Php TAX substract & add back

Question

I have a shop where the client enters the product price with the tax in his country. When i save it i need to remove the tax and keep only the base.

When i calculate the tax back for the clients from the same country in some cases the total price will be different with 0.01 , 0.03 then the original price.

I tried to calculate with 2 to 4 digits but it never works on all cases.

Ex: price with tax = 157, tax is 7.7% => base = 145.78 and tax amount 11.22

When i calculate the tax to the previous price base i get this:

Base: 145.78 , tax is 7.7% => 11.23 = price is 157.01

Php code i use:

$baseprice = round($fullprice / (1+($taxperc/100)),2);

On the frontend:

$tax = round($baseprice * ( $taxperc / 100 ),2);
$total = $tax + $baseprice;

Anyone have a suggestion about how to fix this ?

Thanks!

Update: after some research I found a solution is to use round with PHP_ROUND_HALF_EVEN

Update2: first solution doesnt work. 1-9 works fine, 10 -tax + tax =10.01

Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Obsidian Age, Mar 27 '18 at 23:04
You might also want to use a money library, e.g. http://moneyphp.org/en/latest/ — k0pernikus, Mar 27 '18 at 23:05
IMO the Serverside should store both the base price and added tax. Rounding issues will not be avoidable otherwise as to how floats work. — k0pernikus, Mar 27 '18 at 23:08
I store only the base price because the tax rate changes from time to time, and the old tax will have no use. — Emanuel, Mar 27 '18 at 23:09
You store only the current base price on the product. Yet on the actual order a buyer bought you then can store both the price and tax applied when bought. Prices and tax rates may change, an order once issued should not. — k0pernikus, Mar 27 '18 at 23:12
On the order of course I store both, but in the product record i store only the base price. — Emanuel, Mar 27 '18 at 23:13

score 0 · Answer 1 · answered Mar 28 '18 at 11:54

0

Today's solution would be to work on the back with full number of digits ( for base price, tax and total ) and only when display for user to round it. :)

answered Mar 28 '18 at 11:54

Emanuel

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4
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score -1 · Answer 2 · answered Mar 27 '18 at 23:19

Seems ( not 100% sure yet, i need to test more ) a solution is to use the banker's rounding PHP_ROUND_HALF_EVEN

$baseprice = round($fullprice / (1+($taxperc/100)),2 , PHP_ROUND_HALF_EVEN);
$tax = round($baseprice * ( $taxperc / 100 ),2 , PHP_ROUND_HALF_EVEN);
$total = $tax + $baseprice;

Php TAX substract & add back

2 Answers2