Percentage Calculator

What does this percentage calculator do?

A percentage expresses a part as a share of a whole split into 100 pieces: 15% means 15 out of every 100. This percentage calculator covers the three questions that make up almost every everyday percentage problem, and it gives each one its own mode:

• % of a number: what is 15% of $2,450?
• What percent?: 36 correct answers out of 45, what score is that?
• Percent change: rent went from $1,250 to $1,500, how big is the increase?

The tool does more than print one number. You also see the working, a plain sentence that reads the answer back, and four extra rows. For example, the first mode shows the number with the percentage added and taken off. The change mode shows the percentage you would need to get back to the old value.

I use this math most in ad reports. Typical questions: which share of the budget went to each campaign? How far did the conversion rate move against last month? How large is the price increase in a quote? Everything runs in your browser, so the values you type never reach a server.

How do you work out percentages in your head?

When the tool is not at hand, you can still make a solid estimate. The trick is to split a hard percentage into easy pieces:

• 10%: divide by 10; 10% of 2,450 is 245.
• 5%: take half of 10%; for 2,450 that is 122.5.
• 1%: divide by 100; 1% of 2,450 is 24.5.
• 15%: add 10% and 5%: 245 + 122.5 = 367.5.
• 20%: double 10%, a handy shortcut for tips in the US.

There is also a swap shortcut: a% of b equals b% of a, because both come down to a × b ÷ 100. So if 8% of 50 feels awkward, work out 50% of 8 instead; both give 4. The Swap button in the tool shows this in one tap.

That said, keep mental math for sanity checks. I never paste a head calculation straight into a quote, an invoice or a report. Instead, I use it to ask "does this number look right?" and then confirm the final figure with the calculator.

How should you report a change in a rate that is already a percentage?

Conversion rate, click-through rate, interest and inflation are already percentages. When one of them moves, two different figures appear, and both are correct. The Eurostat glossary reserves the term percentage point for comparing two percentages.

Worked example: if a conversion rate rises from 2% to 3%, the gap is 1 percentage point. The same move is a 50% increase as a percent change. The first figure sounds modest and the second sounds impressive, so your choice shapes how readers judge the result.

In my reports I follow a simple rule:

• State the gap in percentage points first and add the percent change in brackets: "1 point (+50%)".
• Give absolute numbers for small bases. Going from 2 sales to 4 is a 100% increase; still, on its own it proves very little.
• Name the periods you compare: last month, or the same month last year?

In 'Percent change' mode, enter the old and new rate; the "Difference (new − old)" row gives the percentage points and the large figure gives the percent change. I explain which metrics belong in a report in my article on digital marketing KPIs.

Why don't successive percentages add up?

Each new percentage applies to the result of the previous step, so percentages multiply instead of adding. Forgetting that the base moves is one of the most expensive percentage mistakes I see.

Worked example: a $1,000 price that rises 20% and then falls 20% does not return to $1,000. First it becomes $1,200; then 20% of 1,200, which is $240, comes off, and the price ends at $960. The net effect is a 4% decrease.

Stacked discounts follow the same logic:

• 20% off plus an extra 10% off is not 30%; 1,000 × 0.80 × 0.90 = 720, so the total discount is 28%.
• Three yearly raises of 10% add up to 33.1%, not 30%, because 1.1 × 1.1 × 1.1 = 1.331.
• Going from 100 to 150 is a 50% increase; however, going back from 150 to 100 is only a 33.33% decrease.

You can handle this in the tool in two ways. To go step by step, use the minus row in '% of a number' mode. For example, type the 800 from the "1,000 − 20%" row as the next number. To see the overall effect, enter the first and last value in 'Percent change' mode. For quick promo pricing, the discount calculator also does the job.

How do you find the original price before a markup, tax or discount?

Intuition often fails here. Worked example: if a price tag of $1,440 includes a 20% markup, taking 20% off 1,440 gives $1,152, which is wrong. The correct answer is 1,440 ÷ 1.20 = $1,200, because the 20% sat on top of 1,200, not 1,440.

The tool handles this without extra steps:

1. Choose '% of a number' mode.
2. Enter the final price in the first field and the added percentage in the second.
3. Read the "Before the 20% change" row.

To find the list price behind a discounted price, enter the percentage with a minus sign. Worked example: if a price after 25% off is $750, enter 750 and −25; the "Before the −25% change" row then shows $1,000.

Sales tax and VAT work the same way, but for tax amounts, preset rates and invoice wording the VAT calculator is the better fit. If you want list prices, discounts and taxes planned in one consistent pricing table for an online store, take a look at the e-commerce consulting service my team and I offer.

Using a percentage calculator for marketing metrics: CTR, conversion rate and budget share

Most metrics I track in digital marketing are one number divided by another. All of them fit the 'What percent?' mode; you only need to pick the right number as the total:

• Click-through rate (CTR): clicks ÷ impressions × 100. Worked example: 540 clicks from 18,000 impressions is 3%.
• Conversion rate: conversions ÷ clicks × 100. In the same example, 27 conversions means 5%.
• Budget share: campaign spend ÷ total spend × 100.

For month over month comparisons, switch to 'Percent change'. The same formula sits behind official price statistics; the US Bureau of Labor Statistics computes CPI changes exactly this way. In practice, I never compare a holiday or sale month with an ordinary one, otherwise the change measures the calendar rather than performance.

Percentages alone do not show profit. To see how far revenue covers ad spend, use the ROAS calculator. For reading these figures together, see my guide on how to read a digital marketing report. To see how my team and I run campaigns on these metrics, visit the Google Ads management page.

Should you write 25% or 25 %?

In everyday English the percent sign follows the number with no space: 25%. That is also how this calculator shows results on the English page. Scientific and technical writing sometimes follows a different rule: the SI Brochure from the BIPM says a space separates the number and the symbol, as in 25 %.

Other languages have their own habits:

• German: a space between number and sign, 25 %, with a decimal comma.
• Turkish: the sign comes first with no space, %25, and decimals also take a comma.

Separators change too. In English you write 1,234.56; in German or Turkish the same number is 1.234,56. On this page the calculator reads a comma followed by exactly three digits as a thousands separator and a dot as the decimal point. So 10,000 means ten thousand and 1.5 means one and a half. A lone comma with one or two digits after it, such as 1,5, still counts as a decimal. Pick one style per document and stay with it; mixed formats cause more errors than any formula.