Bi-weekly vs monthly mortgage: the thirteenth-payment trick. Form MC-10
Pay half your monthly payment every two weeks and you make 26 half-payments a year — 13 full payments instead of 12. That one extra payment a year goes straight at principal, and principal is what interest is charged on. On the seeded $450,000 example ($360,000 loan, 6.5%, 30 years): 625 bi-weekly payments, done in 23.97 years, $108,503.22 less interest than paying monthly. The comparison panel below is open — change the numbers and watch the savings move.
Tool № 10 · Monday, September 28, 2026 · free, always
26 half-payments is 13 full ones. The 13th is pure principal attack.
Input slip · Form MC-10
Two tabs, one ledger. Affordability solves your income backwards into a max purchase price; Monthly payment prices a specific house down to the dollar. Everything recalculates as you type, and everything stays in your browser — nothing is sent anywhere.
Prefilled at the Freddie Mac 30-year average for the week of Sept 24, 2026 — replace it with your actual quote.
Set to 6.5% for the worked example below (the Freddie Mac 30-year average for the week of Sept 24, 2026 was 7.03%) — replace it with your actual quote.
Posted entry · Form MC-10-R
More doors into the same ledger Form MC-10
Every angle on the mortgage, same arithmetic underneath:
Where $108,503 comes from Form MC-10
Monthly: 360 payments of $2,275.44 in principal and interest, $459,160.16 in lifetime interest. Bi-weekly: 625 payments of $1,137.72, $350,656.94 in lifetime interest. Same loan, same rate — the only difference is timing.
The mechanism is boring, which is why it works. Interest accrues daily on the balance; every extra dollar of principal paid this year is a dollar that never accrues interest again. Thirteen payments a year instead of twelve is an 8.3% overpayment applied entirely to principal, compounding for decades.
The catch is plumbing, not math: your lender has to credit the half-payments promptly, and some servicers hold them until a full payment assembles. If yours won’t play, add one-twelfth of P&I to each monthly payment marked principal-only — nearly the same result, no permission needed. Skip third-party bi-weekly services that charge fees for the privilege.
Common questions Form MC-10
How does paying bi-weekly save interest?
26 half-payments a year equal 13 full monthly payments instead of 12. The extra payment attacks principal directly, and less principal means less interest charged in every period after.
Is bi-weekly the same as paying a little extra each month?
Nearly. Adding one-twelfth of your P&I to each monthly payment makes the same 13th payment over the year. Bi-weekly just automates it — and pays the extra slightly earlier, which saves a touch more.
Will my lender let me pay bi-weekly?
Some do, some don’t, and some hold half-payments until they total a full one — which kills most of the benefit. Ask your servicer how partial payments are credited before switching.
Should I pay for a bi-weekly payment service?
Usually not. Third-party services charge setup and per-payment fees for something you can do free: your lender’s own bi-weekly plan, or a monthly principal-only extra.
Does bi-weekly help if I have PMI?
Yes, indirectly: faster principal paydown crosses the 80%-of-price line sooner, so PMI drops off earlier — on top of the interest savings.
Is this financial advice?
No. It’s amortization arithmetic on numbers you type: it doesn’t check your credit, approve loans, or know your local tax rate. A lender’s pre-approval and your own budget have the final word.