Three different percent questions
Read the question before you pick the mode. The same 20 can mean a share, a change, or the original price. Decide first whether you are asking for a share of a number, a change from an earlier number, or the original price before a discount. The same 20 gives a different answer in each mode. The sample amounts on the page are not a quote from a bank, a card issuer, or an employer. Replace them with the figures on your statement and run the page twice to see which field moves the result.
A share of a number
20% of 80 is 16. That mode does not ask what the number used to be.
Tips and commissions are usually this question.
20% of 80 is 16. That question does not ask for an earlier price. Tips, fees, and a simple tax share use this mode. The discount and sales-tax pages add fields when the story is a price.
A change
From 80 to 100 is a 25% increase, not 20. The base is the starting number.
A drop from 100 to 80 is a 20% decrease. The two directions are not mirrors unless you say which number is the base.
From 80 to 100 is a 25% increase, not 20%. The base is the starting 80. From 100 to 80 is a 20% decrease. The two directions are not mirrors until you say which number is the base.
Working backward
If 80 is the price after 20% off, the original is 100, not 96.
The reverse mode on the percentage page, and the extract mode on the sales-tax page, answer that kind of question.
If 80 is the price after 20% off, the original is 100, not 96. Divide 80 by 0.8. The reverse mode on the percentage page, and extract mode on the sales-tax page, answer that kind of question.
Related tools
These figures are estimates you can recompute from the formula on the page. They are not a lender quote, a tax bill, a payroll statement, or financial advice.