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VAT calculation


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Ever since I changed the display of my product prices i.e. "less tax" rather than "incl tax" I have noticed that the invoice TOTAL (incl tax) is always off by a few cents or tens of cents

They may have been off before although I hadn't previously noticed it and, unlike now, customers had never complained about it

I first toyed around with the rounding parameters, but finally reinstated the default recommended settings - incidentally I don't have any rounding modules installed

I then upgraded PS from to and obtained no improvement

Visually the TOTAL (less tax) on my invoices is always correct

What seems to upset matters is the VAT calculation

At the moment I have to use Excel to calculate the correct TOTAL (incl VAT) and then adjust the shipping cost ("my lever") in order to ensure that the (GRAND) TOTAL is correct – tedious and time consuming

From what I have read it might be a PHP issue, combined with the fact that PS retains six figures after the decimal point to store its product prices.

The issue occurs whenever there are 'multiple' quantities of a product in the order e.g. 50 units
I assume that these six decimal figures could affect the result of VAT calculation

Any suggestions or recommendations ?




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  • 3 weeks later...

Prestashop offers different ways to calculate tax. These are options in the backoffice. You might try one of the other methods.

This has to do with rounding. You can calculate the VAT per product unit, per product (so only once if you order 10 of it) or over the total amount. The smaller the unit the more the VAT is likely to deviate from the total order calculation.


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