In the formula, "r" stands for the monthly interest rate. Interest rates provided by lenders is the cost you pay each year to borrow money expressed as a percentage.
When lenders advertise personal loan interest rates, it's typically a fixed rate, meaning the rate would be steady for the entire length of the loan. Note that what you are shown are "estimated interest rates." Rates may change after consumers apply for a loan and the lender evaluates their creditworthiness.
Many personal loans use "simple interest" instead of "compound interest." Simple interest loans are based only on the principal amount of a loan. Compound interest (which is used in many home mortgages and credit cards) is more complicated and may be more expensive.
Credit scores (based on your credit report) have a major impact on the interest rate a lender will offer. If you have a solid credit history, on-time payments, and a low debt-to-income ratio, you may have a good credit score. Expect lenders to offer you more favorable interest rates.
If your financial history is not as solid and your credit score range is on the lower side, anticipate higher interest rates. Over the life of the loan, you will pay more interest. You might consider speaking with a financial advisor, who might be able to help you raise your credit score before you apply. In turn, you may be offered better loan options.