How to convert currency in your head

Not a trick for one currency. A method you can apply to any of them, in about five minutes, on the plane.

Every traveller eventually meets the same problem: a price, a queue, and no useful phone. The fix is not a better app. It is a rule simple enough to run while someone is still talking to you.

Step 1 — round the rate to something you can divide by

Take the real rate and replace it with the nearest number you can divide by without thinking. If a dollar buys 33.4 Thai baht, your rule is "divide by 33" — or better, "divide by 3 and drop a zero". If a dollar buys 26,160 Vietnamese dong, your rule is "drop three zeros, divide by 26".

The goal is not accuracy. It is speed with a known error.

Step 2 — work out which way the rule lies to you

This is the step people skip, and it is the one that matters. Your rounded rule will always read slightly high or slightly low. Know which.

If you divide by a number larger than the true rate, your answer comes out too small — things look cheaper than they are, and you overspend. If you divide by a number smaller than the true rate, prices look dearer than they are and you are pleasantly surprised at the till.

Every cheat sheet on this site states the direction and the size of that error, because a rule without an error band is a guess wearing a uniform.

Step 3 — anchor on three amounts, not the whole range

Do not try to convert arbitrary numbers. Memorise three prices you will actually meet — a coffee, a short taxi ride, a decent dinner — and judge everything else relative to those. Most real decisions are comparisons, not calculations. You do not need to know that 340 baht is $10.18. You need to know it is about two coffees and a taxi.

Step 4 — learn the zero-counting trap first

For high-denomination currencies — dong, rupiah, kip, Colombian pesos — the arithmetic is easy and the zeros are the hazard. Mistaking 50,000 for 500,000 is a ten-times error and it happens to careful people, because the notes look similar and the numbers are long.

Before the division, count the digits. Locals often help by dropping the thousands entirely when speaking, which is faster for them and a trap for you.

Step 5 — re-check it when you land

Rules rot. A currency that moved 20% since your last trip will make a memorised rule quietly wrong in the expensive direction. Check once on arrival and you are set for the trip.

The five-minute version

1. Find today's rate.
2. Round it to something divisible in your head.
3. Note whether that makes prices look cheap or dear, and by how much.
4. Memorise a coffee, a taxi and a dinner in local money.
5. Count zeros before you divide.

Then pick your currency from the cheat sheets — the first three steps are already done for you, with the numbers dated.