Percentage Calculator - Calculate %, Change and Add/Subtract

What this percentage calculator does

Pick the mode that matches your question. Mode one finds a portion of a known total. Mode two reverses that question and reports what share one value is of another. Mode three recovers the whole from a known part and a known rate. Mode four measures the relative change between two values. Mode five applies a percentage as an increase or a decrease to a base value, which is the same math used for sales tax, markups, and most kinds of discounts.

Worked example

Use the five default scenarios as a single connected example. Mode one applies 15% to 200 and returns 30. Mode two confirms the inverse: 30 of 200 equals 15%. Mode three recovers the whole when only the part and rate are known: 30 is 15% of 200. Mode four measures a move from 100 to 150, which is a +50% change with an absolute difference of 50. Mode five adjusts 200 by 15%: adding gives 230, subtracting gives 170. The same percentage produces a different absolute amount in each mode because each formula uses a different reference value.

Choosing the right mode

• What is X% of Y: use this when you know a rate and a total, for example computing 7% sales tax on a 200 invoice or a 20% tip on a 50 dinner.
• X is what % of Y: use this when you know two amounts and want their ratio as a percent, such as 30 questions correct out of 200 on a test.
• X is Y% of what: use this when you know a part and a rate but not the whole, for example reverse-engineering an invoice when 30 of tax represents 15% of the total, so the total is 200.
• Percent change: use this to compare a before and after value: a price moving from 100 to 150 is +50%, while a portfolio dropping from 100 to 80 is -20%.
• Add or subtract a percentage: use this for markups, discounts and taxes inclusive of a known rate, such as adding 21% VAT or subtracting a 15% discount.

How signs and zero values are handled

In percent-change mode the calculator divides by the absolute value of the original number, so a move from -50 to -30 reads as a +40% recovery rather than a misleading negative. When the original value is zero the formula has no meaningful denominator and the calculator reports the result as undefined. The same applies in mode two when the whole is zero: the calculator flags the input rather than dividing by zero.

Adding versus subtracting a percentage

A common mistake is to assume that adding 15% and then subtracting 15% returns the original number. It does not. Adding 15% to 200 gives 230; subtracting 15% from 230 gives 195.5, not 200, because the second 15% is applied to a larger base. The calculator makes this concrete by showing the delta separately from the final result so you can compare the absolute amount added or removed.

Rounding and display precision

Results are shown with up to four decimal places. The calculator does the math in full precision and only rounds at display time, so consecutive calculations should not drift. If you need exact two-decimal money figures, treat the raw value as the math result and apply your own rounding rules for invoicing or quoting.

When this calculator is most useful

• Shopping math: calculate the price after a 15% discount, or the markup on a wholesale price.
• Tax and tip math: add a VAT or sales-tax rate, or compute a tip on a restaurant bill.
• Grades and scores: express a number of correct answers as a percentage of the total.
• Performance change: compare a current value with a baseline to see how much has changed.

Common mistakes to avoid

• Treating percent change as symmetric: a drop from 100 to 50 is -50%, but the return trip from 50 to 100 is +100%.
• Adding a percentage to a base and then subtracting the same percentage from the new total, expecting the original number back.
• Forgetting that mode four (percent change) uses the absolute value of the original, which is important for negative starting values.
• Mixing percentage points with percent change: a move from 10% to 12% is 2 percentage points but a +20% relative change.

What this calculator does not do

• It does not compute compound percentage growth over multiple periods; for that, use a compound interest calculator.
• It does not apply tax rules or rounding conventions specific to a country or currency.
• It is a math tool for everyday percent questions, not a financial-advice tool.

Supporting calculators

• Discount math: use the Discount Calculator to apply percent-off, fixed-amount, or buy-X-get-Y discounts on quantities of items.
• Tax math: use the VAT Calculator to add or remove a VAT or sales-tax rate from a price, with per-unit and total breakdowns.
• Return on investment: use the ROI Calculator to compute total and annualized return on an investment from gain or loss data.