Magento tax rounding issue - magento

Question

I got strange rounding issue for VAT in Magento. My product set up is
* product price incl 20% VAT is 183.59
I added 30 items into basket and it would cost 30 * 183.59 = 5507.70. I can see this value in basket/checkout so that's fine. If I have just 1 item in basket it's ok.
Also the final VAT would be 5507.70 * 20 / 120 = 917.95, but I'm getting 918.00
Do you have any idea how to fix this or where would I take a look? Thanks in advance.

Answer 1

In the end I found the solution. I changed System > VAT > Tax Calculation Method Based On from Unit price to Row Total and it works, more details here
The issue which I found is in core/store model. I had to rewrite roundPrice method and change rounding precision there.
public function roundPrice($price)
{
return round($price, 4);
}

Answer 2

Info
Round price in Magento based on previous rounding operation delta.
app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php:1392
app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php:719
protected function _deltaRound($price, $rate, $direction, $type = 'regular')
{
if ($price) {
$rate = (string)$rate;
$type = $type . $direction;
// initialize the delta to a small number to avoid non-deterministic behavior with rounding of 0.5
$delta = isset($this->_roundingDeltas[$type][$rate]) ? $this->_roundingDeltas[$type][$rate] : 0.000001;
$price += $delta;
$this->_roundingDeltas[$type][$rate] = $price - $this->_calculator->round($price);
$price = $this->_calculator->round($price);
}
return $price;
}
Sometimes this can cause an error due to the high delta calculation error ($this->_calculator->round($price)). For example, for this reason, some prices can vary in the range of ±1 cent.
Solution
To avoid this, you need to improve the accuracy of the delta calculation.
Change
$this->_roundingDeltas[$type][$rate] = $price - $this->_calculator->round($price);
to
$this->_roundingDeltas[$type][$rate] = $price - round($price, 4);
Changes need to be made in both files:
app/code/core/Mage/Tax/Model/Sales/Total/Quote/Tax.php:1392
app/code/core/Mage/Tax/Model/Sales/Total/Quote/Subtotal.php:719
Don't modify or hack core files! Do a rewrite!
The solution was tested on different versions of Magento 1.9.x, but maybe this will work in earlier versions.
P.S.
Change roundPrice function, as shown below, can solve the rounding error problem, but it can cause others (for example, some platforms require rounding up to 2 decimal places).
app/code/core/Mage/Core/Model/Store.php:995
public function roundPrice($price)
{
return round($price, 4);
}