Question 1

How do I find what percentage one number is of another?

Accepted Answer

Divide the part by the whole and multiply by 100. For example, if you scored 45 out of 60 on a test, that's (45 ÷ 60) × 100 = 75%. This works for any situation where you're comparing a smaller number to a larger one, test scores, completion rates, you name it.

Question 2

What's the easiest way to calculate a tip?

Accepted Answer

For 10%, just move the decimal point one place left. A $47.50 bill has a $4.75 tip at 10%. Double that for 20% ($9.50). For 15%, find 10% and add half of it, $4.75 + $2.38 = $7.13. Our calculator does this math instantly, but knowing the trick helps when you don't have your phone handy.

Question 3

How do I calculate a discount or sale price?

Accepted Answer

Take the original price and multiply by (100 - discount) / 100. A shirt marked $80 with 25% off: $80 × 0.75 = $60. Or do it in two steps: find 25% of $80 ($20) and subtract it. Either way, you're paying $60. Our add/subtract calculator shows both the discounted price and how much you're saving.

Question 4

Why does percentage change give different results depending on direction?

Accepted Answer

Because you're dividing by different starting numbers. Going from 100 to 125 is a 25% increase (25 ÷ 100). But going from 125 back to 100 is only a 20% decrease (25 ÷ 125). This trips people up all the time. A 50% drop followed by a 50% gain doesn't get you back to where you started, you end up 25% lower.

Question 5

What happens if I try to calculate percentage with zero as the base?

Accepted Answer

You can't divide by zero, it's mathematically undefined. If you're asking what percent 50 is of 0, there's no meaningful answer because you can't have parts of nothing. Our calculator shows 'Undefined' in these cases instead of giving you a misleading number.

Question 6

How do I calculate percentage increase or decrease?

Accepted Answer

Subtract the old value from the new value, divide by the old value, and multiply by 100. If your rent went from $1,200 to $1,350, that's ($1,350 - $1,200) ÷ $1,200 × 100 = 12.5% increase. For decreases, you'll get a negative number, a drop from $1,350 to $1,200 is -11.1%.

Question 7

What's the formula for finding X percent of a number?

Accepted Answer

Multiply the number by X divided by 100. So 15% of 240 is 240 × (15 ÷ 100) = 240 × 0.15 = 36. You can also think of it as moving the decimal two places left on the percentage, then multiplying. 15% becomes 0.15.

Question 8

How do I add tax to a price?

Accepted Answer

Multiply the price by (1 + tax rate / 100). If sales tax is 8.25%, a $50 item costs $50 × 1.0825 = $54.13 after tax. This is the same as finding 8.25% of $50 ($4.13) and adding it. Our calculator shows both the final price and the tax amount.

Question 9

What's the difference between percent and percentage points?

Accepted Answer

Percent is relative, percentage points are absolute. If a rate goes from 5% to 10%, it increased by 5 percentage points but doubled (100% increase). News headlines often mix these up. 'Unemployment rose 2%' and 'Unemployment rose 2 percentage points' mean very different things if unemployment was at 4%.

Question 10

How do I figure out the original price before a discount?

Accepted Answer

Divide the sale price by (100 - discount) / 100. If something costs $60 after a 25% discount, the original was $60 ÷ 0.75 = $80. You can check: 25% of $80 is $20, and $80 - $20 = $60. This works for figuring out pre-tax prices too.

Question 11

Can I calculate compound percentages?

Accepted Answer

Yes, but you need to apply them one at a time. A 10% increase followed by a 20% increase isn't 30%, it's 32%. Start with 100, add 10% to get 110, then add 20% of 110 (22) to get 132. That's why compound interest grows faster than simple interest.

Question 12

How accurate is this calculator?

Accepted Answer

Very accurate, it uses standard floating-point math and displays up to 10 decimal places when needed. For everyday calculations like tips and discounts, you'll get the same results as any financial calculator. Just remember that real-world prices get rounded to the nearest cent.

Free Percentage Calculator

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