Cernarus

Boat Loan Interest Calculator

This calculator estimates payments, total interest, and payoff time for boat loans using either monthly amortization, bi-weekly amortization with 26 payments per year, or an accelerated bi-weekly approach where each payment equals half the monthly payment. Use the method that matches your lender or your intended payment plan.

Results are produced with standard amortization mathematics. Outputs are estimates intended for planning; actual lender calculations may differ due to rounding, payment timing, fees, or different compounding rules.

Updated Nov 15, 2025

Compute bi-weekly payments using 26 equal periods per year and standard annuity formula applied to the bi-weekly periodic rate.

Inputs

Results

Updates as you type

Bi-weekly payment (26/yr)

$153.57

Total interest paid (bi-weekly)

$9,928.23

Equivalent monthly payment (estimate)

$332.74

OutputValueUnit
Bi-weekly payment (26/yr)$153.57USD
Total interest paid (bi-weekly)$9,928.23USD
Equivalent monthly payment (estimate)$332.74USD
Primary result$153.57

Visualization

Methodology

Monthly amortization uses the standard annuity formula with a monthly periodic rate (annual_rate / 12) and n = term_years * 12.

Bi-weekly annuity treats the loan as having 26 equal compounding periods per year and applies the annuity formula with periodic rate = annual_rate / 26 and n = term_years * 26.

Accelerated bi-weekly assumes each bi-weekly payment equals half of the standard monthly payment; because 26 bi-weekly payments produce the equivalent of 13 monthly payments per year, this typically shortens the term and lowers total interest. We estimate the required number of payments by solving the annuity formula for n.

Worked examples

Example 1: $30,000 at 6% for 10 years. Monthly payment is calculated with r = 0.06/12 and n = 120. Bi-weekly annuity uses r = 0.06/26 and n = 260.

Example 2: If you make bi-weekly payments equal to half your monthly payment, you typically pay off the loan faster than the nominal term because you make the equivalent of 13 monthly payments per year instead of 12.

Key takeaways

Choose the method that matches your lender (monthly, bi-weekly annuity, or accelerated bi-weekly) to get comparable estimates.

This tool follows standard amortization math and general engineering and quality guidance (IEEE, ISO, NIST). Treat outputs as planning estimates and confirm final figures with your lender.

Further resources

Expert Q&A

Why are bi-weekly payments sometimes cheaper overall?

Making 26 bi-weekly payments (or making half-monthly payments twice per month plus the extra two payments per year) reduces principal faster and shortens the loan life, which lowers total interest compared with 12 monthly payments of the same nominal monthly amount.

Does this calculator include fees or taxes?

No. This tool calculates principal and interest only. It does not include origination fees, title, taxes, insurance, or other charges. Add those separately when evaluating total cost.

Why might my lender's schedule differ?

Lenders may use different compounding rules, day-count conventions, rounding, payment application order, or include fees. Always confirm with your lender for a definitive payoff schedule.

How precise are results?

Results use floating-point arithmetic and closed-form formulas. They are suitable for planning but may differ from lender statements by small rounding tolerances. See accuracy notes and standards below.

Sources & citations