Cernarus

Mortgage Interest Calculator

This tool models mortgage amortization under accelerated biweekly payment schedules (26 payments per year) and standard monthly schedules. It estimates payment amounts, total interest paid, and time-to-payoff and lets you add a recurring extra amount per biweekly period.

Results are estimates based on standard amortization math. Real-world outcomes can differ due to rounding policies, payment timing, escrow changes, loan servicing rules, and fees. Use the values here for planning and comparison, not as a regulatorily binding figure.

Updated Nov 29, 2025

Provides side-by-side estimates for total interest and payoff time for a standard monthly schedule and an accelerated biweekly schedule (26 payments/year). Calculates estimated interest savings and years saved.

Inputs

Results

Updates as you type

Total interest (monthly)

$184,968.26

Total interest (biweekly)

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Interest saved (monthly vs biweekly)

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Estimated years saved

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Biweekly payoff years

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OutputValueUnit
Total interest (monthly)$184,968.26USD
Total interest (biweekly)—USD
Interest saved (monthly vs biweekly)—USD
Estimated years saved—years
Biweekly payoff years—years
Primary result$184,968.26

Visualization

Methodology

Calculations use classical amortization formulas: periodically compound the annual nominal rate to a per-period rate and compute the fixed payment that amortizes the loan over the chosen number of periods.

Accelerated biweekly schedules assume 26 equal payments per year. Because 26 half-month payments equal 13 monthly payments, the accelerated schedule typically reduces principal faster and shortens the loan term.

Where extra payments are entered, the calculator includes them as fixed additional principal applied each biweekly period and recomputes payoff using logarithmic solution for the number of periods until balance reaches zero.

Worked examples

Example: $300,000 principal, 3.5% APR, 30 years. Standard monthly payment computed with r=3.5%/12 and n=360. Accelerated biweekly computes payments with r=3.5%/26 and n=780 and shows the reduced payoff time and interest.

Example with extra payment: adding a fixed $50 per biweekly period reduces principal faster. The calculator estimates the new payoff periods using the logarithmic solution above.

Key takeaways

Use this calculator to compare accelerated biweekly schedules to standard monthly schedules and to estimate the effect of recurring extra payments.

Results are estimates. For contract-accurate payoff figures, consult your loan servicer and review your loan agreement.

Expert Q&A

Does 'biweekly' always mean faster payoff than monthly?

Not necessarily. Accelerated biweekly (26 payments/year of half the monthly amount) typically results in faster payoff because it effectively makes one extra monthly payment per year. Converting a monthly payment into 24 biweekly payments (not 26) does not speed payoff.

Are these numbers exact for my loan?

These are mathematical estimates that assume the lender applies payments immediately to principal and uses standard amortization. Lender-specific rules for payment posting, rounding, escrow, prepayment penalties, or required minimum payments can alter the result.

Why is an extra fixed amount per biweekly period useful?

An extra fixed principal payment applied regularly reduces loan balance sooner and typically reduces both total interest and term length. This calculator models simple recurring extras per biweekly period.

How should I interpret negative or undefined outputs?

If the periodic payment is lower than the interest-only amount (payment less than or equal to principal multiplied by the periodic rate), the formulas do not produce a valid amortizing solution. In practice you would need to increase the payment or change the term.

Sources & citations