How this instrument works
Two loan offers rarely differ in just one dimension. A credit union might quote a lower rate over a shorter term while a dealer or online lender quotes a higher rate stretched a year or two longer, and neither the rate alone nor the monthly payment alone says which one hands over more money by the time both are paid off. This instrument runs the standard fixed-installment formula on each offer independently, using its own amount, rate, and term, multiplies each result by its own term to get a real total-cost figure, then subtracts one from the other.
The person running this comparison is usually shopping, not analyzing a hypothetical: a car buyer weighing a dealer's in-house financing against a preapproved credit-union loan, someone comparing a shorter personal loan at a steep rate against a longer one at a gentler rate, or a small-business owner sizing up two equipment loans that differ in both rate and repayment length. The instinct is to reach for whichever figure is easiest to compare at a glance, the advertised rate, or the payment that has to fit a monthly budget, but neither prices the full cost of borrowing on its own. A smaller monthly payment can still add up to more overall once the extra months of accruing interest are counted, and a higher headline rate can occasionally cost about the same, or even less, if it finishes sooner.
Total cost here means principal plus interest only, built from the amount, rate, and term entered on each side, and it excludes origination fees, prepayment penalties, and any difference in what each lender rolls into the financed amount. It also assumes both offers repay in equal monthly installments rather than an interest-only or balloon structure. Two loans that finance very different amounts, or run over very different lengths, will always show a large gap here regardless of which rate looks friendlier, so the figure means the most when both offers are financing roughly the same thing.
- Enter the first offer's principal under Loan A amount, $, its quoted rate under Loan A rate, %, and its length under Loan A term, months.
- Enter the second offer the same way, under Loan B amount, $, Loan B rate, %, and Loan B term, months.
- Compare Loan A monthly payment against Loan B monthly payment to see which fits a monthly budget more comfortably.
- Read Loan A total cost and Loan B total cost, principal plus every dollar of interest paid over each full term.
- Check Total cost difference (A − B): a negative figure means Loan A costs less overall, a positive one means Loan B does.
Worked example — 6% for five years against 7.5% for four
Loan A finances $20,000 at 6% over 60 months, with Loan A amount, $ at 20000, Loan A rate, % at 6, and Loan A term, months at 60 — and Loan B finances the same $20,000 at 7.5% over 48 months. Running the formula on each gives Loan A monthly payment of $386.66 against Loan B monthly payment of $483.58 — the second offer costs almost $97 more every month, an obvious-looking gap that would end most comparisons if payment were the only number checked.
Multiply each payment by its own term and the picture changes. Loan A total cost comes to $23,199.36 across 60 payments; Loan B total cost comes to $23,211.75 across 48. Total cost difference (A − B) works out to −$12.38: the first offer costs twelve dollars and change less overall, even though it runs a full year longer and carries the lower rate on paper. The extra twelve months at 6% very nearly matched the extra $97 a month the 7.5%, four-year offer demanded, leaving the two within a rounding error of each other despite looking nothing alike on paper.
Questions
Why compare total cost instead of just the interest rate?
Because the rate alone hides the term. A lower rate paid for longer can cost about the same, or more, than a higher rate paid for a shorter stretch. Total cost difference (A − B) folds the rate and the term together instead of forcing a choice between them, which is the only way to see which offer hands over more money by payoff.
Can Loan A amount, $ and Loan B amount, $ be different?
Yes, nothing here assumes the two loans finance the same amount. Comparing a $15,000 loan against a $20,000 offer is a legitimate use, but Total cost difference (A − B) will then reflect the size gap as much as the rate and term gap, so read it beside the two totals rather than as a pure rate comparison when the amounts don't match.
Why did Loan B cost more per month but almost the same overall?
Because Loan B repays $20,000 in 48 months instead of 60, so each payment absorbs more principal, the debt is retired sooner, and interest has fewer months left to accrue on a shrinking balance. That offsets most of the extra cost from its higher 7.5% rate. In the worked example the two effects very nearly cancel, leaving only a $12.38 gap.
What does a negative Total cost difference (A − B) mean?
It means Loan A is the cheaper offer overall, its Loan A total cost sits below Loan B total cost by that amount. A positive figure means Loan B costs less; a value near zero, like the golden example's −$12.38 on a $20,000 balance, means the two offers are close enough that a fee or a rounding difference elsewhere could flip which one is genuinely cheaper.
Does this include fees, points, or prepayment penalties?
No. Loan A total cost and Loan B total cost are principal plus interest only, computed from the amount, rate, and term entered for each side. An origination fee, a documentation fee, or a penalty for paying either one off early would change which offer is genuinely cheaper and has to be added in separately before comparing.
Is the loan with the lower total cost automatically the better choice?
Not necessarily, and this sheet does not decide that. A slightly higher total cost paired with a much lower monthly payment can still matter more to a tight budget than the last few dollars of total savings. This instrument prices both offers accurately; weighing total cost against monthly affordability is left to whoever is borrowing.
References
- Consumer Financial Protection Bureau — Auto loans resources
- Federal Reserve — Consumer Credit statistical release (G.19)
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.