Percentage Calculator
The three questions people actually ask about percentages, answered at once, including the reverse ones.
Results
What this tool does
Percentage questions come in three shapes and people mix them up constantly. What is fifteen per cent of 250? 250 is what per cent of 800? What is 250 after adding fifteen per cent — and what was it before? This page answers all of them from the same three boxes, including the reverse calculations that catch nearly everyone out: taking twenty per cent off 120 gives 96, but adding twenty per cent back gives 115.20, not 120, because the two percentages are of different numbers.
Formula
v × p ÷ 100 · v ÷ w × 100 · v × (1 ± p ÷ 100) · v ÷ (1 ± p ÷ 100)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
v | The value | — |
p | The percentage | % |
w | Compared against | — |
r | That percentage of the value | — |
IS | The value as a percentage of the other | % |
UP | The value after adding it | — |
DN | The value after taking it off | — |
DF | How far the value is from the other | % |
BK | What it was before that was added | — |
BD | What it was before that was taken off | — |
WH | The whole, if the value is that percentage of it | — |
Worked example
- The value250
- The percentage15 %
- Compared against800
- That percentage of the value37.500000
- The value as a percentage of the other31.250000 %
- The value after adding it287.500000
- The value after taking it off212.500000
- How far the value is from the other-68.750000 %
- What it was before that was added217.391304
- What it was before that was taken off294.117647
- The whole, if the value is that percentage of it1666.666667
Limitations
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.
Frequently asked questions
How do I calculate a percentage in my head?
Find 10 % by moving the decimal point one place left, then build from there. For 25 % of 200: 10 % is 20, so 20 % is 40, and 5 % is 10, giving 50. Percentages are also reversible, which is often quicker: 8 % of 50 is the same as 50 % of 8, which is 4.