How to Use

Use the calculators for mortgage scenario planning.

The ARM and Fixed Projection tools help you compare balance, scheduled principal and interest, total interest, property-tax cash flow, and extra-principal assumptions. They are built for budgeting and education, not lender-grade payoff quotes.

Open ARM Calculator Open Fixed Calculator View Methodology

Quick start

Use the calculator that matches the loan structure you want to model, then adjust assumptions in the left input panel. The chart and summary tables update as inputs change.

Choose the tool.Use the Fixed Calculator for a fixed-rate loan. Use the ARM Calculator for a loan with scheduled rate adjustments or payment recasts.

Enter loan basics.Set the original mortgage amount, current principal balance, mortgage start date, projection start date, first scheduled payment date, term, rate, and base monthly P&I.

Add tax and extra principal assumptions.Enter property tax amount and frequency, then add optional monthly extra principal or a one-time extra principal payment.

Select chart views and scenarios.Switch between remaining balance, cumulative interest, scheduled P&I, monthly cash outflow, cumulative principal, and cumulative extra principal views.

Review outputs.Use the chart, scenario summary table, rate schedule table, warnings, and exports to compare outcomes under the assumptions entered.

Fixed Projection Tool

What it models

The Fixed Projection Tool models a fixed annual interest rate over the remaining loan term. It compares a scheduled-only path against a path with additional principal payments.

Scheduled-only With additional principal

Best for

Estimating payoff timing under a fixed rate.
Comparing scheduled payments against extra-principal strategies.
Estimating interest saved from recurring or one-time extra payments.
Viewing property-tax cash-flow spikes alongside mortgage payments.

Confirm the base loan inputs.Use your current balance and projection start date so the output begins from the right point. Leave auto-calculate enabled unless you need to match a specific payment amount.

Enter the fixed rate.The fixed rate drives monthly interest accrual and the amortization path.

Use the extra-principal fields.Set monthly extra principal, a start date, optional end date, and optional lump-sum amount/date to test accelerated payoff scenarios.

Compare the two lines.The scheduled-only line ignores extras. The with-additional-principal line applies the extra-payment assumptions.

Export the results.Use Save View to store the current configuration as JSON, Download CSV for monthly rows, or Download PDF Chart for a shareable chart export.

ARM Projection Tool

What it models

The ARM Projection Tool models adjustable-rate scenarios using a first adjustment window, subsequent adjustment interval, rate floor, rate ceiling, rate increment, and optional payment recalculation at adjustment dates.

Worst case Neutral Best case

Best for

Estimating payment shock at the first ARM reset.
Comparing best, neutral, and worst rate paths.
Testing whether extra principal could reduce future recast payments.
Viewing reset markers and rate schedules over the modeled term.

Select the ARM structure.Use a preset such as 3/3, 5/1, 7/1, or 10/1. Choose Custom when the first window or subsequent interval does not match a preset.

Confirm the first adjustment date.Use the explicit first adjustment date when you need to match your loan documents. Otherwise, the tool can calculate it from the mortgage start date plus the first window.

Set rate limits and increment.Enter the floor, ceiling, and rate-change increment to define the best, neutral, and worst scenario paths.

Review the rate adjustment schedule.The schedule shows each modeled adjustment date and the rate used for each scenario.

Use scheduled-only toggles when needed.Scheduled-only counterparts use the same rate path and recast timing but ignore extra-principal assumptions.

Use Maintain Payment at Recast.This isolates the extra principal needed before the next recast to keep the projected payment near the current base P&I. It intentionally ignores the Additional Principal section.

The ARM scenarios are mechanical assumptions. Worst case increases by the increment at scheduled adjustments until the ceiling. Best case decreases by the increment until the floor. Neutral holds the initial rate flat while still allowing recast behavior when enabled.

Example uses

Test a monthly extra payment

Enter a monthly extra principal amount, set the start date, and compare the payoff date and cumulative interest against scheduled-only. This helps estimate whether a recurring extra payment meaningfully shortens the loan.

Model a lump-sum principal payment

Enter a one-time extra principal amount and payment date. Use the remaining-balance and cumulative-interest views to see how the lump sum affects the projected payoff path.

Budget for property taxes

Enter the tax amount and frequency to show tax-related cash-flow spikes. This affects cash-flow views only and does not change the amortization balance.

Estimate ARM payment shock

In the ARM tool, set the first adjustment date, ceiling, floor, and increment. Compare scheduled P&I across best, neutral, and worst cases around reset dates.

Plan before an ARM recast

Use Maintain Payment at Recast to estimate the lump sum or monthly extra principal needed before the next reset to keep the projected payment near the current base P&I.

Save and compare configurations

Use Save View to export a JSON configuration before changing assumptions. Load the saved view later to restore a scenario and compare it against a new one.

Reading the outputs

Chart

Select a metric, then hover over a line to inspect payment number, date, rate, scheduled P&I, interest, scheduled principal, extra principal, property tax, total cash outflow, ending balance, and cumulative totals.

Summary tables

Use the tables to compare payoff date, total interest, total principal, total extra principal, peak payment or cash outflow, final rate, and remaining balance by scenario.