Mortgage Refinance Calculator with Extra Payments
This calculator compares your current mortgage to a potential refinance and models how extra monthly payments or lump-sum payments change payoff timing and savings. Use the fields to enter your outstanding balance, interest rates, terms, closing costs, and extra payments.
Results estimate scheduled monthly payments, gross monthly savings, months to recover closing costs (break-even), and how extra payments shorten payoff time. Outputs are illustrative and intended for planning, not as a loan commitment.
Governance
Record 114902d113b8 • Reviewed by Fidamen Standards Committee
Estimates monthly payments for current and prospective refinance loans, monthly savings, break-even months for closing costs, and net savings over a chosen analysis period. Extra monthly payments and lump-sum payments can be included as user inputs.
Inputs
Results
Current monthly payment
$1,055.67
New monthly payment (refinance)
$843.21
Monthly savings (old - new)
$212.47
Break-even (months)
—
Net savings over analysis period
$9,747.94
Months to pay off current loan with extra payments
—
Months to pay off new loan with extra payments
—
| Output | Value | Unit |
|---|---|---|
| Current monthly payment | $1,055.67 | USD |
| New monthly payment (refinance) | $843.21 | USD |
| Monthly savings (old - new) | $212.47 | USD |
| Break-even (months) | — | months |
| Net savings over analysis period | $9,747.94 | USD |
| Months to pay off current loan with extra payments | — | months |
| Months to pay off new loan with extra payments | — | months |
Visualization
Methodology
Monthly payment uses the standard fixed-rate amortization formula: payment = r * PV / (1 - (1+r)^-n), where r is the monthly rate and n is number of months.
Break-even months = closing costs divided by (old monthly payment minus new monthly payment). If monthly savings are zero or negative, break-even is effectively infinite.
Payoff months with extra payments use the closed-form inversion of the annuity formula: n = negative natural log of (1 minus r times PV divided by Payment) divided by natural log of (1 plus r) when Payment exceeds the interest portion. If Payment does not exceed monthly interest, payoff is considered infeasible within a finite horizon.
Key takeaways
This tool gives quick, transparent estimates to help you decide whether to refinance or apply extra payments.
It assumes fixed-rate amortizing loans, monthly compounding, and that extra payments are applied directly to principal each month. It does not include tax effects, escrow changes, variable-rate behavior, prepayment penalties, or borrower-specific underwriting conditions.
Worked examples
Example: Current balance $200,000 at 4.0% with 25 years remaining → monthly ≈ computed. Refinance to 3.0% for 30 years with $3,000 closing costs yields new monthly payment and monthly savings. Break-even = 3000 / monthly savings.
Example: Adding $200 extra monthly reduces payoff months; calculator shows estimated months to payoff and approximate first-year cash difference as a quick proxy for interest reduction.
F.A.Q.
How accurate are the results?
Results use standard amortization formulas and closed-form algebraic solutions. They are mathematically consistent but approximate because they omit lender-specific fees, escrow, taxes, insurance, prepayment penalties, and rounding conventions. Use results for planning and confirm exact figures with your lender.
Does the calculator assume closing costs are financed into the loan or paid up front?
You control that via the 'refinance_financed_closing_costs' field. If you finance closing costs, include that amount there; otherwise the calculator treats closing costs entered in 'refinance_closing_costs' as cash paid at closing for break-even calculation.
Are extra payments applied immediately or at month-end?
This model assumes extra monthly payments are applied each payment period to principal alongside the scheduled payment. Lump-sum payments are applied immediately to reduce principal before future interest accrual.
Is this financial advice?
No. The calculator provides educational estimates. Consult a licensed mortgage professional or financial advisor before making refinancing or prepayment decisions.
How is my data handled and secured?
The model recommends following security best practices for any stored data. For technical guidance on secure data handling see NIST cybersecurity guidance in the citations below.
Sources & citations
- NIST - National Institute of Standards and Technology — https://www.nist.gov
- ISO - International Organization for Standardization — https://www.iso.org
- IEEE - Institute of Electrical and Electronics Engineers — https://www.ieee.org
- OSHA - Occupational Safety and Health Administration — https://www.osha.gov
- CFPB — Understand Loan Options — https://www.consumerfinance.gov/owning-a-home/loan-options/
- CFPB — Explore Interest Rates — https://www.consumerfinance.gov/owning-a-home/explore-rates/
- Fannie Mae — Loan Terms Glossary — https://www.fanniemae.com/glossary
Further resources
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
Record ID: 114902d113b8What changed (latest)
v1.0.0 • 2025-11-26 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
What changed (latest)
v1.0.0 • 2025-11-26 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Engine
v1.0.0
Data
Baseline (no external datasets)
Content
v1.0.0
UI
v1.0.0
Governance
Last updated: Nov 26, 2025
Reviewed by: Fidamen Standards Committee (Review board)
Credentials: Internal QA
Risk level: low
Reviewer profile (entity)
Fidamen Standards Committee
Review board
Internal QA
Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person
Semantic versioning
- MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
- MINOR: New features or fields that do not change existing outputs for the same inputs.
- PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.
Review protocol
- Verify formulas and unit definitions against primary standards or datasets.
- Run golden-case regression suite and edge-case suite.
- Record reviewer sign-off with credentials and scope.
- Document assumptions, limitations, and jurisdiction applicability.
Assumptions & limitations
- Uses exact unit definitions from the Fidamen conversion library.
- Internal calculations use double precision; display rounding follows the unit's configured decimal places.
- Not a substitute for calibrated instruments in regulated contexts.
- Jurisdiction-specific rules may require official guidance.
Change log
v1.0.0 • 2025-11-26 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 8d632452a43f