Fidamen

Personal Loan Interest-Only Calculator

This calculator estimates monthly interest-only payments, the outstanding principal after an interest-only period, and the fixed amortizing payment that would apply if the remaining balance is repaid over the rest of the loan term. Use it to compare scenarios or to validate lender quotes.

Results assume a fixed nominal interest rate, periodic compounding at the payment frequency you specify, and level (fixed) amortizing payments when amortization is selected. Values are estimates for planning purposes only and do not replace lender disclosures.

Updated Nov 8, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record bcb57748454d • Reviewed by Fidamen Standards Committee

Calculates interest-only payments for the initial period, then computes the fixed amortizing payment for the remaining term (standard level-payment amortization).

Inputs

Results

Updates as you type

Monthly interest-only payment (initial period)

$41.67

Monthly amortizing payment after interest-only period

$299.71

Remaining amortization months

36

Total interest paid (all periods)

$1,789.52

Total amount paid (principal + interest)

$11,789.52

OutputValueUnit
Monthly interest-only payment (initial period)$41.67USD
Monthly amortizing payment after interest-only period$299.71USD
Remaining amortization months36—
Total interest paid (all periods)$1,789.52USD
Total amount paid (principal + interest)$11,789.52USD
Primary result$41.67

Visualization

Methodology

Monthly or periodic interest-only payment is calculated as principal multiplied by the periodic nominal rate (annual rate divided by payments per year).

When amortization follows an interest-only period, the remaining balance (equal to the original principal under interest-only) is repaid using the standard level-payment amortization formula. If the periodic rate is zero, the amortizing payment is calculated as remaining principal divided by remaining months.

This tool is developed with quality and numeric accuracy practices in mind. Numeric operations follow IEEE 754 floating-point conventions where supported; development and testing follow quality management principles aligned with ISO 9001, and security best practices reference NIST guidelines. Results are for estimation only; always compare with lender-provided amortization schedules and disclosures.

Worked examples

Example 1: $10,000 loan, 5% annual, 2-year interest-only, 5-year total term, monthly payments. Interest-only monthly ≈ $41.67. Remaining 36 months amortize balance of $10,000 — monthly amortizing payment ≈ $299.71.

Example 2: Zero nominal interest. If annual rate is 0% and an amortization period exists, amortizing payment = principal / remaining months.

F.A.Q.

Does this calculator include fees or APR?

No. This estimator uses the nominal interest rate you enter and does not add origination fees, insurance, or other lender fees. APR (annual percentage rate) includes certain fees and compounding and may differ from the rate used here.

What happens after the interest-only period ends?

If you select 'interest-only then amortize', the calculator computes a fixed payment that repays the outstanding principal over the remaining scheduled months. If you select 'interest-only only', the balance remains outstanding at the end of the period (no amortization is computed).

How accurate are the results?

Calculations follow standard financial formulas and IEEE floating-point conventions, but rounding and lender-specific conventions (daily interest accrual, payment application rules, escrow requirements) can produce small differences. Use results for planning and verify with lender disclosures.

Can periodic compounding differ from payment frequency?

This calculator assumes compounding occurs at the payment frequency you select (payments per year). If a lender uses different compounding or accrual rules, results may differ.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: bcb57748454d

What changed (latest)

v1.0.0 • 2025-11-08 • 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 8, 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 • 2025-11-08 • 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: 7c8eeb582b13