How This Mortgage Calculator Works
We use the standard fixed‑rate amortization formula to compute your monthly payment and simulate payoff month‑by‑month. Add extra payments to see interest saved and how many years you can shave off your mortgage.
- Payment formula: M = P × [ r(1+r)n / ((1+r)n − 1) ]
- Inputs: loan amount, interest rate (APR), term, optional extra payments.
- Outputs: monthly payment, total interest, payoff year, and year‑by‑year table.
Example
On a $400,000 loan at 6.5% for 30 years, the base payment is about $2,528/mo (principal + interest). If you add $200/mo extra from the start, total interest drops by tens of thousands and the payoff moves forward by several years.
Use the Year‑by‑Year table to see how principal vs interest shifts over time. The first years are interest‑heavy; extra payments hit principal immediately and compound in your favor.
Key Features
- Extra monthly payments and lump sums
- Interactive charts: growth, YoY interest/principal
- Detailed amortization table by year
Tips
- Try +$100/mo to see interest saved
- Compare 15y vs 30y terms
- Test rate shocks ±1% to gauge sensitivity
What’s Not Included
- Taxes, insurance, HOA, and PMI vary by borrower and market; treat them as separate budget line items.
- Adjustable‑rate and interest‑only loans aren’t modeled; approximate by changing rate/term manually.
Frequently Asked Questions
How accurate is the monthly payment?
It uses the standard fixed‑rate formula. Actual payments can vary with escrow (taxes, insurance), PMI, and fees, which you can approximate with extra costs.
Can I model extra payments?
Yes — add extra monthly payments in the calculator. The chart updates interest saved and payoff time.
Do you support ARM or interest‑only?
This version focuses on fixed‑rate amortization. You can approximate scenarios by adjusting rate and term; ARM support is on our roadmap.
How are taxes/insurance handled?
They’re not part of the core amortization formula and vary by market. Add them to your budget separately; year‑by‑year costs help you plan.