Percentage Calculator
The three “of” questions in one place: X% of Y, X is what percent of Y, and X is Y% of what — all three answered for the same pair of numbers.
Percentage Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Quantities from zero up — no signed change here.
- No compounding or repeat application.
- No percentage-point conversions (Decrease page).
In short: With 15 and 80: 15% of 80 = 12; 15 is 18.75% of 80; 15 is 80% of 18.75. The mode picks which answer leads; the other two never hide. A 20% tip on $43.50 is $8.70; 30% of $80 is $24.
Formula
fork 1: A% of B = A ÷ 100 × B
fork 2: A is (A ÷ B) × 100 % of B
fork 3: A is B% of A ÷ (B ÷ 100)
[('1% = 0.01', 'the percent sign is a division by 100'), ('÷ B', 'fork 2 measures A against the whole'), ('÷ (B/100)', 'fork 3 reconstructs the base'), ('three forks', 'most percent errors are fork errors')]
Worked Example
- Decide which of the three questions you are really asking.
- Enter A and B; the chosen fork leads.
- Check the other two forks on the second card.
- Tips and taxes add; discounts subtract.
- For change over time, use the Increase or Decrease page.
15 and 80: fork 1 → 12; fork 2 → 18.75%; fork 3 → 18.75. Tip: 20% of 43.50 = 8.70; discount: 30% of 80 = 24.
Strengths & Limits Of This Model
Where this engine is strong
- All three forks answered from one input pair
- Fork errors named as the classic percent mistake
- Zero denominators refused with the fork named
Where it stops
- No stacked percents
- No signed changes
Practical Use Cases
Tips & tax
Fork 1 on the bill.
Scores & shares
Fork 2 against the total.
Pre-discount prices
Fork 3 reconstructs the base.
Sale checks
Is 30% off what it claims?
Classroom drills
All three forks, one input pair.
Methodology & Editorial Standards
Three forks on one pair: multiply (A/100 × B), divide (A/B × 100, B ≠ 0), or reconstruct the base (A ÷ (B/100), B ≠ 0). The chosen mode leads as the hero; the other two print on the comparison card. Negative quantities refused — signed change belongs to the Increase and Decrease pages. Zero denominators refused per fork with the reason named.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Percentage Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I calculate a percentage of a number?
Divide the percent by 100 and multiply: 15% of 80 is 0.15 × 80 = 12. The percent sign is literally an instruction to divide by a hundred.
How do I find what percent one number is of another?
Divide and scale: 15 ÷ 80 = 0.1875, times 100 = 18.75%. The whole you are measuring against sits in the denominator.
How do I find the original number before a percentage was taken?
Un-divide: if 15 is 80% of something, that something is 15 ÷ 0.80 = 18.75. This is the fork that reconstructs a price before a discount or a tax.
Why do my percent answers come out wrong so often?
Almost always the wrong fork: 15% of 80 and 15 is what percent of 80 are different questions with different answers (12 versus 18.75%). Decide the direction first.
How much should I tip at 20 percent?
Multiply the bill by 0.20 — on $43.50 that is $8.70. For a 15–20% band, a fifth to a fifth-and-a-quarter of the bill.
Is a discount the same math as a tip?
The multiplication is identical — 30% of $80 is $24 — but the discount SUBTRACTS from the price while a tip adds to it. Same percent, opposite ledger.
How do I turn a decimal into a percent?
Move the point two places right: 0.075 is 7.5% and 1.5 is 150% — per-cent literally means per hundred. Going back, divide by 100: 7.5% of a number is 0.075 times it.
Where do percentage increases fit?
Not here — change over time (80 up to 100) has its own law with the original value in the denominator. The Percentage Increase and Decrease pages own that question.