How to calculate ROI

[MicroCode]

Heyz!

I am here building a Bitcoin Website, but I stumbled across an issue. I need to make a PHP Formula, which returns CURRENT ROI (Percentage) for the moment.
For example: 100$ Investment and CURRENTLY earned 42$.

I need a formula, so I can programmatically calculate the ROI Percentage.

I did ofcourse try myself different version how I could achieve it, in a simple tests:

Code:

<?
$investment = 100;
$percent = 2.4;
$earned = 50;
$ROI = ($investment - $earned)/$investment; // Formula
echo 'Investment: '.$investment.'$<br />Percent: '.$percent.'%<br />Earned: '.$earned.'$<br /><b>ROI: '.$ROI.'%</b>';
?>

Please help me with the right formula. I am very bad when it comes to Percentage calcs.. ^^

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[MicroCode]

Wait a second. I just tried to use this formula, and I think it might work.. but I need a confirmation from expert..

Code:

$ROI = ($investment * $earned)/100;

[PremiumCodeX]

Wait a second. I just tried to use this formula, and I think it might work.. but I need a confirmation from expert..

Code:

$ROI = ($investment * $earned)/100;

I think, it is

Code:

(($earned - $investment) / $investment)

, which gives a non-percent value.

And the percent value is

Code:

(($earned - $investment) / $investment) * 100

Where earned is the amount you have after earning...

[cryptofirm]

Wait a second. I just tried to use this formula, and I think it might work.. but I need a confirmation from expert..

Code:

$ROI = ($investment * $earned)/100;

try this formula 

Code:

 %ROI = ($earned * 100) / $Investment 

i saw a few investments program use that formula
remember the ROI is in percent (%) not in $

regards