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amortization-schedule

Loan amortization math — the fixed-rate monthly payment, the per-period interest/principal split, the remaining balance, and lifetime interest. The arithmetic behind every mortgage, auto loan, and asset-backed loan schedule.

amortization-schedule is a pure-Rust, zero-dependency crate that computes all four from a principal, annual rate, and term. It is the math engine behind the AssetLoanCalculator amortization tool, a free calculator for asset-backed loans.

Why amortization matters

An amortizing loan is paid down in equal installments, but each payment's interest/principal mix shifts over time. Early payments are mostly interest; late payments are mostly principal. The schedule makes that shift explicit:

Month Payment Interest Principal Remaining balance
1 $665.30 $583.33 $81.97 $99,918.03
60 $665.30 $541.33 $123.97 $92,563.05
180 $665.30 $336.36 $328.94 $57,364.84
360 $665.30 $3.88 $661.42 $0.00

($100,000 loan at 7% over 30 years.) Knowing the schedule tells you how much lifetime interest you pay (~$139,508 on that loan) and how fast you build equity.

Install

[dependencies]
amortization-schedule = "0.1"

Quick start

use amortization_schedule::{monthly_payment, total_interest, schedule};

// $100k loan, 7%, 30 years
let payment      = monthly_payment(100_000.0, 0.07, 360);  // ≈ 665.30
let lifetime_int = total_interest(100_000.0, 0.07, 360);   // ≈ 139,508.89
let rows         = schedule(100_000.0, 0.07, 360);         // 360 rows

let (interest, principal_paid, remaining) = rows[0];       // first period
// interest ≈ 583.33, principal_paid ≈ 81.97, remaining ≈ 99,918.03

Build a full schedule interactively

The crate gives you the raw math. For an interactive table with extra payments and export, use the AssetLoanCalculator amortization tool.

Features

  • Pure Rust, no deps — f64 arithmetic, single file, no features to enable.
  • Monthly payment — standard fixed-rate amortization formula.
  • Per-period split — interest, principal, remaining balance per period.
  • Full schedule — Vec of every period, paid down to zero.
  • Lifetime interest — total cost of the loan above principal.
  • MIT licensed — embed anywhere.