Biweekly vs monthly, explained

The calendar math

There are 52 weeks in a year (plus a leftover day). A true biweekly plan takes half of your regular P&I every two weeks:

• 26 half-payments = 13 full monthly payments
• You were only required to send 12
• The 13th payment is extra principal

That extra is about 1/12 of your monthly P&I, every month, if you prefer to think in monthly drafts. On a $2,212 payment, the biweekly-equivalent extra is about $184. AmortSheet’s calculator has a button that fills that amount in for you.

Why the loan ends years sooner

The extra payment is applied to principal, so the balance that next month’s interest is charged on is smaller. Over 30 years, one extra P&I per year typically knocks four to six years off a conventional fixed-rate loan — the exact number depends on rate and how early you start. The interest saved is the interest that would have been charged on those retired dollars.

Nothing about the biweekly calendar itself changes the rate. If you sent the same 13th payment as a single extra in December, and the servicer applied it the same day, the economics would be nearly identical.

Three products that get called “biweekly”

1. True biweekly (rare at large servicers)

Half the P&I is drafted every 14 days and applied when it arrives. Because interest on most closed-end U.S. mortgages is computed monthly, not daily, the mid-month arrival does not save a meaningful extra slice of interest beyond the 13th payment. The benefit is still the extra principal.

2. “Accelerated biweekly” held in suspense

Some companies draft every two weeks but only apply a full monthly payment on the due date. The second half sits until the next cycle. You still get 13 applications a year if they actually remit the extra — but you have given them an interest-free float. Read the agreement. Third-party biweekly services sometimes charge a setup fee for this.

3. DIY monthly extra

You keep the regular monthly draft and add P&I ÷ 12 as extra principal. Same annual principal, no third-party fee, and you stay on the statement you already understand. This is what AmortSheet models.

What this calculator does not pretend

AmortSheet is a monthly ledger. It does not simulate daily interest, mid-cycle application, or a servicer that holds funds. If your note is a simple-interest loan that accrues daily (some HELOCs, a few credit-union products), a true biweekly draft can save a little more than the monthly-equivalent extra. Ask the servicer how interest is computed before you pay a company to “set up biweekly.”

A clean way to compare

1. Enter your principal, rate, term, and first payment on the calculator.
2. Note the baseline payoff and total interest.
3. Click “Apply that extra monthly” to load the biweekly equivalent.
4. Read the new payoff date and interest saved.
5. Export both CSVs if you want to show a spouse, advisor, or loan officer the rows.

If the interest saved is smaller than the fee a biweekly service charges, skip the service and send the extra yourself. For more on lump sums versus habits, see theextra payments guide.