Percentage change compares a difference with its original value. The starting value is the reference point.
The underlying idea
Signed change = (new − original) ÷ original × 100. A positive sign indicates an increase; a negative sign indicates a decrease. A zero original value makes this formula undefined.
Use the original as denominator
Signed change = (new − original) ÷ original × 100. A positive sign indicates an increase; a negative sign indicates a decrease. A zero original value makes this formula undefined. Choose measurements that describe the same object, system, or period; changing the reference changes what the result means.
Worked example
Going from 80 to 100 is a 25% increase. Going back from 100 to 80 is a 20% decrease. The absolute change is 20 in both cases.
Put the method into practice
- Write down the quantity you need to find.
- Identify the measurements and their units.
- Convert units before doing the calculation.
- Apply the formula and keep unrounded intermediate values.
- Check whether the result is reasonable for your situation.
Try the Percentage Change Calculator with your own values.
Common mistakes
Mixing units, using the wrong reference value, and rounding too early can all change the result. A calculator cannot repair inaccurate measurements. For project estimates, distinguish the calculated base quantity from allowances and supplier rounding.
What to learn next
Explore this topic’s related answers below for focused examples and alternative ways to check a result.
