Currency Minor Units

Why the exponent matters

Payment systems carry amounts as integers in the currency's smallest unit. ISO 8583 DE 4 is twelve digits with no decimal point; EMV tag 9F02 is six bytes of BCD; virtually every payment API takes an integer. The exponent is what tells you where the decimal point belongs.

Get it wrong and the error is not subtle. Treating JPY as two-decimal makes every amount 100 times too small. Treating KWD as two-decimal makes it 10 times too large. Both are real production incidents that happen regularly, because the default assumption — two decimal places — is right for about 85% of currencies and silently wrong for the rest.

The exceptions worth memorising

Zero decimals: JPY, KRW, VND, CLP, ISK, PYG, UGX, RWF, VUV, XAF, XOF, XPF, BIF, DJF, GNF, KMF. If you handle Japanese or Korean payments, this is the one that will find you.

Three decimals: BHD, IQD, JOD, KWD, LYD, OMR, TND. Gulf and North African dinars. A thousand fils to the dinar.

Four decimals: CLF and UYW, both index units rather than everyday currencies, but they appear in ISO 4217 and will break naive parsers.

Alpha versus numeric codes

ISO 4217 defines both a three-letter code (GBP) and a three-digit numeric code (826). Card messages overwhelmingly use the numeric form — EMV tag 5F2A and ISO 8583 DE 49 — while APIs usually take the alpha form. EMV pads the numeric code to four digits in BCD, so GBP appears as 0826. This lookup accepts either form and both paddings.

Fund codes are not currencies

Codes like CLF, MXV, CHE and UYI are accounting units, not things you can charge a cardholder in. If one appears in a transaction you are almost certainly looking at a data error rather than an exotic settlement arrangement.

A caveat on ISO 4217 exponents

The exponent describes the currency's formal minor unit, which occasionally diverges from practice. MGA and MRU are formally two-decimal but their subunit is one fifth rather than one hundredth. Some zero-decimal currencies were historically subdivided and old systems may still carry two decimals for them. When a settlement figure disagrees with the exponent, the scheme's own rules win.