Fidamen

Personal Loan Variable Rate Calculator

This calculator helps you estimate how a personal loan with a variable interest rate may behave when the index changes. It models typical adjustable-rate mechanics: index + margin, periodic adjustment limits, lifetime caps, optional initial fixed periods, and interest-only options.

Use the results for planning and comparison. Outputs are estimates based on the inputs and modeling assumptions. They do not replace lender disclosures or legally binding loan documents.

Updated Nov 11, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 523b4fff1bc7 • Reviewed by Fidamen Standards Committee

Standard amortizing loan where payments are calculated on the projected interest rate after applying index, margin, periodic cap, and lifetime cap.

Inputs

Advanced inputs

Interest-only options

Results

Updates as you type

Payment per period

—

Projected post-adjustment APR

7.50%

Total payments

60

OutputValueUnit
Payment per period——
Projected post-adjustment APR7.50%%
Total payments60payments
Primary result—

Visualization

Methodology

Rates are modeled as index + margin. After an adjustment, the uncapped new rate is computed as index + margin. Periodic caps and lifetime caps are applied in sequence to limit changes. Periodic caps restrict movement per adjustment; lifetime caps limit total change versus the starting rate.

Payments for amortizing schedules use standard installment math: payment = P * r / (1 - (1+r)^-n). Interest-only payments are calculated as principal * (periodic rate). This tool assumes compounding aligned with the payment frequency.

Modeling and controls adhere to secure development and data integrity practices recommended by NIST SP 800-series for input validation and by ISO/IEC software quality standards for transparency. Results are informational and should be checked against official loan statements.

Worked examples

Example 1: $10,000 principal, 5-year term, monthly payments, starting APR 7.5%, index 3.0%, margin 4.5%, periodic cap 2%, lifetime cap 6% → tool returns projected APR after cap and estimated monthly payment.

Example 2: Interest-only option for first 12 months: tool returns the lower IO payment during the IO period and the higher amortizing payment once principal amortization begins.

F.A.Q.

Is this calculation guaranteed to match my lender's statement?

No. This tool provides an estimate based on input assumptions and a generalized cap model. Lender-specific adjustments, rounding rules, fee structures, and contract language may produce different results. Always consult your lender's periodic rate notices and the loan agreement.

What is an index and how often does it change?

An index is a published benchmark interest rate (for example, a policy rate or overnight rate). Frequency of change depends on the index and your loan contract. This calculator accepts an index rate input for the next adjustment and applies caps according to your inputs.

How should I use the periodic and lifetime cap fields?

Periodic cap limits how much the rate may change at a single adjustment. Lifetime cap limits the total deviation from the initial APR across the loan's life. Enter the percentage caps as stated in your loan documents.

Are regulatory and security standards considered?

Yes. Development and input validation recommendations reference NIST and ISO guidelines. This tool does not provide legal or financial advice and is for planning only.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: 523b4fff1bc7

What changed (latest)

v1.0.0 • 2025-11-11 • 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 11, 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 • 2025-11-11 • 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: e47973040ce4