How to Calculate Percentage Change

Step 01

Choose the starting value before calculating.

The old value is the baseline against which the difference is measured. Swapping the old and new values changes the denominator, so it answers a different question rather than merely changing the sign.

Record the dates, periods or scenarios represented by both values. A percentage can be arithmetically correct but misleading when the two figures cover different durations or definitions.

Step 02

Apply the percentage-change formula.

Subtract the old value from the new value, divide by the absolute old value and multiply by 100. A positive result is an increase and a negative result is a decrease when the baseline is positive.

• Absolute change = new value − old value.
• Percentage change = absolute change ÷ |old value| × 100.
• Direction comes from the sign of the difference.
• State the baseline whenever the comparison could be ambiguous.

Step 03

Worked example: 80 to 100 and back again.

Moving from 80 to 100 adds 20. Relative to the starting value of 80, the change is 20 ÷ 80 = 25%, so this is a 25% increase.

Moving from 100 back to 80 removes the same 20, but the baseline is now 100. The result is 20 ÷ 100 = 20%, so the reversal is a 20% decrease rather than 25%.

Step 04

Handle zero and negative values explicitly.

There is no finite percentage change from a starting value of zero because the formula would divide by zero. Report the absolute change and explain that no percentage baseline exists.

Negative starting values can occur in balances, temperature differences and accounting measures, but the interpretation depends on the subject. The calculator uses the absolute old value in the denominator; explain the underlying movement rather than relying on the percentage alone.

Step 05

Do not confuse percent with percentage points.

If a rate moves from 40% to 50%, it rises by 10 percentage points. Relative to the original 40%, that is a 25% increase. Both statements can be correct, but they describe different measures.

Keep the unrounded values for the calculation and round only the displayed result. Early rounding can distort small changes or a chain of comparisons.