Well, you're not going to get exactly the same without knowing their formula. But, using the formulas at this

site, I was able to create a formula that gets a result within about a dollar difference from the site you linked using different combinations of values. The problem is determining

exactly **when and how** they calculate the interest and when they are calculating

**when** the monthly contribution is made. Those determinations will change the interest calculations.

$principle = 5000;
$monthly_amt = 200;
$years = 6;
$rate = 3.5;
$periods_per_year = 12;
$periods = $years * $periods_per_year;
$interest = $rate / 100 / $periods_per_year;
$future_value = (pow(( 1 + $interest), $periods) * $principle)
+ ( $monthly_amt * ( ( pow( (1 + $interest), $periods) - 1) / $interest ) );
echo "Start: $ $principle<br>\n";
echo "Monthly: $ $monthly_amt<br>\n";
echo "Years: $years<br>\n";
echo "Rate: $rate %<br><br>\n";
echo "Total: $ $future_value<br>\n";

**Edited by Psycho, 05 December 2012 - 02:29 PM.**