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Using the % key without surprises

Last reviewed on 30 April 2026.

The percent key is one of those buttons that looks simple and behaves differently depending on which calculator you are holding. Type 200 + 10% on a typical office calculator and you will get 220. Type the same keystrokes here and you will get 200.1. Both calculators are working correctly. They are following different conventions, and knowing which convention you have is the difference between a fast estimate and a wrong number.

Two conventions, one symbol

Most modern web and scientific calculators, including this one, treat % as a literal "divide by 100" operator. Wherever you write n%, the calculator substitutes n / 100. 10% becomes 0.1, full stop. So:

200 + 10%   →  200 + 0.1   =  200.1
200 × 10%   →  200 × 0.1   =  20
50 / 5%     →  50 / 0.05   =  1000

Many physical calculators — particularly the ones marketed for office or accounting use — treat % as a context-sensitive operator. 200 + 10% is read as "add 10 percent of the running total", giving 220. The same calculator might compute 200 × 10% as 20 (matching the first convention) but then return something different again for 50 / 5%. The keystrokes look identical; the rules are not.

Neither approach is "the right one". The literal-divide-by-100 reading is consistent and predictable, which is why almost every calculator on a phone or in a browser uses it. The accounting reading saves keystrokes for the most common business calculations. The trouble is that nothing on the keypad tells you which one you have.

How the calculator on this site behaves

The percent key here always means "divide the preceding number by 100". That makes the rules easy to write down:

Worked examples

Tipping at a restaurant

A bill is 78.50 and you want to leave an 18% tip plus the bill total. The clean expression is:

78.50 × (1 + 18%)

The calculator returns 92.63. If you only want the tip itself, type 78.50 × 18% and you get 14.13.

A 25% discount on a 60.00 item

The discounted price is the original times one minus the discount: 60 × (1 − 25%) returns 45. To compute the discount amount alone, 60 × 25% returns 15. To check the work, 45 + 15 equals the original 60.

Sales tax

An item costs 119.99 before tax and the tax rate is 8.25%. The total with tax is 119.99 × (1 + 8.25%), which returns about 129.89. To recover the pre-tax amount from a final price of 129.89, divide by (1 + 8.25%): 129.89 / 1.0825 returns about 119.99.

Markup vs margin

These two phrases sound similar and mean different things. Markup is the percentage added to the cost to set the price; margin is the percentage of the price that is profit.

Percentage change

The change from old value a to new value b is (b − a) / a × 100. From 80 to 100, the change is (100 − 80) / 80 × 100 = 25%. From 100 back to 80, it is (80 − 100) / 100 × 100 = −20%. The asymmetry — up 25%, down 20% — is real and is one of the more common ways an estimate goes off.

Reverse percentage

If a price has been reduced by 20% to give 64, what was the original? Solve x × (1 − 20%) = 64 by dividing: 64 / 0.8 returns 80. The same idea handles "the bill including tip is 92, what was the bill?": 92 / 1.18 returns about 77.97 if the tip was 18%.

Common mistakes

Decision checklist

The percent key is a small convenience. Once you know the convention this calculator follows, it becomes a faster way to type the same thing you would write by hand — and once you know that other calculators may follow a different convention, you stop being caught off guard when the answers disagree.

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