How the percentage calculator works
Last updated 10 November 2026
Percentages come up in prices, pay rises, marks and interest. This calculator handles the five sums people use most.
How it works
- X% of Y = X ÷ 100 × Y.
- A is what percent of B = A ÷ B × 100.
- Change from A to B = (B − A) ÷ A × 100.
- A number plus X% = number × (1 + X ÷ 100).
- The number before X% was added = result ÷ (1 + X ÷ 100).
Worked example
With the starting figures:
- 15% of 200 is 30.00.
Which sum to use
The drop-down box at the top picks the question. Match your situation to one of these.
- X% of a number: the discount amount on a price, or a tip on a bill.
- What percentage one number is of another: a test score out of a total, or your share of a cost.
- Percentage change: how much a price, salary or score has gone up or down.
- A number plus X%: a price after a mark-up or a rise.
- The number before X% was added: the original amount, working back from the new one.
Adding and removing a percentage
Adding a percentage and taking it off again do not cancel out. If a price goes up by a given percentage, then comes down by the same percentage, it ends lower than where it started. The second change applies to a bigger number.
To undo an increase, divide by one plus the percentage, as the last option does. Do not take the same percentage off the new number.
Percentage change
Change is measured against the starting number. A fall followed by a rise of the same size does not return to the start, for the same reason as above. The first number must not be zero, because there is no way to measure a change from nothing.
Common mistakes
- Swapping the first and second numbers. The labels change with the sum, so read them.
- Mixing up percent and percentage points, as the note below says.
- Typing the percentage as a decimal, such as a fraction instead of the percentage. Enter the percentage itself, so fifteen percent is entered as fifteen.
- Using percentage change when you want the difference. Change is a share of the start. The difference is a plain amount.
What this does not cover
The calculator does plain arithmetic. It does not know about VAT, interest, discounts that stack or rounding rules on a till slip. Use the dedicated calculators for those.
A short worked idea
In the worked example the answer is 30.00. Each of the five sums is one multiplication or division, so you can check any result with a basic calculator.
The calculator shows up to four decimals, with trailing zeros removed, so a whole answer appears without a decimal part.
Things to watch
- Percentage points and percent are different: going from 10% to 12% is two percentage points, a 20% rise.
For VAT specifically, use the VAT calculator. Nothing you enter is stored or sent.
Like every tool on MoneyMzansi, this one runs entirely in your browser. There is no account, no upload and no sign-up, so it keeps working even if your connection drops once the page has loaded.