Ledger Math Notes

Most loan questions are a loop and a formula away from a clear answer. These notes build the tools from scratch, check them against the standard formulas, and use them to compare real choices with the same assumptions.

Guides

• 
  Amortization schedules in Python: extra payments, prepaying, and recasting a mortgage

  Every mortgage calculator online gives you the same handful of numbers: the payment, the total interest, maybe a payoff date. What they rarely let you do…

The core formula

For a fixed-rate loan with balance L, monthly rate r, and n payments, the payment is P = r × L / (1 − (1 + r)⁻ⁿ). Each month, interest is r times the balance, and the rest of the payment reduces principal.

General information only, not financial advice.