Negative & Positive Percentage Calculation

Question

Let's say i have 3 sets of numbers and i want the % of their difference.

30 - 60
94 - 67
10 - 14

I want a function that calculate the percentage of the difference between each 2 numbers, and the most important is to support positive and negative percentages.

Example:

30 - 60 : +100%
94 - 67 : -36% ( just guessing )
10 - 14 : +40%

Thanks

Answer 1

This is pretty basic math.

% difference from x to y is 100*(y-x)/x

Answer 2

The important issue here is whether one of your numbers is a known reference, for example, a theoretical value.

With no reference number, use the percent difference as

100*(y-x)/((x+y)/2)

The important distinction here is dividing by the average, which symmetrizes the definition.

From your example though, it seems that you might want percent error, that is, you are thinking of your first number as the reference number and want to know how the other deviates from that. Then the equation, where x is reference number, is:

100*(y-x)/x

See, e.g., wikipedia, for a small discussion on this.

Answer 3

for x - y the percentage is (y-x)/x*100

Answer 4

Simple math:

function differenceAsPercent($number1, $number2) {
    return number_format(($number2 - $number1) / $number1 * 100, 2);
}

    echo differenceAsPercent(30, 60); // 100
    echo differenceAsPercent(94, 67); // -28.72
    echo differenceAsPercent(10, 14); // 40