Rounding miscalcualtion

[dyedblue]

I have a money vouchering form that users input an invoice amount and select percentages (up to three) that the form calculates the amount from each fund for. It works most of the time but I am being thrown off because of a rounding issue that causes the total of the calculated amounts for the 3 funds not to equal the invoice amount. It is off by $.01.

 

I have the following code and have tried several approaches the latest being to take the sum of the 2nd and 3rd amounts and subtract that from the invoice amount to give me the 1st amount. The 1st amount is still often off by $.01. Is there anyone who has experienced a similar problem that could shed some light on the subject for me or that has an easier approach to my problem?

 

//Calculates the Amounts to be charged to each fund based on percentages entered
$percent = 100;

//Calculates the second fund amount
$percentagecalc2=$percentage2 / $percent;
$amtr2=$amt * $percentagecalc2;
$amt2 = preg_replace('/([\d,]+.\d{2})\d+/', '$1', $amtr2);
$amt2 = number_format($amt2, 2, '.', ',');

//Calculates the third fund amount
$percentagecalc3=$percentage3 / $percent;
$amtr3=$amt * $percentagecalc3;
$amt3 = preg_replace('/([\d,]+.\d{2})\d+/', '$1', $amtr3);
$amt3 = number_format($amt3, 2, '.', ',');

//Calculates the first fund adds penny if needed
$amtr1=$amt-($amtr2+$amtr3);
$amt1 = number_format($amtr1, 2, '.', ',');

[lemmin]

Could you also post the numbers that you are using, the expected results and the actual results?

[dyedblue]

*Values passed in from form

$amt=279.71

$percentage2=40

$percentage3=20

$amt2=111.88

$amt3=55.94

 

When these numbers are entered I get $amt1=111.88 and it should be $111.89 (279.71 - 167.82)

[lemmin]

You are rounding before the end of the calculation, which gives the the wrong expected result, the actual result is correct. When you say (279.71 -167.82) = 111.89, that is true, however, the program won't round until the end and it doesn't get 167.82, it gets 167.826, which changes the answer to 111.884, and that rounds down to .88.

[dyedblue]

I understand what you are saying, but I don't see where my error is.

[richardw]

As  lemmin said, round at the end only. Remove your number_format from all but the variable set to display the results.

 

Best

[dyedblue]

In my code ($amtr1=$amt-($amtr2+$amtr3); I use the pre-rounded numbers ($amtr2, $amtr3) to get the first amount. I am using $amt1, $amt2, $amt3 to display the amounts so I don't want to take off the rounding for them, but as far as I can tell I only use the pre-rounding amounts to do the calculations.

[corbin]

To get the most accurate results, you would want to round at the end....

 

Example:

 

$num1 = 5.5;

$num2 = 5.5;

$preround = round($num1) + round($num2);

echo $preround; //12

echo round($num1 + $num2);11