The zero problem

The arithmetic in these countries is easy. The digits are the hazard.

In a number of countries an ordinary lunch costs six figures. Vietnam, Indonesia, Laos, Colombia, and to a lesser extent Korea, Hungary and Chile all put travellers in front of numbers longer than they are used to reading at speed.

The exchange arithmetic is usually simple — often just dropping zeros. The danger is not the maths. It is misreading 50,000 as 500,000, which is not a small error but a ten-times one, and it happens to attentive people because the notes look similar and the numbers are long.

How the mistake actually happens

• Similar note designs. Vietnam's 20,000 and 500,000 notes are the classic pairing, close in colour and easily confused in poor light.
• Locals drop the thousands when speaking. "Twenty" means 20,000. Fast, normal, and a trap for a visitor who hears the literal number.
• Menus drop them too. "45" on a warung board means 45,000 rupiah.
• Digit grouping differs. Some countries use full stops as thousands separators and commas as decimals, so "1.500" is fifteen hundred and not one and a half.

Habits that prevent it

1. Count digits before you convert. Not the value — the number of digits. It takes half a second and catches the error that matters.
2. Learn the notes by colour and size on day one. Lay them out once in your room. Ten minutes, whole trip covered.
3. Say the number back. "Fifty thousand?" — a vendor will correct you immediately and without irritation.
4. Use the biggest note as an anchor. Know what the largest note is worth in your own currency and every other note is a fraction of it.
5. Pay attention when receiving change, not just when paying. Change is where the wrong-note error usually lands.

The cheat sheets for these currencies lead with the zero-dropping step for exactly this reason, and each one notes the specific hazard in that country.