Loan calculator — what most calculators leave out
Almost every loan calculator answers one question: what is my monthly payment? That is the least useful number in the whole transaction. It tells you nothing about which of two offers is actually cheaper, what the lender's fees really cost you, or how much you would save by paying a little extra.
This calculator answers the questions that change decisions:
- True APR including fees. Origination fees, points and closing costs are folded in and solved numerically, giving the number lenders are legally required to quote and the only fair way to compare offers.
- What overpayments actually save. Recurring extra payments and one-time lump sums, with the exact interest saved and new payoff date.
- Interest-only and balloon structures, including the size of the payment jump when an interest-only period ends and the lump sum a balloon leaves you owing.
- Every compounding and payment frequency, converted correctly rather than by dividing the rate by twelve.
How this compares to a typical loan calculator
| Feature | Typical loan calculator | This calculator |
|---|
| Payment & amortization schedule | Yes | Yes, grouped by year |
| All compounding frequencies | Yes | Yes, incl. continuous |
| All payment frequencies | Yes | Yes |
| Extra payments & lump sums | Rarely | Recurring + one-time |
| True APR including fees | Usually a separate tool | Built in, solved numerically |
| Biweekly vs monthly savings | No | Shown automatically |
| Interest-only loans | No | With payment-jump warning |
| Balloon loans | No | With balloon amount |
| Compare multiple loans | No | 3 at once, cheapest flagged |
| Affordability by DTI | Usually a separate tool | Built-in tab |
| Balance chart + yearly split | Static pie chart | Animated, with overpayment overlay |
| CSV export & shareable links | No | Yes |
| Ads / trackers | Usually ad-supported | None |
One thing this tool deliberately leaves out: deferred lump-sum loans and zero-coupon bonds, which some calculators include. They are lending instruments rather than consumer loans, so they sit outside what this tool is for. Everything it does calculate has been checked against the standard worked examples used across the industry and matches them exactly.
The amortization formula
M = P × [ i(1+i)n ] ÷ [ (1+i)n − 1 ]
Where P is the principal, i the periodic interest rate and n the total number of payments. Each payment covers the interest accrued on the outstanding balance, and whatever remains reduces the principal.
Getting the periodic rate right
The subtle part is i. If your rate compounds monthly and you pay monthly, i = rate ÷ 12 is correct. But if compounding and payment frequency differ — a rate compounded daily but paid monthly, say — dividing by twelve is wrong. The correct route is via the effective annual rate:
EAR = (1 + r∕m)m − 1 then i = (1 + EAR)1∕p − 1
with m compounding periods and p payments per year. For continuous compounding, EAR = e^r − 1. A 6% nominal rate is an effective 6.168% compounded monthly and 6.184% compounded continuously. This calculator always uses the exact conversion and shows both figures under the results.
True APR — the only fair comparison
The interest rate covers the cost of the money. The APR covers the cost of the loan, including origination fees, discount points and closing costs. Two lenders can advertise the same rate while one is materially more expensive.
There is no closed-form solution for APR. It is the rate at which the present value of all your future payments equals the amount you actually receive after fees, found numerically. This calculator solves it by bisection and annualises using the US Regulation Z convention. With zero fees the APR comes out exactly equal to the nominal rate, which is a good sanity check on any APR tool.
Worked example: borrow 200,000 over 30 years at 6.5% with 4,000 in fees. The payment is 1,264.14 either way, but the true APR is 6.695%, not 6.5% — about 0.2 percentage points of hidden cost.
Why extra payments work so well
An extra payment reduces the principal immediately, so every future period accrues less interest. The saving compounds, which is why small overpayments early in a loan are far more powerful than large ones late.
On a 250,000 loan over 30 years at 6%, an extra 200 a month saves roughly 86,000 in interest and clears the loan about 7.8 years early. The same 200 added in the final five years barely moves the total.
The biweekly trick
Paying half the monthly amount every two weeks produces 26 half-payments a year — the equivalent of 13 monthly payments, not 12. That thirteenth payment goes entirely to principal. On the same 250,000 loan it saves around 63,000 and cuts about 5.5 years. It is not magic, and you can get the identical result by simply adding one-twelfth of your payment each month.
Interest-only and balloon loans
Both lower your payment now in exchange for a problem later, and both deserve careful modelling.
With an interest-only loan the balance does not fall at all during the interest-only period. When it ends, the entire principal must be repaid over a shorter remaining term, so the payment rises sharply. The calculator shows both payments and the size of the jump.
A balloon loan calculates payments as if the loan ran much longer than it does, leaving a large lump sum due at maturity. You must refinance, sell the asset, or produce that cash. The calculator shows exactly what the balloon would be.
How much can you afford to borrow
Lenders assess DTI (debt-to-income): the share of gross income going to debt payments. Two ratios matter — front-end (housing only, traditionally under 28%) and back-end (all debts, commonly capped near 36%, sometimes up to 43%).
The Affordability tab converts income, existing debts and a DTI cap into a maximum payment and the loan that payment supports. Treat it as a range: it excludes property tax, insurance, PMI and maintenance, which routinely add 20–40% on top of the payment.
Deep links — URL parameters
The address bar stays in sync, so any scenario is shareable:
?amount=250000&rate=5.5&years=30
Optional: months, extra, fees, type (amortized|io|balloon), pay, comp, currency.
Privacy — nothing leaves your browser
Every calculation runs locally in JavaScript. Your loan amounts, income and fees are never transmitted, stored or logged. No account, no ads, no trackers on your inputs, and the page works offline once loaded.