Biweekly mortgage calculator: monthly vs half-payments every payday

Paid every two weeks, you get 26 paychecks a year. Paid monthly, your mortgage takes 12 payments a year. Half a mortgage payment out of every paycheck is 26 halves, which is 13 whole payments. That thirteenth payment is the entire trick, and it is a good one.

What the calculator does

Put in your current balance, your interest rate, and the principal-and-interest part of your payment (the P&I line, not the whole thing with taxes and insurance). The calculator amortizes the loan two ways:

• Twelve payments a year, the way the loan was written.
• Twenty-six halves a year, with the thirteenth payment landing as two extra halves, one in each of the two months a year that carry a third payday, applied entirely to principal.

The difference between the two columns is what biweekly is worth on your loan: interest you will not pay, and months you will not owe.

On a new $300,000 loan at 6.5%, the P&I payment is $1,896. Paid monthly, the loan runs 30 years and costs about $382,600 in interest. Paid as halves, it ends about 5 years and 9 months early and costs about $296,900. That is roughly $86,000 of interest that stays in your pocket, in exchange for one extra payment a year that you were going to have anyway, because two of your months have three paychecks.

Why the extra halves go to principal

A mortgage payment has two parts. Interest is what the lender charges for the month on the balance you owe. Principal is what actually shrinks the balance. Early in a loan most of the payment is interest, so anything extra you pay above the required amount goes straight to principal, and next month's interest is calculated on a slightly smaller balance. Do that every year and the effect compounds in your favor.

The thirteenth payment is pure extra. Nothing about it is owed, so all of it reduces the balance, and every later month's interest is smaller because of it.

The suspense account, and why it matters

Here is the part most biweekly explanations skip. When you send half a payment two weeks before the due date, most servicers do not apply it. They park it in an unapplied-funds account, sometimes called suspense, and wait for the second half. When both halves are in, they apply one full payment on the due date, exactly as if you had paid monthly.

Two things follow from that:

1. You do not save interest from paying early. The balance is not reduced when the first half arrives; it is reduced when the full payment is applied. Anyone telling you biweekly saves money "because interest is calculated more often" is describing a loan you do not have.
2. The whole benefit is the thirteenth payment. In a three-paycheck month, the third half arrives with no payment left to complete. What the servicer does with it depends on their rules and your instructions. Many apply it to principal when it accumulates to a full payment; some apply it only if you ask; a few will treat it as an early next-month payment unless you tell them otherwise.

So before you start, ask your servicer one question in writing: "When I have paid more than my monthly payment in a month, how do you apply the extra, and how do I make sure it goes to principal?" The answer decides whether biweekly does anything at all on your loan. Ample models this exactly the way the servicer describes it, not the way a brochure does.

Do it yourself, for free

You do not need a program. There are three ways to get the thirteenth payment, and all three produce the same amortization:

• Halves every payday, if your servicer accepts partial payments and applies the extra to principal.
• One extra principal payment a year, sent in whichever month is easiest, marked "apply to principal."
• One-twelfth extra every month: divide your P&I payment by 12, add that to each monthly payment, and mark the extra as principal. On a $1,896 payment that is $158 a month.

Companies sell biweekly "programs" with an enrollment fee and a charge per debit. They draft your account every two weeks, hold your money, and forward one payment a month to your servicer, plus the extra once a year. The math is identical to doing it yourself, minus the fees. If your servicer offers a free biweekly draft with extra-to-principal, that is fine; if anyone wants money for the service, decline.

When biweekly is the wrong move

The thirteenth payment is a good use of a third paycheck if the rest of your month is already covered. It is a poor use if:

• You carry a card balance at 20% or more. Extra dollars save more there than on a 6% mortgage, every time.
• Your checking buffer is thin. A prepayment you cannot take back is worse than a buffer you can spend.
• You are close to selling or refinancing. Interest saved accrues over years; a loan that ends in two does not have years.
• Your loan has a prepayment penalty. Rare on recent loans, but read the note.

Half payments and escrow

If your servicer drafts halves, they usually draft half of the total payment, escrow included, because the tax and insurance bills do not care how you are paid. The calculator's half-payment figure adds your escrow if you enter it. Only the P&I half does anything for the balance; the escrow half is passing through.

What Ample does with this

Ample knows your payday calendar, so it knows which two months carry the third half and plans them ahead. The Home engine runs this same amortization on your real balance, shows the monthly and biweekly columns side by side, and flags the month the extra half will land so it never surprises the buffer. When the third half puts you at 80% loan-to-value, it drafts the letter to drop mortgage insurance, too.