Sources and Formula Changelog

Last reviewed: July 11, 2026.

This page identifies the main external sources used for MortgageCalculatorPlus.com calculations and educational content and records material changes to calculator meaning or formulas. A source listed here does not endorse ARSIDIAN LLC or the Website.

Mortgage-rate reference

Freddie Mac Primary Mortgage Market Survey (PMMS). We use the official Freddie Mac PMMS as the reference for a national weekly fixed mortgage rate when a market default is needed. Freddie Mac calculates the average from qualifying mortgage applications submitted through Loan Product Advisor by lenders across the country. The current methodology focuses on conventional, conforming, fully amortizing purchase applications for an owner-occupied single-unit property, generally with a 75%–80% loan-to-value ratio and a borrower FICO score of at least 740. Freddie Mac's methodology note describes the selection criteria and limitations.

This is a national weekly average for a defined profile, not a personalized quote. It does not promise that a borrower can obtain that rate and is not a lender commitment. It may not represent a refinance, government-backed loan, investment property, second home, jumbo loan, borrower with a different credit profile, or loan with a different down payment. PMMS also does not publish average points and fees under its current methodology. Actual rates can change with credit, loan-to-value, product, property, location, points, fees, lender, and market timing.

Federal Reserve Economic Data (FRED). For historical display or retrieval, we may use the Federal Reserve Bank of St. Louis MORTGAGE30US series. FRED republishes the Freddie Mac PMMS series; it is a distribution source, not a separate personalized-rate source. We retain the observation date with the cached default and do not describe it as a live lender quote.

Consumer mortgage disclosures

Consumer Financial Protection Bureau (CFPB). We use the CFPB's explanations of the difference between a mortgage interest rate and APR and its Loan Estimate explainer for consumer-facing terminology. Our APR calculator is an educational cash-flow estimate. A lender determines which charges must be included in its official disclosures.

Loan limits and federal programs

Federal Housing Finance Agency (FHFA). Conforming loan limits are taken from FHFA's official conforming loan limit datasets. County-level values and the applicable calendar year should be used when a state contains both baseline and high-cost areas.

U.S. Department of Housing and Urban Development (HUD). FHA limits and program references are checked against HUD's FHA mortgage limits lookup and current HUD guidance. FHA and conforming limits are different concepts and should not be substituted for one another.

Housing, property-tax, and insurance data

U.S. Census Bureau American Community Survey (ACS). State calculator defaults use the 2024 ACS 1-year median value of owner-occupied housing units (B25077) and median real-estate taxes paid (B25103). The displayed property-tax proxy is B25103 divided by B25077. It is a ratio of statewide medians, not a statutory rate, parcel-level effective rate, or prediction of a specific bill. ACS values are survey estimates with margins of error; they are not current listings, sale prices, appraisals, or forecasts.

National Association of Insurance Commissioners (NAIC). State pages use the statewide HO-3 total average premium from Table 4 of the NAIC report Dwelling Fire, Homeowners Owner-Occupied, and Homeowners Tenant and Condominium/Cooperative Unit Owners Insurance: Data for 2022. This historical average combines different coverage amounts, deductibles, homes, risks and carriers. It is not inflation-adjusted to the current year, does not represent every separate hazard policy, and is not an insurance quote.

Tax information

Internal Revenue Service. General explanations involving home mortgage interest refer to the current IRS Publication 936, Home Mortgage Interest Deduction. Our amortization tool estimates interest paid. It does not determine deductible interest, whether a taxpayer itemizes, the applicable debt limit, or tax savings.

State housing programs

Official state housing finance agencies (HFAs). Program names, eligibility summaries, assistance amounts, and links should be verified on the responsible agency's official site. The National Council of State Housing Agencies directory is used to locate state agencies, but the individual HFA remains the primary source for current program terms. Users should confirm funding availability and eligibility directly with the agency or an approved participating lender.

How we handle source dates

Time-sensitive values are stored or displayed with an observation year or date where the page design permits.
A weekly rate is not converted into a claim about today's personalized pricing.
Annual loan-limit and ACS releases retain their relevant calendar year or vintage.
If a primary source revises its methodology or data, we may update affected pages and distinguish that refresh from correction of a Website error.
Property tax, insurance, and housing-program information can vary within a state; broad defaults are labeled as estimates and should be replaced with local figures.

Formula changelog

July 11, 2026 — calculation and disclosure update

Main purchase calculator: expanded the monthly result from principal and interest to a transparent housing-payment estimate that includes property tax calculated from home price, homeowners insurance, PMI, and HOA. PITI and additional charges are identified separately.
Mortgage-amount pages: pages labeled as a “$N mortgage” now use $N as loan principal rather than treating it as a home price and automatically reducing it by a 20% down payment.
APR calculator: replaced a term-independent fee markup with a cash-flow calculation. The new method calculates amount financed after entered points and finance charges, then solves for the monthly rate whose scheduled-payment present value equals that amount. APR therefore changes appropriately with term.
Mortgage interest tool: changed the output and language from a claimed tax deduction or tax savings to estimated mortgage interest paid. The tool no longer implies that amortization interest is automatically deductible.
Affordability and income scenarios: changed the central result from a loan-only amount to a home-price scenario that reflects down payment and displayed PITI-related assumptions. Low-value salary-page inventory was reduced separately; that routing change does not alter the formula.
Rate disclosure: documented the Freddie Mac PMMS default as a dated national weekly average for its defined profile, not as a personalized quote or lender offer.
Public documentation: published the calculator methodology, validation examples, source register, and this formula changelog.

This is the first public formula changelog for the current static version of the Website. We have not reconstructed a version-by-version history before July 11, 2026. Future material formula changes will be added here with their effective date; routine market-data refreshes may update an observation date without receiving a separate formula entry.

Questions and corrections

For the formulas themselves, see our Calculator Methodology. To report an incorrect value, broken source, or reproducibility problem, follow the Corrections Policy and include the affected URL and inputs.