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Does anyone know whether exchanges usually round values or truncate them instead?

Different exchanges support different resolutions for prices and amounts. For instance, BTC China supports 2 decimals for prices and fiat amounts (in CNY) and 4 decimals for BTC amounts.

Now, if I buy, say, 0.625 BTC at price 1621.37, I would get 1013.35625 CNY, but since fiat amounts are allowed to have only 2 decimals, that number will lose the extra digits. But will it become 1013.36 or 1013.35 (that is, rounded or truncated?).

This can be verified empirically for any given exchange, but I was hoping someone already knew the answer, either for specific exchanges or a general rule that most exchanges follow (if that were the case).

  • Based on greediness of banks, I would say they would floor the number, so just cut off the extra decimals :P – Mathias711 Apr 16 '15 at 12:33
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Cointrader rounds it:

All calculations are performed to 16 decimal places. The final fee for dollars is then rounded up or down to two decimal places, and final fee for BTC is rounded to 8 decimal places. For example: a fee of $0.114 is rounded down to $0.11, and a fee of $0.116 is rounded up to $0.12.

Coinfloor uses something called "weighted stochastic rounding", whatever that may be:

In order to ensure fairness, Coinfloor uses an approach called "weighted stochastic rounding" to calculate the fees charged to our users when trading. Weighted stochastic rounding over time minimises the effects of rounding errors. To understand why this rounding method is preferable to traditional "round-to-nearest" rounding methods, consider the case of one hundred occurrences of a 4.71 fee, each rounded according to the nearest whole number. Using the weighted stochastic rounding method, the expected value of the sum of these rounded figures is 471, which is exactly equal to the ideal sum of the unrounded figures. Using the traditional method, the expected value of the sum of the rounded figures would be 500, which is far from ideal as it results in a much higher fee charged.

I didn't find the information on any other sites, but I think they will use round

  • Thanks for the info. I wasn't referring to the treatment of excess decimals in fees, though, but those in the amounts themselves. But maybe the same policies apply to both. – Juan May 4 '15 at 16:47

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