There is no solution to this. Among other things, it would be dependent
on the rules of the jurisdiction whether the tax is supposed to be
figured per item or on the total of all taxable items. So what would fix
it in one case would break it in the other. It simply is true that
rounded(a) + rounded(b) may not equal rounded(a+b). One of the things I
used to do in my working days was to calculate the correct "fuzz"
(maximum discrepancy) for comparison tests << effective equality >>
It is not only with this that we have a problem. Those of us who keep
books to exact amounts but must file with governmental agencies on a
whole dollar basis know how much "fun" it is deciding which amounts to
juggle up or down (what is the LEAST alteration) as necessary to the
book amounts to make the whole dollar report come out right. This this
year, for one organization, fudging two amounts by a few cents was
enough to make the 990-EZ come out right.
BTW, there are analogous problems with things like figuring amortization
tables for loans )) how much of each payment is interest and how much
principle). There is no "right" way to do this and so any function used
by a program might not match what the particular bank says. In cases
like these we can't ask that the program be "fixed" to agree.
Michael D Novack
On 5/22/2018 9:11 PM, Matthew Pounsett wrote:
I've got an off-by-a-cent error on an invoice due to the way taxes are
calculated. GnuCash (2.6.21) is calculating the tax on each individual
taxable item, then summing that up, and adding that value to the subtotal.
This causes an error because each individual tax calculation is rounded
prior to subtotalling.
Given a tax rate of 13%, GnuCash is doing this:
Item Tax
277.50 36.08
92.50 12.03
Subtotal: 370.00 + 48.11 = 418.11
When the real calculation should be:
Item Tax
277.50 36.075
92.50 12.025
Subtotal: 370.00 + 48.100 = 418.10
This could be fixed by GnuCash not rounding tax amounts until the final
total, but typically I believe the calculation done is to add up all the
taxable items and apply the tax calculation to that subtotal, so that it's
not necessary to track many decimal places to avoid a rounding error.
I did some searching through the mailing lists and I see a lot of reported
issues about rounding errors in GnuCash 2.x. I can work around this by
putting in an extra line item to correct the taxes, but it would be nice
not to have to do that. Is this something that's been fixed in 3.x? I've
been avoiding the update because I haven't had time for a careful
migration, and testing whether I'm affected by any of the various issues
that have been reported since its release. But, this might be a reason to
set that time aside.
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