Helping Home Buyers Make Smarter Decisions

Editorial Standards

Every calculator and guide on PrimeMortgageCalc follows a documented editorial process designed to produce accurate, unbiased, clearly explained financial information.

Who Writes Our Content

PrimeMortgageCalc content is produced and reviewed by people with backgrounds in mortgage lending, financial planning, residential real estate, and software engineering. Our contributors have worked inside the U.S. mortgage industry and are familiar with FHA, VA, USDA, conventional, jumbo, and non-QM loan products, as well as the closing process and RESPA/TRID disclosures.

Editorial contact: For corrections, methodology questions, or content suggestions, email [email protected]. For the full methodology behind our calculators, see How We Calculate. For the legal disclaimer covering our tools, see Disclaimer.

How We Make Money

PrimeMortgageCalc is funded by advertising and affiliate partnerships. We may earn a commission when visitors click on certain outbound links to lender or real estate partner sites, at no extra cost to you. This compensation does not influence our calculators, our rankings, or our editorial content. Advertisements are clearly labeled and visually separated from editorial content. See our Privacy Policy for details.

What PrimeMortgageCalc is

PrimeMortgageCalc is a free, browser-based set of mortgage calculators and explanatory guides. There is no account, no signup, no credit check, and no subscription, and the arithmetic runs on your device: the loan amount, price, rate, taxes, and insurance you type are processed in your own browser and are never sent to our servers. That is the whole product philosophy, and it explains most of the design decisions below.

The toolset covers the numbers a buyer, a homeowner, or a real estate professional actually needs to settle an argument. The mortgage calculator builds a full principal-and-interest schedule for any price, down payment, rate, and term, with taxes, insurance, PMI, and HOA added as separate lines. Around it sit the specialized calculators: affordability and debt-to-income for budgeting, down payment and PMI for the cash side, refinance for break-even analysis, extra payments and biweekly for payoff strategies, and program-specific calculators for FHA, VA, and USDA loans with their own premium and fee schedules. The guides, including the amortization guide, the extra payments guide, and this page's editorial documentation, exist to explain what the calculators are doing and where their outputs stop being useful.

What we are not is equally important. We are not a lender, a broker, or a lead generator: no figure you enter is transmitted to us, sold to anyone, or matched against your contact details, because we never receive it. We do not quote rates, lock rates, rank lenders, or accept payment for placement in any comparison. Every rate that appears on this site is an illustration, clearly bounded by the assumptions stated next to it, and the benchmark used throughout worked examples is 6.5% on a $400,000 loan over 30 years so that figures can be compared across pages. Corrections, methodology questions, and content suggestions go to [email protected], and the full formula documentation lives on the methodology page.

Why accuracy matters

Mortgage math looks small until it is multiplied by 360 months. The errors that matter are not dramatic; they are quarter-points, missing premium lines, and outdated program rules that quietly move a payment by a few hundred dollars a month.

• A quarter-point of rate is $65.40 a month and $23,545 over the loan. On a $400,000 loan at 6.5% the payment is $2,528.27; at 6.25% it is $2,462.87. A calculator that displays a payment rounded to the nearest $50 has already thrown away more money than most people negotiate off a rate.
• A missing or mispriced mortgage insurance line changes the budget decision. On a $380,000 loan at 95% loan-to-value, PMI at 0.45% is $142.50 a month and at 0.70% is $221.67, a $79.17 difference, $950 a year. A buyer who approves a payment without that line has approved a payment that does not exist.
• Getting the removal schedule wrong is a five-figure mistake. FHA mortgage insurance at 3.5% down lasts the life of the loan, while conventional PMI can be requested off at 80% loan-to-value. On a $386,000 FHA loan the premium is $273.42 a month; assuming it behaves like conventional PMI and ends after 11 years understates the cost by 19 years of payments, roughly $62,000.
• Strategy figures have to be reproducible. The $111,892 that $200 a month of extra principal saves on the benchmark loan is not a marketing number: it falls out of a month-by-month loop anyone can rebuild in a spreadsheet, and it should match to the dollar every time it is published.

This is why every figure on the site is derived from a documented formula, checked against an independent implementation, and dated when it was last reviewed.

Our editorial process, step by step

Every calculator and guide on this site passes through the same seven stages before it is visible, and returns to stage three on a schedule.

1. Scope the question and the governing rules. Before any code is written we identify what decision the tool must support and which authority governs it: HUD for FHA mortgage insurance and loan limits, the VA for funding fees and entitlement, USDA Rural Development for guarantee fees, the FHFA for conforming limits, and the CFPB for disclosure timing and consumer-facing definitions. Program-specific pages cite the publication they follow rather than restating a rule from memory.
2. Implement the published formula in the calculator component. The calculation is written from the source equation, not copied from another website: the standard amortization payment formula for fixed-rate tools, a month-by-month loop for schedules and prepayment, and program fee schedules applied as the agency publishes them. Assumptions that are judgment calls, such as default tax and insurance figures, are declared on the methodology page with the reasoning behind them.
3. Hand-check the output against the equation. A person sits down with the formula and a calculator and reproduces the result independently. For the benchmark case, $400,000 at 6.5% over 360 months, the expected payment is $2,528.27, month one splits into $2,166.67 of interest and $361.60 of principal, and the balance lands on $399,638.40. If the component disagrees with the hand check by even a cent, it does not ship.
4. Cross-verify against two independent references. The same inputs are run through a spreadsheet implementation of the formula and through at least one external reference point: a major lender's public calculator, a government agency's published table, or an industry-standard text. The methodology page publishes the resulting verification table so readers can repeat the check themselves.
5. Editorial and compliance review. Sarah Mitchell, Senior Mortgage Analyst, reviews the subject matter and the program rules; James Carter, Financial Content Editor and CFP, reviews educational content for accuracy and compliance. Reviews cover the arithmetic, the wording of disclosures, and whether any claim implies that we lend, quote, or advise.
6. Publish with its documentation attached. Every guide carries a visible last-reviewed date, a link to the methodology, and a link to the disclaimer, and every rate figure is labeled as an illustration with the assumptions stated beside it. If a number cannot be sourced, it does not appear.
7. Re-verify on a schedule and on any external change. Formulas are reviewed quarterly, calculator defaults are checked against conforming benchmarks quarterly, program fees are updated within 30 days of a published change, and loan limits are reloaded each January when HUD and the FHFA release the new figures. Corrections reported by readers are reproduced against an independent implementation and shipped in the next build, with a dated note on the affected page when the change is material.

How our calculators are calculated

Every fixed-rate calculator on the site uses the same equation that lending systems and spreadsheets use. The monthly payment is the amount that leaves the balance at exactly zero after the final scheduled payment:

M = P × [r × (1 + r)ⁿ] / [(1 + r)ⁿ - 1], where r is the annual rate divided by 12 and n is the number of monthly payments.

Worked example: $400,000 at 6.5% for 30 years

Step 1: r = 6.5% / 12 = 0.00541667 and n = 360. Step 2: (1 + r)³⁶⁰ = 6.991798. Step 3: r × 6.991798 = 0.0378722, and (1 + r)³⁶⁰ - 1 = 5.991798. Step 4: 0.0378722 / 5.991798 = 0.00632068. Step 5: $400,000 × 0.00632068 = $2,528.27, the payment the calculator displays.

Step 6: the first payment charges $400,000 × 0.00541667 = $2,166.67 of interest, so principal is $2,528.27 - $2,166.67 = $361.60 and the balance becomes $399,638.40. Step 7: that loop repeats 359 more times; total payments are 360 × $2,528.27 = $910,178, of which $510,178 is interest on $400,000 of principal. Step 8: the same equation with n = 180 returns $3,484.43 for a 15-year term and $227,197 of interest, which is the check that the term, not a rate change, produced the difference.

Three extensions cover the rest of the toolset. Biweekly mode divides the payment by two and applies 26 half-payments a year, which is 13 full payments. ARM scenarios recompute the payment at each adjustment date using the new rate and the months remaining on the original term. Taxes, insurance, PMI, and HOA are never part of this equation: they are user inputs with declared defaults of $3,500 a year for property tax and $1,350 a year for insurance, added as separate lines so the loan math stays auditable. Calculations run at full double precision and round only for display, so a schedule's visible columns may differ from a running total by a few cents while the payment line matches a lender's to the cent.

Verification and review in practice

The table below is the working verification log for the benchmark cases that appear across the site. Each row is computed by the shipped component and independently by hand or spreadsheet; any row that fails is blocked from release.

Verification cases and expected results

The second table is the maintenance schedule. Nothing about mortgage rules is static, so each content type has a named trigger and cadence rather than a general promise to "keep things updated."

Review cadence by content type

Our standards for 2026

• 2026 loan limits are loaded and dated. The conforming baseline for a single-family home is $832,750 with a $1,249,125 high-cost ceiling, announced by the FHFA on November 25, 2025, and FHA limits run from a $541,287 floor to the same $1,249,125 ceiling for case numbers assigned on or after January 1, 2026 under Mortgagee Letter 2025-23, published December 11, 2025.
• The 6.5% benchmark on $400,000 remains the illustrative default for 2026, chosen because it sits inside the range of conforming 30-year fixed rates observed across recent years. It is labeled as an illustration everywhere it appears, and it is not a quote.
• Methodology version 2.0 was last updated September 8, 2026, which is also the review date carried by the guides published on this site. Rounding conventions, the verification table, and the corrections policy were added in that revision.
• Program fee schedules are re-checked within 30 days of publication. For 2026 that includes VA funding fee tiers of 2.15% first use with no down payment, 1.50% at 5% down, and 0.5% for an IRRRL, along with FHA's 1.75% upfront premium and USDA's 1% upfront and 0.35% annual guarantee fee.
• Rate figures are re-reviewed quarterly and never presented as live data unless a page explicitly labels them with a source. Any page showing a rate shows the assumptions next to it.
• Every guide publishes a last-reviewed date in its header, so a reader can tell in one glance whether a figure was checked this quarter or last year.
• Corrections are dated and shipped in the next build. Reports to [email protected] are reproduced against an independent implementation before we answer, and material fixes carry a note on the affected page.

Common mistakes when using calculator output

• Treating an illustrated rate as a quote. The 6.5% benchmark produces $2,528.27 on $400,000, but your rate depends on credit, loan-to-value, property type, and lock period. Enter your actual quote before comparing offers.
• Keeping the default tax and insurance figures. The defaults of $3,500 and $1,350 a year are national approximations. A county charging 1.8% of value on a $400,000 home bills $7,200, which is $300 more a month than the default shows.
• Comparing principal and interest instead of the full payment. Taxes, insurance, PMI, and HOA routinely add $500 to $900 a month. Budget and ratio decisions have to use the complete figure, which is why the calculators display the components separately.
• Assuming the default premium equals your pricing. PMI defaults to a mid-range 0.45% to 0.5% and FHA to the published schedule; your credit band and starting loan-to-value move the real number by hundreds of dollars a month.
• Reading a result as an approval. Nothing here runs underwriting: no credit check, no employment verification, no reserve test, no appraisal. A lender's decision can differ from any figure on this site for reasons that never appear in an input field.
• Using the annual rate as the monthly rate. 6.5% a year is 0.541667% a month. Any spreadsheet a reader builds should divide by 12 first, then compound, and the payment formula on this site shows both steps explicitly so the arithmetic can be checked line by line.

Frequently asked questions

Is PrimeMortgageCalc really free? Yes. Every calculator and guide is free with no account, no credit check, and no subscription. The site is funded by advertising and affiliate partnerships, which are clearly labeled and separated from editorial content, and no payment is tied to which product a visitor chooses.

Do you collect the numbers I enter? No. Calculations run locally in your browser, so loan amount, price, rate, taxes, and insurance never reach our servers and are never sold or shared. We do not sell leads, and nothing you type is transmitted to a lender or a broker. See the privacy policy for the full detail.

How do you verify a calculation before publishing it? Each result is computed by the shipped component, reproduced by hand from the published formula, and cross-checked against a spreadsheet implementation plus an independent external reference. The benchmark case of $400,000 at 6.5% must return $2,528.27 with a first payment of $2,166.67 interest and $361.60 principal before a page ships.

How often is the content updated? Formulas and default assumptions are reviewed quarterly, program fee schedules within 30 days of any published agency change, and loan limits each January when HUD and the FHFA release the new figures. Every guide carries a visible last-reviewed date; the current cycle is September 8, 2026.

Why does my lender's number differ from yours? Usually the inputs differ. Lenders price to your credit, loan-to-value, occupancy, and lock period, and their escrow estimate uses your county's tax bill and your insurance quote rather than national defaults. Load the rate, taxes, and insurance from your Loan Estimate and the payment should match to the cent.

Do you recommend lenders or rank loan products? No. We do not originate, quote, or lock anything, we do not rank lenders, and no lender pays for placement. The comparisons on this site are structural: program rules, fee schedules, and arithmetic that any borrower can reproduce independently.

How do I report a number that looks wrong? Email [email protected] with the page URL, the exact inputs you used, our result, and your reference result. Every report is reproduced against an independent implementation before we reply, confirmed errors ship in the next build, and material corrections are dated on the affected page.

Are the guides written by licensed professionals? Content is produced and reviewed by people with backgrounds in mortgage lending and financial planning, including a Senior Mortgage Analyst and a CFP who reviews educational content for accuracy and compliance. The material is general information, not personalized financial advice, and the disclaimer covers the limits of that.

Related calculators and guides

• Mortgage Calculator - payment, full amortization schedule, and the component breakdown behind every figure on this site.
• How We Calculate - the formulas, the defaults, the verification table, and the corrections policy.
• Amortization Guide - how each payment splits between principal and interest, with the worked $400,000 example.
• Affordability Calculator - the 28/36 ratios applied to your income and debts.
• Refinance Calculator - break-even month for a new rate against your closing costs.
• Disclaimer - what these tools are and what they are not, in plain language.