# How can you tell if a TFN is a real TFN?

So why is a TFN rejected when you enter it in the Australian Taxation Office portal?  Simple you have entered a wrong number so you double check the number and you simply then re-enter the correct number.

But hang on – you did enter it correctly first time and it was rejected.  How can you tell if the TFN is a real TFN?

Embedded in each TFN is a TFN algorithm (also known as a check algorithm).  In the case of TFN’s one of the digits is dependent on the other digits and this digit just happens to be the last digit.

For the algorithm to work each number starting from the left has to have a weighting and the weightings are 1 4 3 7 5 8 6 9 10.  Perhaps it is easier to explain by example.

Let’s use a TFN 185 931 361 and see if it is in fact a true TFN.  Using the weightings mentioned above we have:

1×1, 8×4, 5×3, 9×7, 3×5, 1×8, 3×6, 6×9, 1×10.

When you add these together we come up with 216.

The last key to the puzzle is to divide by 11 and if the number is divisible 11 then what you are looking at is a real TFN.

Don’t ask me why I not a mathematician – I am an accountant – I just know it works!!! 