Magento – Rounding Error Problem

magento-1.7roundingtax

In Magento 1.7 there is a well known rounding error issue, which I thought I have solved using the fix suggested for example here

However, ever since I have applied the fix, there is now a very strange new calculation error, not for taxes, but already for the items in cart. It only happens when the amount of an item in the cart is more than one.

Example: 5 times an item with a price of 1.18 € each equals 5.88 € in cart and not 5.90 € as supposed.

Once I disable the rounding error fix again, price calculation for the articles works again, bit than tax is being calculated wrong.

Any idea how to fox this?

Best Answer

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);

$this->_roundingDeltas[$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);
}

Related Solutions

Tax Calculation Method Based On – Preferred Setting in Magento 1.9

This is probably related to the famous magento rounding bug.

As a solution you can copy /app/code/core/Mage/Core/Model/Store.php file to /app/code/local/Mage/Core/Model/Store.php and modify the roundPrice() method to round with 4 digits:

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

Magento – Magento 2 – rounding issues with voucher discount applied

There are two possible fixes:

1st solution (needs more testing though):

Admin > Configuration > Sales > Tax > Calulation Settings
set Tax Calculation Method Based On to Row Total (worked with my test environment, described in my question)

2nd solution: Create a custom module with a di.xml which contains (vendor in the example is "Koseduhemak", Module "MagentoBugFixes"). I just post the relevant parts:

<preference for="Magento\Directory\Model\PriceCurrency" type="Koseduhemak\MagentoBugFixes\TaxRounding\Model\PriceCurrency" />
    <type name="Magento\Catalog\Pricing\Price\RegularPrice">
        <arguments>
            <argument name="priceCurrency" xsi:type="object">Koseduhemak\MagentoBugFixes\TaxRounding\Model\PriceCurrency</argument>
        </arguments>
    </type>
    <type name="Magento\Tax\Model\Calculation">
        <arguments>
            <argument name="priceCurrency" xsi:type="object">Koseduhemak\MagentoBugFixes\TaxRounding\Model\PriceCurrency</argument>
        </arguments>
    </type>
    <type name="Magento\Quote\Model\Quote\Item">
        <arguments>
            <argument name="priceCurrency" xsi:type="object">Koseduhemak\MagentoBugFixes\TaxRounding\Model\PriceCurrency</argument>
        </arguments>
    </type>

Create a file: Koseduhemak\MagentoBugFixes\TaxRounding\Model\PriceCurrency:

<?php
namespace Koseduhemak\MagentoBugFixes\TaxRounding\Model;

class PriceCurrency extends \Magento\Directory\Model\PriceCurrency
{
    const DEFAULT_PRECISION = 3;
    public function round($price)
    {
        return round($price, self::DEFAULT_PRECISION);
    }
    public function convertAndRound($amount, $scope = null, $currency = null, $precision = self::DEFAULT_PRECISION)
    {
        return parent::convertAndRound($amount, $scope, $currency, $precision);
    }
}

What it does: Rounding to three decimal places. Some people even suggest to round to four decimal places: https://magento.stackexchange.com/a/82444/67243 (If you want to round four decimal places, just change const DEFAULT_PRECISION = 3; to const DEFAULT_PRECISION = 4;.

EDIT: I spoke with the PayPal support and they told me that the PayPal Plus module / PayPal itself rounds to four decimal places. So you should also use four decimal places in your code (fix magento to do that).

However, this works only for modules / stuff that is using \Magento\Directory\Model\PriceCurrency class for rounding. Custom extension have something similar hard coded to two decimal places maybe...

My complete code can be found at Github (along with some other BugFixes especially for Magento 2.1.9): https://github.com/koseduhemak/MagentoBugFixes

Hope I could help someone.