Fidamen

Mortgage Payment Interest-Only Estimator

This estimator helps you understand monthly interest-only payments, the effect of an interest-only period followed by amortization, and the estimated total interest paid across both phases. It makes explicit assumptions and shows the sources of each calculation so you can validate the results against loan documents.

Use this tool to compare scenarios (short IO period vs longer IO period, or IO with a balloon) and to preview the payment shock when amortization begins. This tool does not replace lender disclosures or a formal amortization schedule provided by your lender.

Updated Nov 13, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record fb28bd4e7324 • Reviewed by Fidamen Standards Committee

Calculates interest-only payments for the IO period, then the amortized monthly payment required to fully repay the principal over the amortization period, and aggregates estimated interest.

Inputs

Advanced inputs

Amortization inputs

Results

Updates as you type

Monthly interest-only payment

$1,125.00

Monthly payment after IO (amortization)

$1,667.50

Estimated total interest (IO + amortization)

$267,749.23

OutputValueUnit
Monthly interest-only payment$1,125.00USD
Monthly payment after IO (amortization)$1,667.50USD
Estimated total interest (IO + amortization)$267,749.23USD
Primary result$1,125.00

Visualization

Methodology

Calculations assume interest-only payments during the interest-only period, meaning principal is not paid down during that phase unless stated otherwise.

Monthly interest-only payment is computed as principal multiplied by the monthly interest rate. For amortization after the IO period, the standard annuity amortization formula is applied to the remaining principal to compute the level monthly payment that fully repays principal over the amortization term.

This estimator follows best practices for software quality and data handling: use of scientific expression with explicit units, reproducible formulas, and recommendations for secure handling of personal data consistent with NIST and ISO guidance. Outputs are estimates and should be verified against lender-provided amortization schedules.

Worked examples

Example 1: Principal $300,000, APR 4.5%, IO 5 years, amortization 25 years. The tool will show monthly interest-only payments for 60 months and the monthly amortizing payment required thereafter, plus estimated total interest.

Example 2: Principal $200,000, APR 3.8%, IO 3 years, balloon option selected. The tool will show monthly interest-only payments and the outstanding principal that becomes due at the balloon maturity.

F.A.Q.

Does the tool include taxes, insurance, or fees?

No. This estimator calculates interest and principal repayment only. Taxes, insurance, homeowner association fees, lender fees, and escrow are not included and must be added separately.

What happens if the interest rate is zero or changes over time?

The formulas assume a fixed interest rate for the entire schedule. If the rate is zero the amortizing payment reduces to principal divided by months. For adjustable-rate loans or varying rates, results are approximate and you should request an amortization schedule from your lender that reflects rate adjustments.

How accurate are the estimates?

Estimates are based on standard mathematical formulas and are subject to rounding and the assumptions you provide. They are not loan offers. Check lender disclosures for precise figures. For guidance on secure handling and quality controls, see the referenced standards.

Is this tool compliant with regulations?

This estimator provides informational calculations only. It is not a substitute for legally required lender disclosures. Use lender documents for regulatory compliance and consult a qualified professional for binding advice.

Sources & citations

Further resources

Versioning & Change Control

Audit record (versions, QA runs, reviewer sign-off, and evidence).

Record ID: fb28bd4e7324

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: 6466fcb3d74e