How to Calculate Percentage Increase and Decrease: Formula and Inputs

Finding the adjustment before changing the value

The entered percentage is converted into an amount by multiplying it by the starting value. That amount is added for the increased result and subtracted for the decreased result.

The spread is the distance between those two results, equal to twice the change amount. Negative starting values or percentages follow the same arithmetic and may reverse the intuitive direction.

One percentage amount, added or subtracted

Two directions around one starting number

The default example calculates one percentage amount and uses it to show both the increased and decreased result around the same starting value.

Equal amount does not mean reversible percentage

The two adjusted values sit equally far from the original in absolute terms. Returning from the decreased value to the original requires a different percentage because the base has changed.

The changed-base trap

A 20% decrease followed by a 20% increase does not return to the start. The second operation applies to the smaller new base.

• The entered percentage is applied once to the original value.
• Increase and decrease outputs use the same starting base.
• No minimum, maximum or rounding rule is imposed beyond display formatting.

Calculations related to percentage increase and decrease

The following tools examine neighbouring parts of the same calculation without changing the inputs or assumptions used above.