. This calculator goes the other direction: you know the percent and the whole, you want the part.
Can I use negative percents?
Yes, entering a negative percent is mathematically valid and just flips the sign of the result. −20% of $100 is −$20: a decrease of $20. Y plus X% with a negative percent becomes the same as Y minus |X|%, and vice versa. In practice, most people just enter a positive percent and read whichever output (plus or minus) matches their question. Negative percents are useful when chaining calculations or working from a formula that produced a signed percent change.