If I have a $6.00 purchase and the tax rate is 9.75%, then the calculated taxes is 0.585. In my app, I would round that up to 0.59; however, when I integrate to Square, it looks like Square is calculating the taxes at 0.58, so I’m getting a payment amount doesn’t match order amount error. Does Square always round down taxes if it’s at a half?

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Square uses a method called Bankers’ Rounding, also known as Rounding Half to Even. Under this method, half-values round to the closest even integer. This means that if the tax calculation results in a half-value, it will round down if the preceding number is even and round up if it’s odd. So in your case, since 0.585 is a half-value and the preceding number (5) is odd, it will round down to 0.58. This is why you’re seeing a discrepancy between your app’s calculation and Square’s calculation.

*This answer was reviewed by @Bryan-Square.*