Identify the two different conversions
With DCC, the merchant’s conversion produces the home-currency amount shown on the terminal. Paying the local-currency price leaves the conversion to the card’s applicable terms. Visa says a DCC offer should show both currency amounts, the conversion rate and any additional fee or markup, and you should be able to accept or decline it.
Work through a USD and EUR example
Imagine a EUR 90 purchase. Assume your card converts at 0.90 EUR per USD, with a separate 2% fee and USD 1 fixed fee. The estimated local-currency payment cost is 90 ÷ 0.90 × 1.02 + 1 = USD 103.00. If the comparable DCC total is USD 108.00, paying in euros costs USD 5.00 less under these assumptions. These are invented figures, not a card recommendation or a live offer.
Avoid counting a fee twice
Enter the rate as local-currency units per one home-currency unit. If the card rate already includes the conversion markup, do not also enter that markup as a separate percentage. The DCC input should be the complete expected home-currency cost: add any separate issuer charge applicable to that option. Check your card terms rather than assuming that DCC removes every foreign-transaction fee.
Treat the card result as an estimate
The final card conversion depends on the rate and charges actually applied. If you only know an indicative rate, use the result as a comparison estimate and check the statement afterwards. A positive difference in this tool means DCC costs more; a negative difference means the entered DCC total costs less.
Keep control of the choice
Read the currency and amount before approving payment, and keep the receipt. If the terminal does not give clear information, ask for clarification. Visa recommends declining the conversion offer if its required details are missing or you feel pressured. Open the calculator and select “Fill in an example” to reproduce the USD 103 versus USD 108 comparison.