Magento tax rounding issue

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);
}