Percentage Calculator

Three percentage questions, one calculator

Almost every real-world percentage problem is one of three questions, and this calculator handles each separately so you never have to remember the formula:

• What is X% of Y? — the everyday one. What is 18% of a $54 bill? (Answer: $9.72.) Use it for tips, tax, and discounts.
• X is what percent of Y? — turns two numbers into a percentage. You scored 43 out of 50 — what percent is that? (86%.) Use it for grades and shares of a total.
• Percentage increase or decrease — how much something changed. A price went from $80 to $100 — that is a 25% increase. From $100 to $80 is a 20% decrease (note the two are not the same).

The mistake almost everyone makes

Percentage increase and decrease are not symmetric, and it trips people up constantly. If a $100 item drops 20% to $80, adding 20% back does not return you to $100 — it gives $96, because the 20% is now calculated on the smaller $80. Whenever a value goes down and then up (or vice versa), calculate each step from its own starting point rather than assuming the percentages cancel.

Everyday situations it covers

• Working out a tip or splitting tax on a restaurant bill.
• Checking a sale price — is "30% off $120" really worth it? ($84.)
• Converting a test score into a percentage grade.
• Measuring growth — followers, revenue, or weight change month over month.

Frequently asked questions

How do I add tax or a tip quickly?
Use "What is X% of Y" to find the amount, then add it to the original — or simply calculate the total directly (a 15% tip on $40 is $6, so $46 total).

Why is a 50% increase then a 50% decrease not back to the start?
Because each percentage is taken from a different base. 100 up 50% is 150; 150 down 50% is 75, not 100. The decrease applies to the larger number.

Is the calculation done privately?
Yes — everything is computed in your browser, with no numbers sent anywhere.