Mortgage Amortization Calculator with Bi-Weekly Payments
This calculator computes scheduled periodic payments, total money paid, total interest, and estimated payoff time for a mortgage using monthly, bi‑weekly, or weekly payment frequencies. You may add a fixed extra principal amount per period to see how extra payments accelerate payoff and reduce interest.
Results are estimates for planning purposes. Actual lender schedules, day‑count conventions, escrow payments, fees, and payment posting times can change final outcomes. Use values from your loan documents for best accuracy.
Governance
Record 19ffe8d5f4b9 • Reviewed by Fidamen Standards Committee
Inputs
Results
Scheduled periodic payment (including extra principal)
$1,347.13
Estimated total paid over life of loan
$484,968.26
Estimated total interest paid
$184,968.26
Estimated number of payments until payoff
—
Estimated payoff time (years)
—
| Output | Value | Unit |
|---|---|---|
| Scheduled periodic payment (including extra principal) | $1,347.13 | — |
| Estimated total paid over life of loan | $484,968.26 | — |
| Estimated total interest paid | $184,968.26 | — |
| Estimated number of payments until payoff | — | — |
| Estimated payoff time (years) | — | — |
Visualization
Methodology
The tool converts annual interest rate to an effective periodic rate given the chosen payment frequency (monthly = 12, bi‑weekly = 26, weekly = 52). It uses the standard fixed‑payment amortization formula for installment loans when interest rate is nonzero; for a zero interest rate the principal is divided evenly across periods.
To estimate accelerated payoff from extra principal, the calculator treats extra payments as applied directly to principal each period and solves the discrete amortization equation to estimate the number of periods required to reduce principal to zero. Numerical rounding is applied to present user‑friendly results.
Worked examples
Example 1: $300,000 principal, 3.5% APR, 30 years, bi‑weekly payments with $0 extra. The calculator returns the scheduled bi‑weekly payment, total interest over the loan, and payoff time equal to the nominal term.
Example 2: Same loan but $50 extra per bi‑weekly payment. The calculator estimates reduced total interest and a shortened payoff time; results illustrate how small recurring extra contributions accelerate payoff.
F.A.Q.
Why does the bi‑weekly option often show interest savings vs monthly?
Bi‑weekly frequency increases the number of payments per year (26 vs 12), and because some lenders apply payments more frequently to principal, the effective amortization can reduce interest. This calculator assumes each periodic payment is applied when scheduled and extra principal is applied directly to principal.
Are these results exact for my loan?
No. Results are precise within the calculator's mathematical model but are estimates. Actual payoff and interest depend on your lender's day‑count conventions, rounding rules, payment posting times, fees, and whether the lender treats bi‑weekly plans as accelerated monthly equivalents. Always confirm with your loan servicer.
What about taxes, escrow, PMI, or fees?
This tool calculates principal and interest only. Taxes, insurance, mortgage insurance (PMI), and other fees are excluded and must be added separately when budgeting.
What numerical limits or failure modes should I watch for?
If the periodic payment (including extra principal) is less than or equal to the interest accrued each period, the loan will not amortize (interest may capitalize). The calculator flags this by returning the original term as a ceiling in that scenario; verify inputs to ensure the payment exceeds periodic interest.
How should I interpret small differences versus lender statements?
Small discrepancies can arise from rounding differences, payment application timing, and daily interest accruals. Use lender amortization schedules for legal amounts.
How accurate is the payoff time when extra payments are used?
The payoff time formula solves the discrete amortization relation and then rounds up to an integer number of payments. This estimate is consistent with standard amortization mathematics but may differ from an exact servicer schedule by one period due to rounding and payment posting timing.
Sources & citations
- NIST - Numerical Methods and Software Quality Principles — https://www.nist.gov/publications
- ISO - Quality Management Systems (example guidance) — https://www.iso.org/iso-9001-quality-management.html
- IEEE Standards Association - Software and Numerical Standards — https://standards.ieee.org
- OSHA - General Guidance on Validation and Documentation Practices — https://www.osha.gov/laws-regs
- 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: 19ffe8d5f4b9What changed (latest)
v1.0.0 • 2025-11-13 • 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 • 2025-11-13 • 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 13, 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 • 2025-11-13 • 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: 0e2a28803488