Mortgage Calculator with Amortization Schedule & Refinance Break-Even

Full amortization schedule and refinance break-even, no lead-capture wall.

Mortgage calculators typically show a payment number and stop there. This one walks the entire loan month by month — all 360 rows for a 30-year term, not a three-point preview — and lets you drop milestones onto that timeline: an extra principal payment in month 40, a refinance in month 84, an ARM's first rate reset in month 61. Each milestone changes every row after it, which is the only way a break-even calculation means anything.

How to use the mortgage calculator

Enter the home price, down payment, interest rate and term in years, then your assumed property tax rate (percent of the price per year) and annual homeowners insurance premium.
Read the five result boxes: loan amount, monthly payment (P&I), estimated monthly PITI, total interest and the payoff year.
Click anywhere on the principal-vs-interest chart, or press Add milestone by month, to pin an extra principal payment or a refinance at that month; the schedule recalculates from that row forward.
Tick Compare current loan vs. a proposed refinance and enter the proposed rate, term and closing costs to see the monthly savings and the break-even in months.
Optionally switch on discount points, PMI or the ARM comparison in their own panels, then use Print / Save as PDF to keep a one-page summary.

The monthly mortgage payment formula, with a worked example

Every row of the schedule comes from the standard fixed-rate formula M = P·r(1+r)^n / ((1+r)^n − 1), where P is the loan amount, r the monthly rate (annual rate divided by twelve) and n the number of monthly payments. Each month, interest is the remaining balance times r and the rest of the fixed payment reduces principal. Change the amount, rate, or term and the whole schedule recalculates from row one, since each month's interest depends on the balance carried forward.

With the calculator's starting values — a $450,000 home, $90,000 down, 6.35% over 30 years — P is $360,000, r is 0.0635 ÷ 12 ≈ 0.00529 and n is 360, which gives a principal-and-interest payment of about $2,240 a month and roughly $446,417 of interest over the full term. The 1.1% tax assumption adds $412.50 a month and the $1,800 insurance premium adds $150, so the estimated PITI box reads about $2,803.

The simulator caps any single run at 1,200 months (100 years) as a hard backstop, so a milestone that keeps re-extending the term can never spin the schedule into an infinite loop. In practice a normal amortization finishes in the 180-to-360-month range it's built for.

How extra principal payments shorten your loan

A lump sum pinned as an extra-principal milestone comes straight off the balance that month, so every later row charges interest on a smaller number while the monthly payment stays the same. The saving compounds: less interest each month means more of the unchanged payment goes to principal.

On the default loan, a single $20,000 extra payment in month 40 moves the payoff from month 360 to month 317 — 43 payments sooner — and cuts total interest from about $446,417 to about $369,757, a saving of roughly $76,660. Try the same $20,000 in month 120 and the saving is smaller, because there are fewer remaining months for the lower balance to work over.

Extra principal, refinance, and ARM resets are not the same milestone

An extra-principal milestone applies a one-time lump sum to the balance in the month you pick, on top of that month's normal payment, shortening every remaining row.
A refinance milestone swaps in a new rate and, if you set one, a new term — the schedule re-amortizes the remaining balance over that new term from the refinance month forward.
An arm-adjustment milestone changes only the rate; it always re-amortizes over what was left of the ORIGINAL term, never a new one — an adjustable-rate reset changes your payment, not your payoff date.

Reading your mortgage results

Monthly payment (P&I) is principal and interest only. Est. monthly PITI adds one twelfth of the yearly tax (your tax rate times the home price) and one twelfth of the insurance premium; PMI is reported in its own panel and is not folded into that box.
Total interest is the sum of every row's interest charge, so it already reflects any extra payments or refinance milestones you pinned.
Payoff shows the year the balance reaches zero — Yr 30.0 for the default loan, Yr 26.4 after the $20,000 extra payment in month 40.
With PMI switched on, the panel shows the first month's charge and the month it drops off: 10% down ($45,000) at the default 0.55% starts at about $185 a month and ends in month 107.
Break-even is closing costs divided by monthly savings, or Never when the proposed payment isn't lower. Compare mode prices the proposed loan on the original loan amount (price minus down payment), not on today's remaining balance.

Why PMI doesn't vanish exactly at 20% equity

PMI is only added to the schedule when the down payment you entered was below 20% of the purchase price — a down payment at or above that line skips PMI from row one. Once it's running, PMI is removed automatically the month the scheduled balance first drops below 78% of the ORIGINAL purchase price, not 78% of whatever the home is worth today or of the current balance measured against itself.

That 78%-of-original-price rule mirrors the automatic-termination point set by the U.S. Homeowners Protection Act for the loan's original schedule, different from the lender-discretion 80% cancellation point some servicers apply on request with a fresh appraisal — the simulator only models the automatic date, not a manual cancellation you'd have to request.

Reading the refinance and points break-even

Break-even on a refinance is closing costs divided by the drop in monthly payment: at $4,800 in costs and $200 saved a month, that's 24 months before the refinance pays for itself. With the calculator's own compare defaults — 5.5% over 30 years and $6,000 of closing costs — the proposed payment is about $2,044, roughly $196 below the current $2,240, so break-even lands at about 30.6 months. Discount points use the identical division, framed as upfront cost against the monthly savings the lower rate buys — there is no separate points formula, it reuses the refinance break-even math with a zero comparison payment.

If the proposed payment isn't actually lower than the current one, the break-even is undefined rather than a negative or infinite number — a milestone or rate change that doesn't save you anything monthly will simply report no break-even at all.

What this simulator is not

It has no idea what your actual lender will quote for closing costs, appraisal fees, title insurance, or origination points — you supply those numbers, and a wrong one produces a wrong break-even.
Property tax and homeowners insurance enter only as two flat assumptions — a yearly tax rate on the home price and a fixed annual premium — added on top of P&I for the PITI estimate. They don't change the amortization rows, don't rise over time, and aren't part of the refinance or points break-even, which compare P&I only.
It is not a Loan Estimate or Closing Disclosure and carries none of the regulatory formatting or accuracy guarantees those documents do under TILA-RESPA.
Before signing anything, run your actual numbers past a licensed loan officer or a fee-only advisor — amortization math is the easy part of a mortgage decision.

Questions people ask about Mortgage & Refinance Simulator

Should I refinance my mortgage?: The calculator can't decide for you, but it shows the key test: how many months of lower payments it takes to recover the closing costs. If you expect to keep the loan well past that break-even, the refinance usually pays off; if you might sell or refinance again sooner, it may not. Restarting a 30-year term can also raise total interest even when the monthly payment falls, so compare the total interest figures too.
How much are closing costs on a refinance?: This tool has no lender data, so it uses whatever figure you type into the closing costs field ($6,000 by default). A commonly quoted rule of thumb puts refinance closing costs at roughly 2% to 5% of the loan amount, but the only reliable number is the one on your lender's Loan Estimate.
What's the difference between APR and interest rate?: The interest rate is what sets your monthly payment. APR folds certain upfront fees and points into an annualized cost, so it is usually a little higher than the rate. Enter the note interest rate here, not the APR, because the amortization formula needs the rate that actually accrues on the balance each month.
How much does PMI cost?: It depends on your lender, credit profile and down payment, so the calculator lets you set the annual PMI rate yourself; the default is 0.55% of the balance per year. On a $450,000 home with 10% down at 6.35%, that starts at about $185 a month and stops in month 107, when the balance falls to 78% of the original price.
Why didn't my PMI drop off at exactly 20% equity like I expected?: The schedule removes PMI when the balance crosses 78% of the ORIGINAL purchase price, the automatic-termination point under federal law, not 80% of current value. If your home has appreciated, you may be able to request earlier cancellation from your servicer with a new appraisal, but that manual path isn't modeled here.
Does adjusting my ARM's rate change my final payoff date?: No. An arm-adjustment milestone changes the rate and re-amortizes only what was left of the loan's original term, so your payoff month stays fixed. A refinance milestone is the one that can change your payoff date, and only if you enter a different new term for it.
How is the refinance break-even period actually calculated?: It's closing costs divided by the reduction in your monthly payment, in months. If the new payment isn't lower than the old one, there is no positive break-even to compute, so the simulator reports that rather than a misleading negative or infinite figure.
Is any of my loan or personal information sent anywhere?: No request ever leaves your device while you use this tool — every calculation is a plain JavaScript function running in your browser tab, with no server call involved at any point. You can confirm that yourself by checking your browser's network activity while entering numbers.
Can I use this instead of getting a real Loan Estimate?: No. This models the arithmetic of amortization, PMI timing, and break-even, but it has no relationship to the disclosures, fees, or rate lock a lender is legally required to provide. Confirm real terms with a licensed loan officer before you commit to anything.

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Content reviewed 2026-09-27.