How to use it
- Pick what you want to work out, for example "% increase" or "Price after discount".
- Fill in the two values. The labels change to match the calculation you picked.
- The answer appears straight away, with the working underneath. Use Copy result to paste it elsewhere.
The formula
X% of Y = X ÷ 100 × Y · % change = (New − Old) ÷ Old × 100
Every mode is a rearrangement of the same relationship: part = percentage ÷ 100 × whole. The working panel shows the exact formula for the mode you choose.
Worked example
A salary rises from 1,200 to 1,380 a month.
Increase = (1,380 − 1,200) ÷ 1,200 × 100 = 15%.
If it later falls back to 1,200, the decrease is (1,380 − 1,200) ÷ 1,380 × 100 = 13.04% — not 15%, because the starting value is different.
Assumptions
- Percentage change is measured against the original (first) value.
- Percentage difference compares two values symmetrically, against their average.
- Negative values are not accepted, because most money calculations do not use them.
Common mistakes
- Assuming a 20% rise followed by a 20% fall brings you back to the start. It leaves you 4% lower.
- Dividing by the new value instead of the original when calculating change.
- Adding percentages from different bases, such as two discounts, instead of applying them one after the other.
Questions
What is the difference between percentage change and percentage difference?
Change has a direction — from an old value to a new one — and is divided by the old value. Difference has no direction and is divided by the average of the two values.
Why can’t the original value be zero?
A change from zero has no defined percentage, because the formula would divide by zero. The calculator tells you when this happens instead of showing a misleading number.
How do I add VAT to a price?
Choose "Price after increase" and enter the VAT rate, for example 15 or 16.
Estimates only. Your actual figures depend on the provider’s terms.