Converting currency is one multiplication or division, but the mistake most people make is not the arithmetic. It is using an outdated rate, reading the rate backwards, or forgetting the fee that the provider takes on top.
How it works
An exchange rate is quoted as a pair, such as EUR/USD = 1.08, meaning one euro buys 1.08 dollars. To convert euros to dollars, multiply by 1.08. To convert dollars to euros, divide by 1.08. The mid-market rate is the rate banks use between themselves; the rate you actually get includes a margin and possibly a fixed fee, so the delivered amount is usually a little less than the mid-market calculation.
Step by step
- Find a live mid-market rate for your currency pair and note the time.
- Write the rate in words: one unit of your starting currency equals how many units of the target.
- If the rate is quoted as target per starting unit, multiply. If it is quoted the other way, divide.
- Compute the mid-market amount.
- Subtract the provider margin and any fixed fee to get the delivered amount.
- Compare two providers on the delivered amount, not the headline rate.
Worked example
Convert 500 euros to dollars at EUR/USD 1.08. Mid-market amount = 500 x 1.08 = $540. If the provider charges a 1 percent margin and no fixed fee, delivered amount = 540 x 0.99 = $534.60. Converting back, $534.60 / 1.08 = 495 euros at the mid-market rate, which shows the round-trip cost of the margin.
Common mistakes
- Multiplying when you should divide, which inverts the conversion.
- Using a rate from last week as if it were live. Major pairs can move a percent in a day.
- Comparing headline rates without fees. A better rate with a big fee can deliver less.
- Forgetting that cash exchange at an airport or hotel usually carries a far worse margin than a bank transfer.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
For travel, convert a rough budget at the mid-market rate, then add a few percent for margins and fees. For large transfers, get a firm quote with the delivered amount in writing before sending money.