Simple Currency Converter

How Currency Conversion Works

Currency conversion multiplies an amount in one currency by the current exchange rate to get the equivalent in another currency. The complexity lies in understanding which rate applies and what fees are hidden in it.

Basic conversion formula:

Amount in Target Currency = Amount in Source Currency × Exchange Rate

Worked example:

Converting $1,000 USD to Euros at a rate of 0.92 EUR per USD:

$1,000 × 0.92 = €920

Converting €920 back to USD at 1.087 USD per EUR:

€920 × 1.087 = $1,000 (round trip, no fees)

Bid-ask spread:

Banks and exchanges make their money on the gap between the rate they buy at and the rate they sell at, and the gap always runs against you in both directions. That is the point of it.

• Mid-market rate (the “real” rate, what you see on Google): 1.087 USD per EUR
• Bank selling rate (what you pay to buy euros): 1.130, so you hand over more dollars per euro
• Bank buying rate (what you receive when you sell euros): 1.045, so you get back fewer

Notice the direction. Buying costs more than mid and selling returns less than mid. If it ran the other way, as it is often mistakenly written, the bank would lose money on every single transaction.

The spread here is (1.130 − 1.045) ÷ 1.087 = 7.8% round trip, or roughly 3.9% each way. On a $1,000 conversion that one-way cost is about $39, and nothing on the receipt will call it a fee. Airport booths run wider still.

Cross-rate calculation:

To convert between two currencies that don’t have a direct quote (e.g., BRL to JPY):

BRL/JPY = BRL/USD × USD/JPY

If 1 USD = 4.97 BRL and 1 USD = 149 JPY:

1 BRL = 149 ÷ 4.97 = 29.98 JPY

Best practices to minimize fees:

• Use Wise (formerly TransferWise) or Revolut. Both quote close to the mid-market rate and charge a small explicit fee instead of hiding it in the rate.
• Avoid airport exchange booths, where spreads of 8 to 15% are normal. You are paying for the convenience of being trapped.
• When abroad, always choose to pay in local currency. If the terminal offers to charge you in your home currency, that is dynamic currency conversion (DCC) and it adds 3 to 7%.
• Credit cards with no foreign transaction fees (Chase Sapphire, Capital One Venture) typically offer near-mid-market rates