Multiply 1 by your chosen percentage. The basic mathematical approach for calculating a percentage increase is to subtract the second number from the first number. For example if the products value has increased by 15 from its last month value which was 250 then the new value of the product will be calculated by the following formula.
Percent Change New Value Old Value Old Value.
If you want to decrease an amount by a specific percentage Excel can help. Add this number to your original number. Check if everything went alright. One common way to calculate percentage change with negative numbers it to make the denominator in the formula positive.