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Conventions

r = APR / 12 (monthly periodic rate).
Standard payment: PMT = P * r / (1 - (1 + r)^-n)
Negative amortization guard: if M ≤ r * B, the payment doesn’t cover interest.

Monthly amortization (baseline)

Interest: I_t = B_{t-1} * r
Principal: Pr_t = max(M - I_t, 0)
Extras (if any) are applied to principal after the split: Extra_t = ExtraMonthly + ExtraAnnual_t + ExtraOneTime_t
Balance update: B_t = max(B_{t-1} - Pr_t - Extra_t, 0)
Stop when B_t ≤ $0.01; final period is capped to exactly clear the loan.

Inferring remaining term

If remaining term isn’t provided but M is known, we infer n = ceil( ln(M / (M - r*B)) / ln(1 + r) ).

Extras

Monthly extra: added each month.
Annual extra: added when month_in_year = extra.annualMonth.
One‑time extra: added at month = extra.oneTimeMonthOffset.
Extras are capped if they would overpay the final balance.

Biweekly

Exact daily: accrue daily at APR / (365 or 360), apply half‑payment every 14 days.
Approx 13× monthly: equivalent to adding M/12 extra per month (≈ 13 payments/yr).

Refinance

At refi month k: remaining balance B* becomes new principal P2 = B* +