COMPARISON · MONEY · MORTGAGES
On a $400,000 loan at 6.5% for 30 years, the monthly schedule pays $510,177.95 in interest over 30 years — the biweekly schedule pays it off in 24.15 years with $392,682.25 in interest. Biweekly saves $117,495.71 and shaves 5.85 years. The catch: it wins only because 26 half-payments equal 13 monthly payments a year — and adding 1/12 extra to each monthly payment does the identical thing, for free. Run your own numbers below.
Enter your loan amount, annual rate, and term. The calculator amortizes the standard monthly schedule (P&I via the amortization formula) and simulates a biweekly schedule — half the monthly payment every 14 days at the annual rate divided by 26 — until the balance hits zero, then names the interest saved and years shaved.
Biweekly wins because 26 half-payments = 13 monthly payments a year. That 13th payment is one full extra monthly payment of principal per year, and it attacks the balance early — which is where interest savings come from. At the defaults ($400,000 at 6.5% for 30 years): monthly P&I is $2,528.27, total interest $510,177.95 over 30 years; biweekly pays $1,264.14 every 14 days, finishes in 24.15 years, and total interest is $392,682.25 — biweekly saves $117,495.71 and shaves 5.85 years. But you can get the identical payoff by adding 1/12 of your payment (about $210.69 here) to each monthly payment yourself — no biweekly plan, no setup fee, and you keep control of the extra dollars.
Setup: $400,000 at 6.5% for 30 years (360 months).
Line 1 — the monthly payment. M = P × (r/12) / (1 − (1 + r/12)^−360) = $400,000 × 0.0054167 / (1 − 1.0054167^−360) = $2,528.27 P&I. That is the baseline everything else is measured against.
Line 2 — the monthly schedule. $2,528.27 × 360 = $910,177.95 paid; minus $400,000 of principal = $510,177.95 in interest over the full 30 years.
Line 3 — the biweekly simulation. Half the payment ($1,264.14) every 14 days, with interest accruing at 6.5% ÷ 26 per period. The balance hits zero after 628 payments = 289.85 months = 24.15 years. Total interest: $392,682.25.
Line 4 — the verdict. $510,177.95 − $392,682.25 = $117,495.71 saved; 360 − 289.85 months = 5.85 years shaved. Biweekly wins — but the win is the extra principal, not the payment rhythm.
Branch A — $400,000 at 5.9% for 15 years (biweekly barely helps): monthly P&I $3,353.86, total interest $203,693.91 over 180 months. Biweekly: 344 payments = 158.77 months (13.23 years), interest $176,560.55 — saves $27,133.36 and shaves 1.77 years. Same mechanism, far smaller prize: a 15-year loan already pays down fast, so the extra 1/12 has less interest left to kill.
Branch B — $400,000 at 7.5% for 30 years (bigger savings): monthly P&I $2,796.86, total interest $606,868.89 over 360 months. Biweekly: 606 payments = 279.69 months (23.31 years), interest $446,472.63 — saves $160,396.27 and shaves 6.69 years. Higher rates make every extra principal dollar worth more, so biweekly's edge grows with the rate.
Branch — the error case: enter a rate over 25% or a term over 50 years and the calculator refuses — those are typos, not mortgages.
| Output | Formula | What it means |
|---|---|---|
| Monthly P&I | P × (r/12) / (1 − (1 + r/12)^−360) | $2,528.27 at the defaults — the standard amortized payment, the baseline for both schedules. |
| Monthly-schedule interest | M × months − P | $510,177.95 at the defaults — every dollar of interest a full-term borrower pays. |
| Biweekly payoff | simulation: half-payment every 14 days at r/26 | 289.85 months (24.15 years) at the defaults — 628 half-payments instead of 360 monthly ones. |
| Biweekly interest | total paid under biweekly sim − P | $392,682.25 at the defaults — what the faster payoff actually costs in interest. |
| Interest saved | monthly-schedule interest − biweekly interest | $117,495.71 at the defaults — the headline number to decide on. |
| Years shaved | (360 − biweekly months) ÷ 12 | 5.85 at the defaults — how many years sooner the loan dies. |
The math covers principal and interest only — taxes, insurance, PMI, and escrow are not in these numbers (they are identical under both schedules anyway). It assumes a fixed rate; adjustable-rate loans need a different model. The biweekly simulation pays half the monthly P&I every 14 days with interest accruing at the annual rate divided by 26 — real lender biweekly plans vary in exactly how they credit payments, so your servicer’s result can differ slightly. It does not model prepayment penalties (check your loan terms) or opportunity cost — extra principal earns your mortgage rate, guaranteed, but money invested elsewhere could earn more (or less). This is educational math, not financial advice.
Bright Side Kit. "Biweekly vs Monthly Mortgage Calculator | Bright Side Kit." https://brightoolbox.com/biweekly-vs-monthly-mortgage/ Accessed October 23, 2026.Data sourcesYou pay half your monthly P&I every 14 days instead of the full amount once a month. Since a year has 52 weeks, you make 26 half-payments — the equivalent of 13 monthly payments. That 13th payment is pure extra principal, and extra principal early in the loan is what saves the interest.
It is not the same — you pay one-twelfth more per year. 26 half-payments = 13 monthly payments, i.e. 108.33% of a normal year of payments. The extra 8.33% goes straight to principal, cutting the balance the interest is charged on. The rhythm (every 14 days) barely matters; the extra money does everything.
Essentially yes. Adding $210.69 (1/12 of $2,528.27) to each monthly payment at the defaults pays the loan off in about the same 24 years with about the same interest — and you skip the biweekly setup fee, keep the extra dollars under your control, and can stop anytime. Most servicers let you designate the extra as principal-only.
Often yes — many lenders and third-party plan providers charge a one-time enrollment fee (sometimes a few hundred dollars) and occasionally a per-payment fee. Those fees eat into the interest savings. That is exactly why the DIY route — adding 1/12 to your monthly payment — is usually the better deal: same math, zero fees.
Barely. At the Branch A defaults ($400,000 at 5.9% for 15 years) biweekly saves $27,133.36 and shaves only 1.77 years — a 15-year loan already pays down so fast that the extra 1/12 has little interest left to kill. The biweekly edge is biggest on long, high-rate loans (Branch B at 7.5%: $160,396.27 saved).
No — read the fine print. Some "biweekly" plans just split your monthly payment into two halves on the 1st and 15th without adding the 13th payment — those save almost nothing (the interest saved is only from paying half a few days early). The savings come from 26 half-payments = 13 monthly payments; if the plan doesn't deliver that extra payment a year, skip it.