Mortgage Payment Calculator with Extra Payments
This calculator estimates your scheduled mortgage payment and the impact of additional payments. Enter the loan amount, interest rate, term, and any extra payments you plan to make. The tool reports the scheduled monthly payment, an adjusted monthly payment reflecting extra contributions, the estimated time to pay off the loan, and approximate interest saved.
The calculator treats yearly extras as divided evenly across months and spreads a one-time extra evenly across the remaining term for an approximate impact. See methodology and accuracy notes for details and limitations.
Governance
Record 40a97e4c02b0 • Reviewed by Fidamen Standards Committee
Inputs
Results
Scheduled monthly payment (no extras)
$1,432.25
Effective extra payment per month (monthly + yearly/12 + spread one-time)
$0.00
Estimated monthly payment including extras (applied monthly equivalent)
$1,432.25
Estimated months to payoff with extras (approximate)
360
Estimated years to payoff with extras (approximate)
30
Total interest paid over original term (no extras)
$215,608.52
Estimated total interest paid with extras (approximate)
—
Estimated interest saved by making extras (approximate)
—
| Output | Value | Unit |
|---|---|---|
| Scheduled monthly payment (no extras) | $1,432.25 | USD |
| Effective extra payment per month (monthly + yearly/12 + spread one-time) | $0.00 | USD |
| Estimated monthly payment including extras (applied monthly equivalent) | $1,432.25 | USD |
| Estimated months to payoff with extras (approximate) | 360 | months |
| Estimated years to payoff with extras (approximate) | 30 | years |
| Total interest paid over original term (no extras) | $215,608.52 | USD |
| Estimated total interest paid with extras (approximate) | — | USD |
| Estimated interest saved by making extras (approximate) | — | USD |
Visualization
Methodology
Core formula: standard fixed-rate amortization where monthly rate r = annual_interest_rate / 12 / 100 and scheduled payment P = r*L / (1 - (1+r)^-n) with L = principal and n = term_years * 12.
Extra payments are converted to a monthly equivalent: monthly extras are added directly; yearly lump sums are divided by 12; one-time extras are distributed across remaining months as an approximation. The calculator then estimates payoff months by solving the closed-form amortization relation for n: n = -ln(1 - r*L / P_extra) / ln(1 + r), where P_extra is the monthly payment including extras.
Standards and quality: calculations and numerical stability follow software quality and numerical-method guidance. This product references general quality and numerical accuracy standards (for example NIST guidance on numerical computation and software assurance, applicable ISO quality-management principles, and IEEE numeric representation considerations) to inform validation, rounding, and error-handling policies. This is not a regulatory disclosure or financial advice.
F.A.Q.
Are extra payments applied directly to principal?
This calculator models extra payments as additional amounts that reduce outstanding principal and thus shorten the amortization period. The one-time extra is approximated by spreading its effect across remaining months; if your servicer applies a one-time extra in a different way, actual payoff may differ.
Can I model irregular extra payments?
This tool supports monthly extras, yearly lump-sum extras (converted to monthly equivalents), and a single one-time extra. For irregular or advanced schedules (biweekly, variable timing, changing rates, or recast scenarios), use an amortization schedule tool or consult a loan servicing statement.
How accurate are the payoff and interest saved estimates?
Estimates use closed-form amortization formulas and make approximations for converting non-monthly extras. They are generally accurate for fixed-rate loans with extras applied regularly. Accuracy may degrade if interest is compounded differently, if fees or escrow changes apply, or if the servicer applies extra payments with specific allocation rules. See the methodology section for limitations.
What if the calculated monthly payment with extras is less than monthly interest?
If the monthly payment including extras does not cover the monthly interest (r * loan_amount), the amortization formula is invalid and the tool cannot produce a meaningful payoff time. In such cases, increase payment amounts or contact your loan servicer.
Sources & citations
- National Institute of Standards and Technology (NIST) — Numerical Software and Computation Guidance — https://www.nist.gov
- International Organization for Standardization (ISO) — Quality management principles — https://www.iso.org
- IEEE Standards Association — Numeric and software considerations — https://standards.ieee.org
- U.S. Occupational Safety and Health Administration (OSHA) — Software and operational quality guidance (organizational) — 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: 40a97e4c02b0What changed (latest)
v1.0.0 • 2025-11-27 • 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 • 2025-11-27 • 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 27, 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 • 2025-11-27 • 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: 5e420746b336