Loan Comparison Calculator: Side-by-Side
Compare two loans side-by-side. Calculate monthly payment differences, lifetime interest savings, break-even on discount points, and total costs with Excel export.
Interactive WebAssembly Calculation Engine
This tool runs 100% in your local web browser tab. All amortization schedules, debt yield calculations, and tax deferral projections are computed client-side in WebAssembly. Zero bytes are uploaded to remote servers.
Use the interactive form above to adjust parameters, toggle extra principal schedules, stress-test interest rates, and export full reports to Excel (.xlsx) or CSV.
Underwriting Methodology & Mathematical Formulation
Our calculation algorithms strictly adhere to institutional lending conventions, CFPB disclosure rules, and statutory IRS definitions:
- Standard Annuity Formula: Monthly payment M = P * [r(1+r)^n] / [(1+r)^n - 1], calculating exact interest and principal allocations down to the penny.
- Zero-Roundoff Double Precision: Computations execute using standard IEEE 754 floating-point and integer math to match commercial bank loan documents.
- Sensitivity Analysis: Real-time stress testing against interest rate fluctuations, vacancy rate increases, and balloon maturity horizons.
Frequently Asked Questions
How do discount points affect my loan comparison?
One mortgage point costs 1% of the loan amount and typically lowers your interest rate by 0.25%. To determine if paying points is worthwhile, divide the upfront cost of the points by the monthly payment savings. If you plan to keep the loan longer than the break-even period (typically 36 to 60 months), buying points saves money.
Why can a 15-year loan save hundreds of thousands in interest?
A 15-year loan features higher monthly principal and interest payments than a 30-year loan, but cuts the compounding timeline in half and usually carries a lower interest rate (0.5% to 1.0% lower). This combination drastically reduces total lifetime interest payments.
How is the break-even point on loan upfront fees calculated?
Break-even in months = (Total Upfront Closing Costs of Loan B - Total Upfront Costs of Loan A) / (Monthly Payment of Loan A - Monthly Payment of Loan B). It tells you how many months you must hold the loan before lower monthly payments compensate for higher upfront origination fees.
What is the benefit of making extra monthly principal payments?
Any extra dollar paid directly toward principal shortens the repayment schedule and eliminates the compound interest that would have accrued on that dollar across remaining years. Even an extra $100/month can shave 3 to 5 years off a 30-year term.