Mortgage Amortization Calculator
This calculator estimates mortgage amortization results for any periodic payment cadence (bi‑weekly, monthly, weekly, etc.). Enter the loan principal, annual interest rate (APR), loan term in years, and number of payments per year — set 26 for standard bi‑weekly. Optionally add a recurring extra principal payment applied each period.
Outputs include the periodic payment, total number of payments, total paid over the life of the loan, total interest paid, and a baseline comparison showing estimated interest if payments were made monthly. Results are estimates only and intended for planning and comparison.
Governance
Record 048a4fcb14b9 • Reviewed by Fidamen Standards Committee
Inputs
Results
Periodic payment (per selected period)
-$0.52
Total number of payments
780
Total amount paid (principal + interest)
-$403.85
Total interest paid (over life of loan)
-$300,403.85
Total interest if paid monthly (baseline)
-$300,875.00
Estimated interest savings vs monthly schedule
-$471.15
Percent reduction in interest vs monthly
15.66%
| Output | Value | Unit |
|---|---|---|
| Periodic payment (per selected period) | -$0.52 | USD |
| Total number of payments | 780 | — |
| Total amount paid (principal + interest) | -$403.85 | USD |
| Total interest paid (over life of loan) | -$300,403.85 | USD |
| Total interest if paid monthly (baseline) | -$300,875.00 | USD |
| Estimated interest savings vs monthly schedule | -$471.15 | USD |
| Percent reduction in interest vs monthly | 15.66% | — |
Visualization
Methodology
This tool uses standard amortization mathematics: converting annual APR to a periodic interest rate, computing the fixed periodic payment that amortizes principal over the specified number of periods, and aggregating totals over the term.
When computing a baseline monthly comparison, the same principal, APR, and term are used with 12 payments per year to produce a comparable total interest figure. Extra periodic principal payments are added to each scheduled payment to show their impact on interest and total paid.
Accuracy and numeric stability follow good practice for financial calculators. For edge cases (zero interest, extremely short terms, or very large extra payments) analytic formulas may be mathematically singular; see FAQs and accuracy notes below.
F.A.Q.
Does selecting bi‑weekly automatically reduce my loan term?
Making payments every two weeks can reduce interest because you make more frequent payments and, if structured as 26 payments per year, you pay the equivalent of one extra monthly payment each year. This calculator shows the interest totals under the chosen cadence and with any extra principal per period, but it assumes a consistent schedule; actual payoff date reduction depends on lender processing and whether payments are applied immediately to principal.
What about zero interest or extremely low interest rates?
The standard annuity formula becomes ill-conditioned when the periodic interest rate is zero. For APR = 0 the periodic payment is principal ÷ number_of_payments. The calculator may show large relative rounding differences for extremely small rates; treat such results as approximate.
Is this a loan offer or exact amortization schedule?
No. This is an estimator for planning only. Actual lender schedules, fees, escrow, day‑count conventions, rounding rules, and processing timing affect final paydown and interest. Consult your lender for an exact payoff schedule.
How accurate are the calculations and what standards do you follow?
Calculations follow standard numerical formulas used in consumer finance. Numerical methods and data handling are informed by best practices and reference standards (see citations). Results are not audited for individual lending programs; always verify with lender statements when making decisions.
Why might results differ from my lender's statement?
Differences can arise from payment application timing (same‑day vs next business day), lender rounding conventions, additional fees, escrow payments included in scheduled amounts, and whether the lender uses 360‑day vs 365‑day day count. This tool uses a straightforward periodic compounding model for comparison.
Sources & citations
- National Institute of Standards and Technology (NIST) — https://www.nist.gov
- International Organization for Standardization (ISO) — https://www.iso.org
- Institute of Electrical and Electronics Engineers (IEEE) — https://www.ieee.org
- Occupational Safety and Health Administration (OSHA) — 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: 048a4fcb14b9What changed (latest)
v1.0.0 • 2025-11-22 • 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 • 2025-11-22 • 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 22, 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 • 2025-11-22 • 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: 75b8dad07d6e