MortgageBoost Calculator
Free mortgage calculator with extra payment scenarios, amortization charts, and lender comparison for the US and UK. See how overpaying your mortgage can save you thousands in interest and cut years off your loan. JavaScript is required to use the interactive calculator.
How Extra Mortgage Payments Work
When you make extra payments on your mortgage, the additional amount reduces your outstanding principal directly. Because interest is calculated on the remaining balance, a lower principal means less interest accumulates each month. This creates a compounding effect: each extra payment slightly lowers every future interest charge, accelerating your payoff.
On a $300,000 mortgage at 6.5% over 30 years, your standard monthly payment is roughly $1,896. Adding just $200 per month extra can cut the loan term by over 5 years and save more than $60,000 in total interest paid.
Key strategies:
- Monthly overpayments — small consistent amounts compound significantly over time
- Annual lump-sum payments — applying a bonus or tax refund each year
- One-time lump-sum — inheritance or windfall applied directly to principal
- Bi-weekly payments — results in one extra full payment per year automatically
Top US Mortgage Lenders (July 2026)
| Lender | Rate | Type |
|---|
| Rocket Mortgage | 6.45% | 30-Year Fixed |
| Navy Federal Credit Union | 6.50% | 30-Year Fixed |
| PenFed Credit Union | 6.55% | 30-Year Fixed |
| Chase Home Lending | 6.60% | 30-Year Fixed |
| U.S. Bank | 6.65% | 30-Year Fixed |
Top UK Mortgage Lenders (July 2026)
| Lender | Rate | Type |
|---|
| Santander | 4.25% | 2-Year Fixed |
| Nationwide | 4.29% | 2-Year Fixed |
| HSBC | 4.31% | 5-Year Fixed |
| Barclays | 4.35% | 2-Year Fixed |
| Halifax | 4.39% | 5-Year Fixed |
Understanding Amortization
Amortization is the process by which each mortgage payment is split between interest and principal repayment. In the early years of a mortgage, the majority of each payment goes toward interest. As the loan balance falls, the proportion going to principal grows. A standard 30-year amortization schedule means it can take over 20 years before you are paying more principal than interest each month.
Extra payments disrupt this schedule in your favour, shifting the balance toward principal repayment much sooner.
About MortgageBoost
MortgageBoost is a free, privacy-first mortgage calculator. All calculations happen entirely in your browser — no data is sent to our servers. We support both US (USD) and UK (GBP) mortgages. The calculator includes extra payment modelling, full amortization tables and charts, PMI estimation, and a live lender rate comparison updated monthly.
Contact: boost.fin.tools@gmail.com