Any Month Mortgage

How we calculate

Every calculator on the site uses one calculation engine. This page explains what it does, how it rounds, and where each program rule comes from, so you can check any number yourself.

The payment and the schedule

A fixed-rate loan is repaid in equal payments. The payment is:

M = P × r ÷ (1 − (1 + r)−n)

where P is the amount borrowed, r the interest rate per payment (the yearly rate ÷ 12 for monthly payments) and n the number of payments. At a 0% rate it's simply P ÷ n. Each payment first pays the interest owed on the balance for that period, and the rest reduces the balance.

Rounding: whole cents, like a lender

The schedule is worked in whole cents. The payment is rounded to the cent, each period's interest is rounded to the cent, and the final payment takes whatever is left so the balance lands on exactly $0.00. Every column of the schedule therefore adds up exactly to its total. Many calculators keep fractions of a cent and only round for display, so their columns don't quite match their totals.

Extra payments

Extra principal, whether every payment or on one chosen payment, is taken off the balance straight away, so it stops costing interest from the next period. The required payment stays the same and the loan ends sooner. The savings shown compare the same loan paid exactly as scheduled. The payoff calculator finds the smallest extra amount, to the cent, that pays the loan off by the date you pick. It searches the same schedule rather than using an approximate formula, so the answer is exact even with one-time extras in the mix.

Dates, and what you owe now

With a first-payment month, each payment gets a date: the 1st of each month for monthly payments, the 1st and 15th for twice a month, and every 14 days for biweekly. What you owe now is the balance after the latest payment due by today's date on your device. It's the balance on the loan's schedule; a servicer's payoff amount adds interest from the last payment to the payoff day, and any fees, so it's usually a little higher.

Payment frequency

Adjustable rates (ARMs)

The rate stays at its starting level for the fixed period, then moves at each adjustment toward the rate you expect (or as far up or down as the caps allow, for the worst and best cases). The first adjustment is limited by the initial cap, later ones by the periodic cap, and the rate never goes above the starting rate plus the lifetime cap or below the floor. Whenever the rate changes, the payment is recalculated on the balance actually owed, over the time left, as lenders do. The ARM presets use Fannie Mae's standard caps for SOFR ARMs: 2/1/5 for a 5/6 ARM, and 5/1/5 for 7/6 and 10/6 ARMs.

Interest-only and balloon loans

An interest-only loan charges just the interest for its interest-only period, then repays the whole balance over the years left, which is why its payment jumps. A balloon loan's payment is figured over the full term, and whatever is still owed at the balloon date is due in one lump sum.

PMI on conventional loans

With less than 20% down, private mortgage insurance is charged at the yearly rate you enter, on the original loan amount. It ends following the federal Homeowners Protection Act:

FHA, VA and USDA fees

These figures change by policy letter and fiscal year. They're kept in one dated place in the code and checked against the official sources; the date is shown on each program's calculator.

APR

The APR is the yearly rate at which the loan's scheduled payments (principal and interest, any balloon, and mortgage insurance) are worth exactly the amount financed. That's the loan less the finance charges paid at closing: discount points, the lender fees you enter, and an FHA, VA or USDA upfront fee (when it's added to the loan, the payments repay it, so it counts too). It's the actuarial method of Regulation Z, the federal Truth in Lending rule (12 CFR 1026.22 and its Appendix J), solved exactly rather than from a table. Extra payments and an early request to cancel PMI aren't part of the loan's terms, so they don't change it. For an ARM the APR follows the rate path you choose, so it's an estimate; a lender discloses one based on the index at closing. A lender's APR can also differ with which fees it counts.

The points calculator compares quotes by each one's position every month (points, plus everything paid, plus what's still owed), as the refinance calculator does, so its break-even counts the faster principal paydown a lower rate brings.

Equity and today's dollars

Below the schedule, the calculator shows your equity at milestone years: the home's value, growing at the rate you assume, less what you'd still owe. It also shows everything paid over the loan in today's dollars: each payment is discounted by the inflation rate you assume, from the start of the loan to when it's paid, and what's paid at closing counts in full. On the $320,000, 6.5% example, the $728,141 paid over 30 years is worth about $548,600 in today's dollars at 2% inflation. A fixed payment gets cheaper in real terms every year. Both rates are assumptions you set, not forecasts.

The tax deduction estimate

With a filing status chosen, the calculator estimates the federal income tax the home saves each year. For each loan year (the first taken as 2026), it adds up what owning lets you itemize:

The saving is what those add over the larger of the standard deduction ($16,100 single, $32,200 married filing jointly, $24,150 head of household in 2026) and what you'd itemize anyway, times your tax bracket. In the 37% bracket each deducted dollar is worth about 35 cents under the 2025 law. The figures come from the IRS's 2026 inflation adjustments (Rev. Proc. 2025-32), Public Law 119-21 and IRS Publication 936, checked on 21 September 2026.

It's an estimate: federal only, one tax rate for every dollar, no alternative minimum tax, no extra amounts for age 65 or blindness, 2026's standard deduction used for later years, and a loan that qualifies (secured by your home and used to buy it). It isn't tax advice; check with a tax professional.

Affordability

The affordability calculator finds the highest price whose full monthly housing cost fits both limits you set: housing as a share of gross income (front-end ratio), and housing plus other debts (back-end ratio). The housing cost is the loan's actual first payment from the same schedule, including property tax, insurance, PMI or FHA insurance and HOA dues. The price is also limited by the program's minimum down payment: 3% for conventional loans and 3.5% for FHA. The answer is found by a search, to the dollar.

Refinancing

The refinance calculator compares keeping your loan with replacing it by each loan's position every month: everything paid so far plus what's still owed, plus closing costs paid in cash and minus any cash taken out. It breaks even the first month the refinance's position is lower. Unlike the common shortcut (costs ÷ monthly saving), this counts how fast each loan pays down principal. It also shows when a longer new term makes the refinance fall behind for good. The comparison covers principal and interest, since tax and insurance don't change with a refinance.

What the results leave out

Real loans also have lender fees, prepaid interest, escrow cushions and exact payment dates, and each lender has its own rounding and cutoff rules. The results are close estimates for planning, not a loan estimate or an offer. Your lender's Loan Estimate and Closing Disclosure are the official figures for your loan.

Tested

The engine has an automated test suite that runs before every release. It checks, for example, that every schedule pays off exactly what was borrowed, that the FHA table matches HUD's, and that PMI ends when the law says it must. Spotted something that looks wrong? Tell us.