Accelerated payments: what the extra payment actually does

A plain bi-weekly payment takes your annual cost and divides it by twenty-six. You pay the same money over a year that you would monthly, in smaller and more frequent instalments. The saving is real and it is tiny: interest accrues on a slightly lower average balance because you are paying a little sooner within each month.

An accelerated bi-weekly payment is calculated differently. It takes your *monthly* payment, halves it, and charges that twenty-six times. Twenty-six halves is thirteen monthly payments, not twelve — so you pay one whole extra monthly payment every year, and every dollar of it goes to principal.

That is the entire mechanism. It is not clever, it is not a compounding effect, and it is not something the lender is giving you. It is a decision to pay about 8% more each year, arranged so that it does not feel like one.

The table below runs the same mortgage three ways. Plain bi-weekly moves the same money as monthly and saves about a thousand and a half in interest across the whole amortization. Accelerated bi-weekly moves one extra payment a year and saves tens of thousands, and takes years off the payoff date.

Both are worth doing if you can. But they are not the same decision, and describing them both as "bi-weekly saves you money" — which is how they are almost always described together — hides the fact that only one of them requires you to find more money. If your budget is tight, plain bi-weekly costs you nothing and buys you almost nothing; that is a fair trade and it should be made knowingly.

There is a second reason to know which one you are on: the prepayment privileges. Lenders apply the payment-increase percentage to the *non-accelerated* payment, dividing the acceleration back out first — so a borrower who assumes the percentage applies to the payment they actually make overstates their allowance by about 8%.