Standard Tier Module
Loan Amortization with Precision Audit
Exact amortization math with a GeoNum drift audit that shows where naive f64 accumulation stops being trustworthy.
See it run - a worked example, 100% in this browser tab
The problem
Lenders and borrowers rely on amortization schedules whose running-balance recurrence quietly accumulates floating-point roundoff over hundreds of payments, and most tools never tell you when the printed figures stop matching the exact math.
The local-first solution
This plugin runs the standard fully-amortizing fixed-rate formula client-side, builds the full per-period schedule against an exact closed-form balance reference, and surfaces a GeoNum drift verdict - all deterministic arithmetic with cited formulas and nothing sent to any cloud.
What it does
Level monthly payment via the present-value-of-annuity formula, including the interest-free (r=0) case
Full per-period principal, interest, and running-balance schedule rounded to cents as a lender would
Exact closed-form balance reference B_k for every payment period
GeoNum drift audit comparing f64 recurrence, GeoNum recurrence, and exact balance
Final-payment residual adjustment so the loan closes at exactly zero
Total paid and total interest computed from the rounded schedule actually paid
Honest scope
The payment formula, per-period arithmetic, and closed-form balance are exact under the standard U.S. monthly-compounding convention (r = APR/12), and the rate and convention used are always shown so a wrong convention cannot silently mis-amortize. Points, origination fees, PMI, escrow, prepayments, ARMs, late fees, and actual/360 or actual/actual day-count accrual are flagged but not modeled. Not financial, tax, or legal advice - confirm the rate, convention, and fees against your loan note and Reg Z disclosure.
Authorities cited
- Standard fully-amortizing fixed-rate level-payment formula: M = P r / (1 - (1+r)^-n), the inverse of the present value of an ordinary annuity, with per-period rate r = nominal annual rate / 12 and n = 12 * years.
- Broverman, S. A. (2017). Mathematics of Investment and Credit, 7th ed. ACTEX. - Amortization of a loan; the outstanding-balance identity B_k = P(1+r)^k - M((1+r)^k - 1)/r (retrospective / prospective methods).
- Brealey, Myers & Allen (2020). Principles of Corporate Finance, 13th ed., McGraw-Hill. - Present value of an annuity and the annuity (mortgage) payment formula.
- Kellison, S. G. (2009). The Theory of Interest, 3rd ed. McGraw-Hill. - Nominal vs effective rates; nominal annual rate compounded monthly (the r = i^(12)/12 convention used here).
- 12 CFR Part 1026 (Regulation Z) / Truth in Lending Act, 15 U.S.C. 1601 et seq. - The disclosed APR folds in finance charges and may differ from the note rate used here; fees / points are NOT modeled (confirm against your TILA disclosure).
- Goldberg, D. (1991). What Every Computer Scientist Should Know About Floating-Point Arithmetic. ACM Computing Surveys 23(1), 5-48. DOI 10.1145/103162.103163. - Accumulated roundoff in iterative recurrences (the precision-audit basis).
- Higham, N. J. (2002). Accuracy and Stability of Numerical Algorithms, 2nd ed. SIAM. DOI 10.1137/1.9780898718027. - Forward error growth of recurrences; conditioning of the running-balance update.
Audit a schedule now
Run an amortization schedule in your browser and route the audited results into a Sandbox workspace or attach them to a Worklog case. Everything computes locally - nothing is uploaded to anyone's cloud.