How this instrument works
A loan repaid in equal installments is priced by one formula regardless of what it funds — a personal loan, a debt-consolidation loan, a medical loan, or any other lump sum a lender hands over with no collateral tied to a specific asset. Fix the amount borrowed, the periodic rate, and the number of payments, and the fixed monthly installment falls out of the arithmetic, because interest is charged only on whatever balance is still outstanding and that balance declines a little more with every payment until it hits zero on the last one.
Most calculators built around this same formula stop at the monthly figure, because that is the number a household budget has to absorb. This sheet leads with a different question: not 'can I afford the payment' but 'what does borrowing this money actually cost me.' Total interest paid sits at the top of the results because it is the one figure a lower monthly payment can quietly hide — stretch the term and the payment drops, but the balance sits outstanding longer, so more interest accrues even though each individual check written is smaller.
The arithmetic here assumes one fixed rate held constant for the whole term and no fees folded into the numbers. A real loan agreement can add an origination fee charged once up front, a prepayment penalty, or a variable rate that resets on a schedule the contract spells out — none of that is priced by Monthly payment, Total paid over the life of the loan, or Total interest paid, which only ever describe principal and interest on the exact figures typed into the three input fields.
- Enter the amount you are borrowing under Loan amount, $ — the sum actually advanced, not a purchase price still needing a down payment subtracted.
- Set the lender's yearly rate under Annual interest rate, %, entered as a whole number such as 8, not 0.08.
- Enter the repayment length in months under Loan term, months.
- Read Monthly payment for the fixed installment, then check Total paid over the life of the loan and Total interest paid to see the full cost.
- Change only the term and watch Total interest paid move — that comparison is the reason this sheet separates the payment from the cost.
Worked example — $20,000 at 8% over four years
Borrow $20,000 (Loan amount, $) at an 8% annual rate (Annual interest rate, %) over 48 months (Loan term, months), a typical horizon for a personal or debt-consolidation loan. The monthly rate is r = 8 ÷ 1200 = 0.006667, and (1.006667) raised to the 48th power works out to about 1.37527. Feeding those into the formula returns a Monthly payment of $488.26, the exact figure this sheet shows for these inputs.
The payment alone understates what the loan costs. Multiply $488.26 by 48 installments and Total paid over the life of the loan comes to $23,436.41, of which Total interest paid accounts for $3,436.41 — a figure a monthly-payment-only calculator never surfaces on its own. Halve the term to 24 months instead and the monthly payment climbs past $900, but total interest falls to roughly $1,709 — proof that a smaller check every month and a cheaper loan are not the same claim.
Questions
Why does this calculator lead with total interest instead of the monthly payment?
Because the monthly payment answers a budgeting question, not a cost question, and the two get conflated constantly when shopping loan offers. Two loans can quote the same monthly payment while one costs thousands more in total interest over a longer term. Total interest paid is placed first here so that comparison is the first thing visible, not something you have to compute by hand from the payment and the term.
Does a lower monthly payment always mean a cheaper loan?
No. On the golden example — $20,000 at 8% — stretching the term from 24 to 48 months cuts the monthly payment roughly in half but roughly doubles total interest paid, from about $1,709 to $3,436. A longer term trades a smaller monthly check for more months of interest accruing on a balance that falls more slowly, which is exactly the trade this sheet is built to expose.
How is this different from a car loan, mortgage, or business loan calculator?
Those sheets net a price down to a loan amount first — subtracting a down payment, a trade-in, or an SBA fee before the formula runs. This one asks for Loan amount, $ directly, because a personal loan, a debt-consolidation loan, or any lender-advanced principal usually starts from a figure already agreed with the lender rather than a price still needing adjustments.
What isn't included in Total paid over the life of the loan?
Origination fees, credit insurance premiums, late charges, and any prepayment penalty are contract terms a specific lender adds, and none of them enter this arithmetic. Total paid over the life of the loan is principal plus interest on exactly what you typed into Loan amount, $ — add real fees from your offer on top before comparing it against a lender's own disclosure.
What happens to these results if my loan has a variable rate?
Every result here assumes one fixed rate for the entire term, so Monthly payment and Total interest paid stay constant across the schedule. A variable-rate loan resets on whatever cadence the contract sets, usually against a published benchmark, so its real total interest will differ from this figure — re-run the sheet with the current rate each time it changes to see the fixed-rate equivalent at that moment.
Should I enter the rate as 8 or as 0.08?
Enter it as a whole percentage — 8 for eight percent — into Annual interest rate, %. The formula divides by 100 and by 12 internally to reach the monthly rate, so typing the decimal form instead understates the rate roughly a hundredfold and returns a Monthly payment barely above principal spread evenly across the term, wrong by a wide margin on any loan of meaningful size.
References
- Consumer Financial Protection Bureau — Loan options and terms
- 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.