Re: Computing sales taxes

Tom Anderson <>
Sun, 9 May 2010 12:22:29 +0100
On Sat, 8 May 2010, Roedy Green wrote:

If you use double to compute sales taxes, and round to the nearest
penny, there is a gotcha. You are supposed to round a precise half
penny up. This won't work properly for percentage like 6% (0.06) (of
$0.75) which is not precise in binary.

There a number of ways to avoid problem. Which would you use?

I'd start off working in thousandths-of-a-penny using integers (or longs
if i thought i might have to deal with amounts above 20 000 dollars).
Multiplying by 6% looks like:

int salesTaxInPercent = 6;
int taxableAmount = ...;
int tax = (salesTaxInPercent * taxableAmount) / 100;

If that got awkward (and it would, after about three minutes), i'd wrap
all amounts in an Amount class that hides all the storage and scaling and
so on. When that in turn got awkward, i'd switch to using BigDecimal
inside the Amount class.

My gut feeling is to keep away from BigDecimal as far as possible, because
it adds a lot of complexity you don't need for this. That feeling could
well be wrong.


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