Identify the original amount first

The calculation is the new amount minus the original amount, divided by the original amount, then multiplied by 100. If a price rises from $15 to $18, the difference is $3. Dividing $3 by the original $15 gives 0.2, or a 20% increase. The denominator answers the question: a change relative to what?

Write the two values and their dates before calculating. A monthly figure and an annual figure are not directly comparable without adjustment. Likewise, a larger package and a smaller one can have different prices without describing the percentage change in the price of the same quantity.

A fall uses the same method

If the price moves from $18 back to $15, the difference is minus $3. Dividing by the new starting point of $18 gives approximately minus 16.67%. The return journey is not a 20% decrease because the base is different. Equal dollar changes need not be equal percentage changes.

This also explains why a 50% fall requires a 100% rise to recover. A hypothetical value of $100 falling to $50 has lost half its starting amount. Getting from $50 back to $100 requires an increase equal to the entire $50 starting point of the recovery.

Check whether the calculation is meaningful

An original value of zero makes the usual percentage-change formula undefined. Do not replace that problem with an enormous invented percentage. Describe the actual change, such as moving from zero contributions to $40, and explain why the ordinary ratio cannot be used.

Negative starting values can also produce results that are difficult to interpret. A movement from a negative balance to a positive one is often clearer in amounts and context than in a headline percentage. The formula's output is not automatically a useful description.

Keep rounding until the end

Use the unrounded values for the calculation and round the final result to a level appropriate to the question. Showing many decimal places does not improve an estimate based on approximate inputs. State whether the figure is an increase or decrease so a missing minus sign does not change the meaning.

Finally, check the result against the amounts. A $3 increase on $15 should be a substantial fraction, while $3 on $1,500 should be small. This rough sense-check catches many misplaced decimals before a percentage is copied into a budget or comparison.