What is a percentage calculator?
A percentage calculator turns everyday ratio questions into clear numbers. You might need to know what share one value is of another, find a percentage of a base amount, apply a percent increase or decrease, or measure how much a value changed from an old number to a new one. Each of those problems uses a different formula, and mixing them up is one of the most common sources of spreadsheet and checkout errors.
The percentage calculator on Yaya Tools covers all four scenarios on one page. Enter your numbers and the result updates instantly. There is no sign-up, no export step, and no server upload—inputs and calculations stay in your current browser session. That makes it practical for quick checks at work, in class, or while comparing prices on your phone.
Percentages appear everywhere: test scores, sales tax, tips, discounts, growth reports, and investment returns. A dedicated percent change calculator saves time because you do not have to rewrite formulas or hunt for the right cell reference each time the question changes. Whether you need to calculate percentage share, compute a percent increase calculator result after a raise, or compare two quarterly figures, the four modes on this page map directly to the math you would write by hand.
How to use this percentage calculator
The tool is organized into four cards, each matching one type of percentage problem. Follow the card that fits your question.
- Open the percentage calculator in your browser.
- Choose the card that matches your question:
- What percent is one number of another? — Enter the part and the total.
- What is a percent of a number? — Enter the rate (%) and the base.
- Increase or decrease by a percent — Enter the original value, the change rate, and select increase or decrease.
- Percent change from A to B — Enter the starting value and the ending value.
- Type your numbers. Results refresh as you type; there is no separate Calculate button.
- If you see an error message, fix the highlighted input (for example, a zero total or a negative change rate).
- Click Reset examples to restore the sample values if you want a clean starting point.
Each card shows the result in place: a percentage for ratio and change-rate questions, or a plain number for “percent of” and increase/decrease questions. You can copy the displayed value into a spreadsheet, message, or report. For shopping scenarios that combine a list price, coupon, and tax, the discount calculator applies a full checkout sequence; this page handles the underlying percentage steps on their own.
Percentage calculator formulas
All four modes use standard arithmetic. The notation below matches what the tool implements.
What percent is the part of the total?
When you want to know what share one value represents of a whole:
Percentage = (part ÷ total) × 100
Example: 25 is what percent of 200?
(25 ÷ 200) × 100 = 12.5%
The total must not be zero. Division by zero is undefined, so the calculator asks for a non-zero total.
What is a percent of a number?
When you know the rate and the base:
Result = base × rate ÷ 100
Example: What is 15% of 80?
80 × 15 ÷ 100 = 12
This is the same structure used for sales tax lines, commission rates, and many fee calculations. The discount calculator builds on this idea for sale prices and coupons; here you get the raw proportion in one step.
Increase or decrease by a percent
When an original value grows or shrinks by a given rate:
New value = original × (1 + rate ÷ 100) [increase]
New value = original × (1 − rate ÷ 100) [decrease]
Example: Increase 100 by 20%:
100 × (1 + 20 ÷ 100) = 120
Example: Decrease 100 by 20%:
100 × (1 − 20 ÷ 100) = 80
The change rate must be zero or positive; choose decrease when you want to subtract that share from the original. This mode answers “what is the new amount after a percent raise or markdown?” It does not, by itself, tell you the percent difference between two arbitrary numbers—that is mode four.
Percent change from A to B
When you compare an old value to a new value:
Percent change = ((new − old) ÷ old) × 100
Example: From 80 to 100:
((100 − 80) ÷ 80) × 100 = 25%
A positive result means an increase; a negative result means a decrease. The starting value (old) must not be zero. This is the formula behind year-over-year growth, price hikes, and score improvements. For compound growth over many periods with contributions, use the compound interest calculator after you have converted rates or proportions here.
Worked examples
These examples walk through each mode with numbers you can verify in the tool.
Example 1: Test score as a percentage
You answered 42 questions correctly out of 50.
(42 ÷ 50) × 100 = 84%
You scored 84%. If the passing threshold is 70%, you cleared it by 14 percentage points—not by “14 percent more than 70%,” which would be a different calculation (see the FAQ on percentage points).
Example 2: Finding a tip before tax
Your pre-tax bill is $64 and you want to estimate 18% for mental math before opening a full tip split.
64 × 18 ÷ 100 = 11.52
Eighteen percent of $64 is $11.52. For a full restaurant bill with tax, tip rate, and splitting among people, the tip calculator applies tax first and then tip on the after-tax subtotal, which matches common U.S. checkout practice.
Example 3: Sale price after a percent markdown
A jacket costs $89.99 and is marked down 30%.
Select decrease, enter 89.99 as the original value and 30 as the change rate:
89.99 × (1 − 30 ÷ 100) = 62.993 → about $63.00
The reduced price is roughly $63 before coupons or tax. To stack a percentage off, a fixed coupon, quantity, and tax in one total, switch to the discount calculator.
Example 4: Measuring revenue growth
Last month’s revenue was $12,400; this month it is $13,640.
((13,640 − 12,400) ÷ 12,400) × 100 ≈ 10%
Revenue rose about 10%. If next month drops from $13,640 to $12,276, the percent change is negative:
((12,276 − 13,640) ÷ 13,640) × 100 ≈ −10%
Using the new month as the denominator would answer a different question. Always divide by the original (starting) value for standard percent change.
Example 5: Salary increase
An hourly rate rises from $22.00 to $24.20.
((24.20 − 22.00) ÷ 22.00) × 100 = 10%
That is a 10% raise. To find the new rate directly from a 10% increase instead, use the increase/decrease card with original 22 and rate 10.
Common mistakes when calculating percentages
Using the wrong denominator. Percent change always divides by the starting value. Growth from 50 to 75 is 50%, not 33.3% (which would use 75 as the denominator).
Confusing “percent off” with “percent of.” Twenty percent off $100 leaves $80. Twenty percent of $100 is $20. Discount labels refer to the removed share; the discount calculator expects the off amount in its discount field.
Applying two decreases as if they add. Cutting a price by 20% and then another 20% does not remove 40% of the original. The second cut applies to the already reduced price: $100 → $80 → $64, which is a 36% total reduction from the start.
Treating percentage points like percent change. Interest rates moving from 4% to 5% is a one percentage point increase, but relative to 4% that is a 25% increase in the rate itself. The FAQ below expands on this distinction.
Rounding too early. The calculator keeps full precision internally and formats results for display. If you round intermediate steps in a spreadsheet, your final figure can drift by a cent or a tenth of a percent. Recalculate from the same inputs when auditing.
Ignoring sign and direction. A drop from 200 to 150 is −25% change. For increase/decrease mode, pick decrease explicitly when you mean a markdown; entering a negative rate is not allowed because the tool separates direction with a radio control.
Expecting this page to compound over many periods. A single percent change or increase/decrease step is linear. Multi-year investment growth with periodic deposits belongs in the compound interest calculator, not in a one-step percent change card.
References
- Percent (NIST Guide to the SI) — SI guidance on expressing ratios and the percent symbol.
- Mathematics of percent change (OpenStax) — Standard percent application formulas used in introductory algebra.