How to convert an annual mortgage rate to a monthly rate

For a nominal annual rate with monthly payments:

monthly rate = annual rate / 12
9% / 12 = 0.75% = 0.0075

On an opening balance of $2,000,000, first-month interest is $2,000,000 x 0.0075 = $15,000. From a payment of $17,994.52, the remaining $2,994.52 reduces principal.

Do not enter 9% as a monthly rate or use Mexico’s CAT as though it were the contractual interest rate. CAT is a broader cost indicator. Effective annual rates also require a different conversion:

monthly effective rate = (1 + annual effective rate)^(1/12) - 1

Keep sufficient decimals and round the displayed payment only at the end. The mortgage calculator expects an annual rate.