How do you calculate a 12 month average balance on a loan?

The answer depends on when interest is calculated, how frequently payments are made, the interest rate being charged and the life time of the loan. There are a number of "interest calculators" available on the Internet that can probably show you the answer - working out the answer from scratch means you'll need to add on the interest for each payment / interest cycle over the 12 months and then you can work out the average. If your using this to calculate your interest then an accurate calculation will depened on how your interest is calculated ie. daily monthly semi-annual, or annual. The simplist answer is take the balance of the loan at the end of each month, add them together and then divide by 12