What is the standard for formatting currency values in JSON?

Question

Bearing in mind various quirks of the data types, and localization, what is the best way for a web service to communicate monetary values to and from applications? Is there a standard somewhere?

My first thought was to simply use the number type. For example

"amount": 1234.56

I have seen many arguments about issues with a lack of precision and rounding errors when using floating point data types for monetary calculations--however, we are just transmitting the value, not calculating, so that shouldn't matter.

EventBrite's JSON currency specifications specify something like this:

{
"currency": "USD", 
"value": 432, 
"display": "$4.32"
}

Bravo for avoiding floating point values, but now we run into another issue: what's the largest number we can hold?

One comment (I don’t know if it’s true, but seems reasonable) claims that, since number implementations vary in JSON, the best you can expect is a 32-bit signed integer. The largest value a 32-bit signed integer can hold is 2147483647. If we represent values in the minor unit, that’s $21,474,836.47. $21 million seems like a huge number, but it’s not inconceivable that some application may need to work with a value larger than that. The problem gets worse with currencies where 1,000 of the minor unit make a major unit, or where the currency is worth less than the US dollar. For example, a Tunisian Dinar is divided into 1,000 milim. 2147483647 milim, or 2147483.647 TND is $1,124,492.04. It's even more likely values over $1 million may be worked with in some cases. Another example: the subunits of the Vietnamese dong have been rendered useless by inflation, so let’s just use major units. 2147483647 VND is $98,526.55. I’m sure many use cases (bank balances, real estate values, etc.) are substantially higher than that. (EventBrite probably doesn’t have to worry about ticket prices being that high, though!)

If we avoid that problem by communicating the value as a string, how should the string be formatted? Different countries/locales have drastically different formats—different currency symbols, whether the symbol occurs before or after the amount, whether or not there is a space between the symbol and amount, if a comma or period is used to separate the decimal, if commas are used as a thousands separator, parentheses or a minus sign to indicate negative values, and possibly more that I’m not aware of.

Should the app know what locale/currency it's working with, communicate values like

"amount": "1234.56"

back and forth, and trust the app to correctly format the amount? (Also: should the decimal value be avoided, and the value specified in terms of the smallest monetary unit? Or should the major and minor unit be listed in different properties?)

Or should the server provide the raw value and the formatted value?

"amount": "1234.56"
"displayAmount": "$1,234.56"

Or should the server provide the raw value and the currency code, and let the app format it? "amount": "1234.56" "currencyCode": "USD" I assume whichever method is used should be used in both directions, transmitting to and from the server.

I have been unable to find the standard--do you have an answer, or can point me to a resource that defines this? It seems like a common issue.

Answer 1

I don't know if it's the best solution, but what I'm trying now is to just pass values as strings unformatted except for a decimal point, like so:

"amount": "1234.56"

The app could easily parse that (and convert it to double, BigDecimal, int, or whatever method the app developer feels best for floating-point arithmetic). The app would be responsible for formatting the value for display according to locale and currency.

This format could accommodate other currency values, whether highly inflated large numbers, numbers with three digits after the decimal point, numbers with no fractional values at all, etc.

Of course, this would assume the app already knows the locale and currency used (from another call, an app setting, or local device values). If those need to be specified per call, another option would be:

"amount": "1234.56",
"currency": "USD",
"locale": "en_US"

I'm tempted to roll these into one JSON object, but a JSON feed may have multiple amounts for different purposes, and then would only need to specify currency settings once. Of course, if it could vary for each amount listed, then it would be best to encapsulate them together, like so:

{
"amount": "1234.56",
"currency": "USD",
"locale": "en_US"
}

Another debatable approach is for the server to provide the raw amount and the formatted amount. (If so, I would suggest encapsulating it as an object, instead of having multiple properties in a feed that all define the same concept):

{
"displayAmount":"$1,234.56",
"calculationAmount":"1234.56"
}

Here, more of the work is offloaded to the server. It also ensures consistency across different platforms and apps in how the numbers are displayed, while still providing an easily parseable value for conditional testing and the like.

However, it does leave a problem--what if the app needs to perform calculations and then show the results to the user? It will still need to format the number for display. Might as well go with the first example at the top of this answer and give the app control over the formatting.

Those are my thoughts, at least. I've been unable to find any solid best practices or research in this area, so I welcome better solutions or potential pitfalls I haven't pointed out.