Percentage Difference Calculation

[Anon]

Greetings, All -

I'm having trouble figuring out how to calculate the percentage change between two numbers: e.g. I'm trying to get the correct percentage change between the numbers 15.25 and 15.90 Technically it is 4.26% (or 4.3 when using the round function)but I keep getting 95.29 - Here is my code:

<?
$first = '15.20';
$close = '15.95';

$perch = (($first/$close)*100)."%";

echo $perch;
?>

The negitive calculation is needed as well when the numbers are reversed.

Thanks for any help!

[Anon]

I think you want

$perch = 100.0*($close-$first)/$first;

or (equivalently)

$perch = 100.0*($close/$first-1);

Note that you have to be sure that $first is > 0. After all, if $first is zero, and $close is anything bigger, then the percentage change is infinite!

There are two reverse operations; one when $close is needed form $first and $perch, and one when $first is needed from $close and $perch. A trivial bit of algebra gives:

$first = $close/($perch/100.0+1);
$close = ($perch/100.0+1)*$first;

[Anon]

Greetings, Grumbler -

Thank you very much! It's perfect.