PHP: Increase / decrease number by a given percent.

This is a short tutorial on how to increase or decrease a number by a given percent in PHP. In this particular example, we will be adding 2.25% to 100 (i.e. we will be increasing 100 by 2.25%).

Let’s take a look at some example PHP code:

<?php

//The percent that we want to add on to our number.
$percentageChange = 2.25;

//Our original number.
$originalNumber = 100;

//Get 2.25% of 100.
$numberToAdd = ($originalNumber / 100) * $percentageChange;

//Finish it up with some simple addition
$newNumber = $originalNumber + $numberToAdd;

//Result is 102.25
echo $newNumber;

The code snippet above is pretty simple:

• We specified that we wanted to increase our original number by 2.25% (always use decimals). We also specified that our original number is 100.
• We worked out what 2.25% of 100 is by dividing the original number by 100 and multiplying it by the percentage decimal. (Read: Calculating the percentage of a given number in PHP).
• We then added that figure onto our original number.

To decrease the figure by 2.25%, we can simply assign -2.25 to our $percentageChange variable (i.e. place a minus sign in front of the figure).

<?php

//The percent that we want to change our number by.
$percentageChange = -2.25;

//Our original number.
$originalNumber = 100;

As you can see, it’s actually pretty simple :)