I'm about to do a balance transfer that has a fee of 4%. It gives me 0% interest for 14 months. Is it just a straight calculation of 4% for the year or is there another calculation that I need to include?
How much interest you effectively pay depends on when you pay off the balance transfer amount. If you pay off in a lump sum after the full 14 months, you have pre-paid 4% simple interest over a 14-month period. In effect, if you transferred a balance of $100, paying $4 as the transfer fee, you have borrowed only $96 of the $100 that was used to pay off the other credit card company. Paying $100 at the end of the 14 month period is an interest rate of 100/96 - 1 = 4.1666% for 14 months which works out to be an annual interest rate (APR) of 3.57%. If you pay off in bits and pieces over 14 months, you will have paid interest at a larger rate.
Assuming you pay your credit card bill in full each month, does paying off each month's new charges count as having paid your credit card bill in full for that month so that new charges in the month following do not get hit with interest from the time they are posted? If not, you will end up paying a lot in these interest charges too. In short, read the fine print of the balance transfer offer very very carefully.
If you keep the whole balance for the whole 14 months and pay it off right before the interest raises, you'll end up with ~3.43% APR.
If you're going to pay off the balance gradually, then the effective APR will be higher.