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.