Do not round pre-tax amounts #901
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When rounding the pre tax amount for VAT calculations, we run into rounding errors on creating the adjustment. Example: We want the tax amount for $16.00 at 19% VAT 16.00 / 1.19 = 13.4453 ~ 13.45 --> 2.55 VAT 13.45 * 1.19 = 16.0055 ~ 16.01 --> 2.56 VAT (wrong wrong wrong) Not rounding the pre tax amount fixes this rounding error. As usual with rounding, there will still be cases where this does not work, depending on how many digits our database stores. They will be much rarer, though. There is a failing test for this, but it's pending for a different reason (in the taxation integration spec, we have the aforementioned rounding error, but it doesn't show up because shipping rates are not tax adjusted because of a missing `contains?` check.
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Yea, I had a chat with @jhawthorn about this IRL and I'm cool with it too. As hawth points out it more affects euro customers than it does NA, and I definitely want to start tracking more things unrounded so I am 👍. Thanks for pulling this bit out @mamhoff This should probably make it into a changelog entry though. |
cbrunsdon
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Feb 24, 2016
Do not round pre-tax amounts
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When rounding the pre tax amount for VAT calculations,
we run into rounding errors on creating the adjustment.
Example: We want the tax amount for $16.00 at 19% VAT
16.00 / 1.19 = 13.4453 ~ 13.45 --> 2.55 VAT
13.45 * 1.19 = 16.0055 ~ 16.01 --> 2.56 VAT (wrong wrong wrong)
Not rounding the pre tax amount fixes this rounding error.
As usual with rounding, there will still be cases where this does
not work, depending on how many digits our database stores. They
will be much rarer, though.
There is a failing test for this, but it's pending for a different
reason - in the taxation integration spec, we have the aforementioned
rounding error, but it doesn't show up because shipping rates are not
tax adjusted because of a missing
contains?check.