Magento 1.9 Rounding Issue Tax Calculation

Question

I have a product priced at £10.82 which gets 20% VAT added to it, leaving the price to be £12.98

This works fine when 1 product is in the cart but when more than 1 (e.g. 7) are added the calculation works like this

£10.82 * 7 = £75.74 (no problem here) £75.74 * 1.2 = £90.888 (which is rounded to £90.89

When £12.98 * 7 = £90.86 (the correct amount). This is the figure I need to show not £90.89.

Answer 1

Change Tax Calculation Method Based On to Unit Price instead of Total under System > Configuration > Tax > Calculation Setting.

This will give you the tax calculation you'd like based on (£10.82 + (£10.82 * 0.2)) = £12.98 (rounded) * 7 = £90.86 rather than £75.74 + (£75.74 * 0.2) = £90.89.

Answer 2

Try copying app/code/local/Mage/Core/Model/Store.php to app/code/core/Mage/Core/Model/Store.php and change:

public function roundPrice($price)
{
    return round($price, 2);
}

for:

public function roundPrice($price)
{
    return round($price, 4);
}