Is there any circumstance in which it is necessary to mark extra payments on a loan as going to “principal and not interest”?
It could be a couple of things besides extra principal: I seem to remember hearing that some (shady?) lenders would just pocket extra payments if you didn't specify where they were headed, but I've also been told that this just isn't true. The mortgage I got last year through Wells Fargo explicitly indicates in its terms that excess payment will be considered against future payments (i.e., pay $500 extra in January and you owe $500 less in February) unless indicated otherwise. It goes on to state that with electronic payments you do not get to specify where excess payment goes, so excess payment made electronically always goes toward future payments. If you want to make excess payments toward principal, you must actually send them a check and your payment stub, with the appropriate box ticked.  This won't be very different for other major banks, I wouldn't imagine. I had a car loan through GMAC and extra money was applied to future payments.   At one point, I received a statement telling me I had 15 months until my next payment was due because I had not marked extra payments as going to principal. I would always presume that given a choice of doing what is in its own self interest verses in the customer's interest, a bank will ALWAYS do the former rather than the latter. Banks are in the business of making money, always presume that their policies, processes and contractual terms will be slanted to maximize their ability to make money.  It's not being evil or anything, it's just business, they are under no obligation to be altruistic or do what's best for you at the expense of their profits. So, especially since it's not exactly a hardship, I would always make extra principal payments using a separate check and clearly mark it as an extra principal payment.
An offset account is simply a savings account which is linked to a loan account. Instead of earning interest in the savings account and thus having to pay tax on the interest earned, it reduces the amount of interest you have to pay on the loan. Example of a 100% offset account: Loan Amount $100,000, Offset Balance $20,000; you pay interest on the loan based on an effective $80,000 loan balance. Example of a 50% offset account: Loan Account $100,000, Offset Balance $20,000; you pay interest on the loan based on an effective $90,000 loan balance. The benefit of an offset account is that you can put all your income into it and use it to pay all your expenses. The more the funds in the offset account build up the less interest you will pay on your loan. You are much better off having the offset account linked to the larger loan because once your funds in the offset increase over $50,000 you will not receive any further benefit if it is linked to the smaller loan. So by offsetting the larger loan you will end up saving the most money. Also, something extra to think about, if you are paying interest only your loan balance will not change over the interest only period and your interest payments will get smaller and smaller as your offset account grows. On the other hand, if you are paying principal and interest then your loan balance will reduce much faster as your offset account increases. This is because with principal and interest you have a minimum amount to pay each month (made up of a portion of principal and a portion of interest). As the offset account grows you will be paying less interest, so a larger portion of the principal is paid off each month. An amortization schedule is often used to produce identical payments for the term (repayment period) of a loan, resulting in the principal being paid off and the debt retired at the end of the loan. This is in contrast to an interest only, or balloon loan. These loans require little or no payment against the balance of the loan, requiring the loan to be paid indefinitely if there is no term, or requiring the loan to be entirely paid off from cash or a new loan at the end of the term. A basic amortization formula can be derived from the compound interest formula:   This formula comes from the Wikipedia article on amortization. The basics of the formula are the periodic payment amount, A (your monthly payment), can be determined by the principal loan, P, the rate, r, and the number of payments, n.  Lenders lend money to make a profit on the interest. They'd like to get back all the money they lent out. Amortization schedules are popular because the fixed low payments make it easier for borrowers to pay the loan off eventually. They also tend to be very profitable for lenders, especially at the start of the term, because they make a lot of profit on interest, just like the start of your mortgage. The principal of a mortgage has more meaning than the principal of a revolving debt credit card. The mortgage principal is fixed at the start, and represents the value of the collateral property that is your home. You could consider the amount of principal paid to be the percentage of your home that you actually own (as part of your net worth calculation).  A credit card has a new balance each month depending on how much you charge and how much you pay off. Principal has less meaning in this case, because there is no collateral to compare against, and the balance will change monthly. In this case, the meaning of the amortization schedule on your credit card is how long it will take you to pay off the balance if you stop charging and pay at the proscribed payment level over the term described. Given the high interest rate on credit cards, you may end up paying twice as much for goods in the long run if you follow your lenders schedule. Amortizing loans are common for consumer loans, unless a borrower is seeking out the lowest possible monthly payment. Lenders recognize that people will eventually die, and want to be paid off before that happens. Balloon and interest only bonds and loans are more commonly issued by businesses and governments who are (hopefully) investing in capital improvements that will pay off in the long run. Thousands of people and businesses have gone bankrupt in this financial crisis because their interest only loans reached term, and no one was willing to lend them money anymore to replace their existing loan. That makes no sense at all. They try to compare and that's exactly the same as comparing apples versus oranges. Mortgage is long-term loan, so for the first many years the huge part of the payment will go to repaying interest, so that ratio 1 will indeed be something like 20% or more despite the fact that the interest rate on the mortgage is much lower - something around 6%. HELOC will have the interest rate of 6%, but it will have the same structure so that you have equal payments, so if you compute that ratio 1 it will be very close to that of the mortgage. The bottom line is - if HELOCs were that great noone would apply for mortgages. You should stick to making extra payments towards the principal on the mortgage. "By the sounds of things, you're not asking for a single formula but how to do the analysis... And for the record you're focusing on the wrong thing. You should be focusing on how much it costs to own your car during that time period, not your total equity.  Formulas:  I'm not sure how well you understand the nuts and bolts of the finance behind your question, (you may just be a pro and really want a consolidated equation to do this in one go.) So at the risk of over-specifying, I'll err on the side of starting at the very beginning. Any financial loan analysis is built on 5 items: (1) # of periods, (2) Present Value, (3) Future Value, (4) Payments, and (5) interest rate.  These are usually referred to in spreadsheet software as NPER, PV, FV, PMT, and Rate. Each one has its own Excel/google docs function where you can calculate one as a function of the other 4. I'll use those going forward and spare you the 'real math' equations.  Layout: If I were trying to solve your problem I would start by setting up the spreadsheet  up with column A as ""Period"". I would put this label in cell A2 and then starting from cell A3 as ""0"" and going to ""N"". 5 year loans will give you the highest purchase value w lowest payments, so n=60 months... but you also said 48 months so do whatever you want.  Then I would set up two tables side-by-side with 7 columns each. (Yes, seven.) Starting in C2, label the cells/columns as: ""Rate"", ""Car Value"", ""Loan Balance"", ""Payment"", ""Paid to Interest"", ""Principal"", and ""Accumulated Equity"". Then select and copy cells C2:I2 as the next set of column headers beginning in K2. (I usually skip a column to leave space because I'm OCD like that :) ) Numbers: Now you need to set up your initial set of numbers for each table. We'll do the older car in the left hand table and the newer one on the right. Let's say your rate is 5% APR. Put that in cell C1 (not C3). Then in cell C3 type =C$1/12. Car Value $12,000 in Cell D3. Then type ""Down Payment"" in cell E1 and put 10% in cell D1. And last, in cell E3 put the formula =D3*(1-D$1). This should leave you with a value for the first month in the Rate, Car Value, and Loan Balance columns. Now select C1:E3 and paste those to the right hand table. The only thing you will need to change is the ""Car Value"" to $20,000. As a check, you should have .0042 / 12,000 / 10,800 on the left and then .0042 / 20,000 / 18,000 on the right.  Formulas again: This is where spreadsheets become amazing. If we set up the right formulas, you can copy and paste them and do this very complicated analysis very quickly.  Payment The excel formula for Payment is =PMT(Rate, NPER, PV, FV). FV is usually zero. So in cell F3, type the formula =PMT(C3, 60, E3, 0). Obviously if you're really doing a 48 month (4 year) loan then you'll need to change the 60 to 48. You should be able to copy the result from cell F3 to N3 and the formula will update itself.  For the 60 months, I'm showing the 12K car/10.8K loan has a pmt of $203.81. The 20K/18K loan has a pmt of 339.68.  Interest The easiest way to calculate the interest is as =E3*C3. That's (Outstanding Loan Balance) x (Periodic Interest Rate). Put this in cell G4, since you don't actually owe any interest at Period 0.  Principal If you pay PMT each month and X goes to interest, then the amount to principal is ""PMT - X"". So in H4 type =-F3 - G3. The 'minus' in front of F3 is because excel's PMT function returns a negative amount. If you want to, feel free to type ""=-PMT(...)"" for the formula that's actually in F3. It's your call.  I get 159 for the amount to principal in period 1.  Accumulated Equity As I mentioned in the comment, your ""Equity"" comes from your initial Loan-to-Value and the accumulated principal payments. So the formula in this cell should reflect that. There are a variety of ways to do this... the easiest is just to compare your car's expected value to your loan balance every time. In cell I3, type =(D3-E3). That's your initial equity in the car before making any payments. Copy that cell and paste it to I4. You'll see it updates to =(D4-E3) automatically. (Right now that is zero... those cells are empty, but we're getting there) The important thing is that as JB King pointed out, your equity is a function of accumulated principal AND equity, which depreciates. This approach handles those both.  Finishing up the copy-and-paste formulas I know this is long, but we're almost done.  Rate // Period 1 In cell C4 type =C3.  Payment // Period 1 In cell F4 type =F3.  Loan Balance // Period 1 In cell E4 type =E3-H4. Your loan balance at the end of period is reduced by the principal you paid. I get 10,641.  Car Value // Period 1 This will vary depending on how you want to handle depreciation. If you ignore it, you're making a major error and it's not worth doing this entire analysis...  just buy the prettiest car and move on with life. But you also don't have to get it scientifically accurate. Go to someplace like edmunds.com and look up a ballpark. I'm using 4% depreciation per year for the old (12K) car and 7% for the newer car. However, I pulled those out of my ass so figure out what's a better ballpark.  In G1 type ""Depreciation"" and then put 4% in H1. In O1 type ""Depreciation"" and then 7% in P1. Now, in cell D4, put the formula =D3 * (1-(H$1/12)).  Paste formulas to flesh out table As a check, your row 4 should read 1 / .0042 / 11,960 / 10,641 / 203.81 / 45 / 159 / 1,319. If so, you're great. Copy cells C4:I4 and paste them into K4:Q4.  These will update to be .0042 / 19,883 / 17,735 / 339.68 / 75 / 265 / 2,148.  If you've got that, then copy C4:Q4 and paste it to C5:C63. You've built a full amortization table for your two hypothetical loans. Congratulations.  Making your decision I'm not going to tell you what to decide, but I'll give you a better idea of what to look at. I would personally make the decision based on total cost to own during that time period, plus a bit of ""x-factor"" for which car I really liked.  Look at Period 24, in columns I and Q. These are your 'equities' in each car. If you built the sheet using my made-up numbers, then you get ""Old Car Equity"" as 4,276. ""New Car Equity"" is 6,046. If you're only looking at most equity, you might make a poor financial decision.  The real value you should consider is the cost to own the car (not necessarily operate it) during that time... Total Cost = (Ending Equity) - (Payment x 24) - (Upfront Cash).  For your 'old' car, that's (4,276) - (203.81 * 24) - (1,200) = -1,815.75 For the 'new' car, that's (6,046) - (339.68 * 24) - (2,000) = -4,106.07.  Is one good or bad? Up to you to decide. There are excel formulas like ""CUMPRINC"" that can consolidate some of the table mechanics, but I assumed that if you're here asking you would have gotten stuck running some of those.  Here's the spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0Ah0weE0QX65vdHpCNVpwUzlfYjlTY2VrNllXOS1CWUE#gid=1" "I think you're right that from a pure ""expected future value"" perspective, it makes sense to pay this loan off as quickly as possible (including not taking the next year's loan). The new student loans with the higher interest rates have changed the balance enough that it's no longer automatically better to keep it going as long as possible. The crucial point in your case, which isn't true for many people, is that you will likely have to pay it off eventually anyway and so in terms of net costs over your lifetime you will do best by paying it off quickly. A few points to set against that, that you might want to consider: Not paying it off is a good hedge against your career not going as well as you expect, e.g. if the economy does badly, you have health problems, you take a career break for any reason. If that happens, you would end up not being forced to pay it off, so will end up gaining from not having done so voluntarily. The money you save in that case could be more valuable to you that the money you would lose if your career does go well. Not paying it off will increase your net cash earlier in life when you are more likely to need it, e.g. for a house deposit. Having more free cash could increase your options, making it possible to buy a house earlier in life. Or it could mean you have a higher deposit when you do buy, reducing the interest rate on the entire mortgage balance. The savings from that could end up being more than the 6% interest on the loan even though when you look at the loan in isolation it seems like a very bad rate." The principal of the loan is the amount you borrow.  The capitalized interest is added to the principal of the loan, because you are not paying this interest as it accrues.  So when you begin payments, the principal of the loan is $5,500 + $436 = $5,936. Using the standard amortization formula (see this page for details), the per-month payment for a ten-year payment plan at 6.8% interest on principal of $5,936 is $68.31.  One hundred twenty payments (each month for ten years) totals $8,197.40. "The credit and debit terms here is, talking from bank's point of view (shouldn't be a surprise, banks are never known to look at things from the customers' POV ;)).  In accounting, a liability (loans, owners capital etc) is a credit balance and asset (cash, buildings and such) is a debit balance.  Your account is a liability to the bank (in accounting parlance that is because they owe you every single penny that is there in your account, btw, in literal parlance too if you really make their life harder ;))  So when the bank accepts money from you, they need to increase their asset (cash) which they will debit (higher debit balance for asset means more assets), and at the same time they also have to account for the added liability by ""crediting"" the deposited money into your account. So when bank says they have credited your account, it means you have more money in your account. Now, if you transfer money from your account to another, or make a payment through your account, your account will be debited and the beneficiary account will be credited(bank's liability towards you reduces) More or less what everyone else said here... but hey, I could also take a swipe at banks ;))" You're looking for the amortization calculation. This calculation is essentially solving for the series of cash flows that will yield a zero balance after the specified term, at the given rate for a loan amount. Once you have solved for the payment amount, you can add additional principal amount to each payment period and see the interest payments and time to zero balance drop.