How to Use This Calculator
- 1
Choose the question you're asking
The three tabs cover the three classic setups: taking a percent of a number, expressing one number as a percent of another, and measuring the change from an old value to a new one.
- 2
Fill in the two numbers
Each mode needs exactly two inputs. Order matters in two of them — X is what % of Y flips if you swap the values, and percent change always measures from the starting value.
- 3
Read the sentence, not just the number
Below the big result, the answer is restated in words — for instance, 15% of 80 is 12 — so you can confirm the calculator understood the question the same way you did.
- 4
Switch modes to sanity-check
The modes are inverses of each other. If 15% of 80 is 12, then asking what percent 12 is of 80 should return 15% — a quick way to verify a result you don't quite trust.
How It Works
All three modes reduce to one idea: a percent is just a fraction with 100 as the denominator. Each question rearranges that relationship differently:
X as a % of Y = (X ÷ Y) × 100
% change = ((new − old) ÷ old) × 100
The first multiplies, the second divides, and the third compares a difference against the starting point. Percent change is the one people trip over most — the denominator is always the oldvalue, never the new one, which is why a rise and a fall between the same two numbers don't produce matching percentages.
Worked example
Percent of a number: 15% of 80 is (15 ÷ 100) × 80 = 12. Handy for tips, taxes, and commissions.
X is what percent of Y: 30 out of 120 is (30 ÷ 120) × 100 = 25%. This is the test-score and market-share question.
Percent change: going from 50 to 65 is ((65 − 50) ÷ 50) × 100 = a 30% increase. Going back down from 65 to 50 would only be a 23.08% decrease — same numbers, different base.
Frequently Asked Questions
How do I calculate a percentage of a number by hand?
Move the decimal point in the percent two places left, then multiply. So 15% of 80 becomes 0.15 × 80 = 12. For mental math, break it into easy pieces: 10% of 80 is 8, and 5% is half of that (4), so 15% is 8 + 4 = 12. This chunking trick handles most everyday percentages without a calculator.
How do I find the percent increase between two numbers?
Subtract the old value from the new one, divide by the old value, and multiply by 100. Going from 50 to 65: (65 − 50) ÷ 50 = 0.30, so a 30% increase. The most common mistake is dividing by the new number — that would give 23.08%, which is the percent decrease going the other direction.
What's the difference between percentage points and percent?
Percentage points measure the simple gap between two percentages; percent measures relative change. If a savings rate climbs from 4% to 5%, it rose 1 percentage point — but that's a 25% increase, because 1 is a quarter of 4. News headlines mix these up constantly, and the difference can make a small move sound dramatic or a big one sound trivial.
How do I reverse a percentage to find the original number?
Divide by the multiplier instead of subtracting the percent. If a $86.40 total already includes 8% sales tax, the pre-tax price is 86.40 ÷ 1.08 = $80 — not 86.40 minus 8%, which gives the wrong answer ($79.49). The same logic un-does discounts: an item that cost $70 after 30% off originally sold for 70 ÷ 0.70 = $100.
How do I calculate my grade as a percentage?
Divide points earned by points possible and multiply by 100 — the second mode of this calculator. Scoring 30 out of 120 on an exam is (30 ÷ 120) × 100 = 25%, while 51 out of 60 is 85%. For a weighted course grade, convert each category to a percent first, multiply by its weight, and add the pieces together.
Why does percent change look huge when the starting number is small?
Because the starting value is the denominator, a tiny base inflates the result. Growing from 2 users to 10 is a 400% increase, while growing from 1,000 to 1,400 — four hundred times more new users — is only 40%. Whenever a claim touts a triple-digit percentage gain, ask what the starting number was before being impressed.