M = payment amount
P = principal, meaning the amount of money borrowed
J = effective interest rate. Note that this is usually not the annual interest rate; see below for an explanation.
N = total number of payments
For example, if the annual interest rate is 5%, and you pay in monthly installments (12 times per year), calculate 5/100 to get 0.05, then calculate J= 0.05 / 12 = 0.004167.
For example, if the loan term is 5 years and you'll be paying in twelve monthly installments each year, your total number of payments will be N = 5 * 12 = 60
Calculate (1+J)-N. First add 1+J, then raise the answer to the power of "-N." Make sure to include the negative sign in front of the N. If your calculator can't handle negative exponents, instead write this as 1/((1+J)N).[2]
In our example, (1+J)-N = (1.004167)-60 = 0.7792
Calculate J/(1-(your answer)). On a simple calculator, first calculate 1 - the number your calculated in the previous step. Next, calculate J divided by the result, using the effective interest rate you calculated above for "J."
In our example, J/(1-(answer)) = 0.004167/(1-0.7792) = 0.01887
Find your monthly payment. To do this, multiply your last result by the loan amount P. The result will be the exact amount of money you need to pay each month in order to pay off your loan on time.
For example, if you borrowed $30,000, you would multiply your answer from the last step by 30,000. Continuing our example above, 0.01887 * 30000 = 566.1 dollars per month, or $566 and 10 cents.
This works for any currency, not just dollars.
If you calculated our example all on one line of a fancy calculator, you would get a more accurate monthly payment, very close to $566.137, or about $566 and 14 cents each month. If we instead paid $566 and 10 cents each month like we calculated with the less accurate calculator above, we would be slightly off by the end of the loan term, and would need to pay a few dollars extra to make up for it (less than 5 in this case).