How every number on this site is worked out
There is no data source behind a percentage calculator, and pretending otherwise would be worse than saying so. This page states the arithmetic, the rounding rule, and the limits.
SOURCE·Arithmetic only — no external rate·REVIEWED JUL 2026
15% of 200 is 30 — because 200 × 0.15 = 30.
In short
- Four operations, four formulas, no lookup tables and no external rates.
- Everything is computed in double-precision floating point and rounded once, at the end, to two decimal places.
- Nothing you type is transmitted anywhere. The pages are static files.
How percent of a number is worked out
- Take the percentage P and divide it by 100. That turns it into a plain multiplier — 15% becomes 0.15.
- Multiply the number N by that multiplier. The number after the word “of” is always the one you multiply.
- Round to two decimal places. Nothing is rounded before this point, so the figure shown is the exact arithmetic rounded once at the end.
- To add the percentage on instead, multiply by (1 + P⁄100); to take it off, multiply by (1 − P⁄100).
result = N × (P ÷ 100)
Full method, rounding and worked examples →
SOURCE·Arithmetic only — no external rate·REVIEWED JUL 2026
Percentages as multipliers
Every percentage question reduces to one of these three multipliers. Learn the middle two columns and most percentage arithmetic becomes a single multiplication.
| Percent | Of a number (×) | Added on (×) | Taken off (×) |
|---|---|---|---|
| 1% | 0.01 | 1.01 | 0.99 |
| 2.5% | 0.025 | 1.025 | 0.975 |
| 5% | 0.05 | 1.05 | 0.95 |
| 10% | 0.1 | 1.1 | 0.9 |
| 12.5% | 0.125 | 1.125 | 0.875 |
| 15% | 0.15 | 1.15 | 0.85 |
| 20% | 0.2 | 1.2 | 0.8 |
| 25% | 0.25 | 1.25 | 0.75 |
| 30% | 0.3 | 1.3 | 0.7 |
| 40% | 0.4 | 1.4 | 0.6 |
| 50% | 0.5 | 1.5 | 0.5 |
| 60% | 0.6 | 1.6 | 0.4 |
| 75% | 0.75 | 1.75 | 0.25 |
| 80% | 0.8 | 1.8 | 0.2 |
| 90% | 0.9 | 1.9 | 0.1 |
| 100% | 1 | 2 | 0 |
SOURCE·Arithmetic only — no external rate·REVIEWED JUL 2026
The four formulas in full
percent of result = N × (P ÷ 100)
percent off to pay = N × ((100 − P) ÷ 100)
saving = N × (P ÷ 100)
what percent P = (A ÷ B) × 100
percent change change % = ((new − old) ÷ old) × 100
Rounding
Every figure is rounded once, at the point of display, to two decimal places, using round-half-away-from-zero. No intermediate value is rounded, so chained results do not drift. Where a value is exact — 25% of 200 — no decimals are shown at all.
What this calculator does not do
- It does not apply GST. New Zealand GST is 15%, but extracting GST already inside a price is ÷23×3, not ×0.15 — a legal definition, not a percentage shortcut. Use the GST calculator for that.
- It does not compound. A percentage applied twice is not double the percentage. Successive changes multiply.
- It does not know your currency. Dollar signs on the discount pages are presentational; the arithmetic is unit-free.
Provenance
Arithmetic only — no external rate. There is no rate, table or dataset behind these pages to go stale — the only thing that could change is the arithmetic, and it will not. The review date below records when the formulas and the worked examples were last checked by hand.
SOURCE·Arithmetic only — no external rate·REVIEWED JUL 2026
The four percentage calculations, and when each one applies
Every percentage question you will ever meet is one of four shapes. Naming the shape first is what stops the classic mistakes — and it is why this site keeps the four apart instead of putting one box on the page and hoping.
P% of N. Tips, commission, deposits, interest. 15% of $200 → $30.
P% off N. Sales and discounts — compute what you pay, not what you save. 30% off $80 → $56.
A is what % of B. Test scores, market share, progress. 42 of 60 → 70%.
% change. Price rises, growth, drops. 80 → 100 is +25%; 100 → 80 is −20%.
The base is everything
Nearly every percentage error is a wrong base — dividing by the new value instead of the old one, or taking a percentage of the discounted price instead of the original. The rule that fixes all of them: the base is the number the percentage is “of”, which in change problems is always the value you started from.
It is also why percentages do not add. A 20% rise followed by another 20% rise is not 40% — it is 44%, because the second rise acts on a bigger base (1.2 × 1.2 = 1.44). Compounding is just this effect repeated.
Mental shortcuts that survive real life
- 10% is one decimal shift. 10% of $87.50 is $8.75. Nearly everything builds from this: 5% is half of it, 20% is double, 15% is one-and-a-half.
- x% of y equals y% of x. 8% of 25 feels hard; 25% of 8 is obviously 2. Same number.
- For discounts, think “what fraction do I pay”. 25% off means you pay ¾. A $59.99 jacket at 25% off is about $45 before you reach for a phone.
Percentage questions, answered
How do I work out a percentage of a number?
How do I take a percentage off a price?
How do I work out what percent one number is of another?
How is percentage change calculated?
Why is a 50% increase not cancelled by a 50% decrease?
What is a percentage point?
Does the calculator store what I type?
Where a percentage usually goes next
Ranked by what people actually reach for after this one — not alphabetically. Each line says what the next tool adds that this one cannot.
GST is a percentage with a legal definition. This one does it to IRD's rules.
JUL 2026 Markup calculatorMargin and markup are not the same number — this one keeps them apart.
JUL 2026 Pay converterSalary to hourly and back, on real working-year hours.
JUL 2026 Invoice generatorA GST-compliant tax invoice with your details on it, as a PDF, with nothing to sign up for.
JUL 2026 Rate calculatorWhat you have to charge per hour once holidays, downtime and overheads are counted.
JUL 2026 Receipt makerProof of payment with the GST split out the way IRD expects to see it.
JUL 2026 Day counterDays between two dates, working days only, and every NZ public holiday.
JUL 2026See all 10 free tools in one list →
These tools are free and always will be. Built by Amit Sharma — an Auckland real-estate agent and marketer followed by 214K across YouTube, Facebook, Instagram and TikTok.
