# how calculate the total of an invoice with several units of the same product

when you calculate invoices with several amounts, the total calculated with an unique invoice of 100 units, must to be the same that if you create 100 invoices of 1 unit each one. if you multiply 100 units * 15.5 and after multiplicate by 13% of tax, the roundings provoque a wrong result 100*15.5 = 1550 >>> The total of the invoice is by this wrong method: 1550 +(1550 * 13%) = 1550+201.5=1751.5. The wrong total tax is 1550*13%=201.5.

## How to calculate the total tax amount.

You have to calculate the tax per unit, and after that, multiply the tax by the amount of units of that product:

 1 UNIT price tax tax without rounding tax rounded amount total tax 15,5 13% 2,015 2,02 1 2,02

Then, if 1 unit is 2,02(with tax) then 110 units have to be = 100 * 2,02 = 202,2

 100 UNITS price tax tax without rounding tax rounded amount total tax 15,5 13% 2,015 2,02 100 202,2 (=2,02*100)

How to calculate the total of the invoice:

 1 UNIT price tax price +tax without rounding price+tax rounded amount total invoice 15,5 13% 17,515 17,52 1 17,52
 100 UNITS price tax price +tax without rounding price+tax rounded amount total invoice 15,5 13% 17,515 17,52 100 1752

As you see the tax total, and the invoice total are not the same depending of the way you make the calculations.

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