Loan Amortization & Mortgage Calculator
Loan Amortization & Mortgage Calculator: Calculates fixed monthly installment M = P * [r(1+r)^n] / [(1+r)^n - 1].
About this loan amortization & mortgage calculator
Loan Amortization & Mortgage Calculator — browser-based utility.
How this tool works
Calculation and parameter relationship: r=annualRate/1200; payment=principal×r/(1−(1+r)^(−months)); r=0: payment=principal/months
- Select “Raw JSON / Advanced” and paste the sample input.
- Click “Run locally”.
- Compare “Raw Output Data” with the example; revise inputs when an error is shown.
Worked example
Scenario: Compute a fixed monthly payment for a 36-month loan and inspect the aggregate interest.
Sample input:
Processing: Convert annualRate to a monthly rate, then use principal × rate / (1 − (1 + rate)^−months), evaluated with log1p/expm1; a zero rate uses principal/months.
Illustrative output:
Limits and verification
Parameter domains and units: principal,annualRate ≥ 0; months ∈ ℤ ∩ [1,1200]; [annualRate] = %/year; [monthlyPayment] = currency/month. Results use binary64 floating-point numbers unless BigInt is stated. Result cards show up to 10 significant digits; raw output retains the serialized value. A null ratio means the model has no meaningful finite value for that ratio; it is not zero.
Examples demonstrate an expected workflow; they do not prove every input or every branch of an external specification. Check important results with an independent source before using them for money, security, compliance, safety, or irreversible file changes.
Browser processing boundary
Tool input is processed by code running in the browser and is not intentionally sent to a CZOA processing API. The page can still request ordinary site assets, analytics, or advertising when those services are enabled. Browser extensions and managed-device software remain outside this tool's control.
Relevant references
These references govern or help explain the format, protocol, or calculation used here. Listing a reference does not claim certification or complete implementation of every optional feature.
- CFPB Regulation Z, Appendix J — Closed-End Credit APR Computations
- ECMAScript 2026, §6.1.6.1 Number Type
Content owner: CZOA Tools · Last reviewed: 2026-09-15 · Review methodology
How to use it
- Select “Raw JSON / Advanced” and paste the sample input.
- Click “Run locally”.
- Compare “Raw Output Data” with the example; revise inputs when an error is shown.
Frequently asked questions
How does the loan-payment calculator compute monthly payment?+
It converts annualRate to annualRate/1200, uses principal and months, and calculates a level payment with a log1p/expm1-stable amortization formula. At zero rate it divides principal by months.
What inputs does the page accept?+
The form and Raw JSON / Advanced mode accept principal, annualRate, and months. The verified input is {"principal":10000,"annualRate":6,"months":36}.
What limits and interpretation apply?+
Months must be an integer from 1 through 1200. Zero, negative values, decimals, and values above 1200 are rejected. Results are JavaScript Number binary64 values. The tool returns aggregate payment and interest, not a payment schedule, taxes, insurance, fees, or lending advice.
What result does the worked example produce?+
For {"principal":10000,"annualRate":6,"months":36}, the tool returns monthlyPayment 304.21937451555124, totalPaid 10951.897482559845, and totalInterest 951.8974825598452.