UK. The rules may have changed now, it's a long time since I implemented the rounding rules here, but the last time I did it required 5 decimals accuracy. The rules also used to specify how you needed to account for line items vs. sub-totals in your invoices to ensure you didn't find any "workarounds" to shave off some pennies of tax (In fact, the last time was while the tax authority was still called the Inland Revenue, which it hasn't for years.)

For aggregate totals of your VAT liability across your total set of invoices, you'd be fine with rounding up to the nearest pound, to the Inland Revenue's benefit. For individual invoices however, you were required to stick to very specific rounding rules.

> For aggregate totals of your VAT liability across your total set of invoices, you'd be fine with rounding up to the nearest pound, to the Inland Revenue's benefit

On personal tax forms you have to round in the taxpayer favour. If your income is 12345.67 you round it to 12345. If your expense (say giftaid) is 12345.67 you round it to 12346.

Surprised it's the other way with VAT, but then I do very little with tax other than click a few buttons and confirm "yes, you have to tax me as I have children".

I'm not sure it's the other way with VAT, as I said it's been many years and the rules may well have changed multiple times.

The key, though, is that with taxes, if at all unsure you've got the rules right, the safest option is to round in the tax offices favour.

It's in general a lot less painful to explain an overpayment than underpayment if something is broken.

Of course, better yet, get it right.

That's for final amounts though, right?

I bet they care about you not throwing away decimals in intermediate calculations for VAT or sales tax.

I think the point is that the integer arithmetic implementation your CPU provides is wrong in at least one jurisdiction, so (for example) the machine code in the article is wrong.