To calculate FD interest, use the formula: Interest = (Principal amount * Interest rate * Time period) / (100 * Number of compounding periods). Determine the principal amount, interest rate, time period, and compounding frequency (e.g., annually, quarterly). Substitute these values into the formula and perform the calculation. The resulting amount is the interest earned on your FD. Keep in mind that actual returns may vary due to factors like rounding and any additional charges. Online FD calculators or consulting with your bank can provide accurate calculations tailored to their specific policies and compounding methods.